Thesis - 東京大学wyvern.phys.s.u-tokyo.ac.jp/f/Research/arch/Thesis_Son.pdfThesis Photoemission...
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Thesis Photoemission study of intermetallic itinerant -electron magnets Jin-Y oung Son Department of Physics ， Graduate School of Science ， University of Tokyo December 2000
Transcript of Thesis - 東京大学wyvern.phys.s.u-tokyo.ac.jp/f/Research/arch/Thesis_Son.pdfThesis Photoemission...
Thesis
Photoemission study of intermetallic
itinerant -electron magnets
Jin-Y oung Son
Department of Physics， Graduate School of Science，
University of Tokyo
December 2000
論文の内容の要旨
論文題目 Phot侃 missionstudy of intennetallic 泊施rant-el即位onmagn出
(金属開化合物遍歴磁性体の光電子分光)
氏名 孫 珍永
1.序
物質の性質はその電子状態で決まるが、電子聞の多体的な相E作用に起因する電子相聞は高温
超伝導、金属一絶縁体転移、磁性など様々な複雑な物性の原因である。特に「遍歴電子磁性体Jとし
て知られる一連の金属開化合物は、電子相関が原因で多様な磁性を示すことから、長い間理論、実
験の両方から研究がされてきた。その中でも 3d遷移金属間化合物は反強磁性、強磁性、常磁性、
メタ磁性、スピンゆらぎなどを含む非常に多様な磁気的性質を示すものとして多くの注目を浴びて
きた。一般的に3d電子は電子相聞が強いため、電子相関効果の研究は金属間化合物の電子状態を
理論、実験の両面から理解する上で重要な役割を果たす。これらの遷移金属間化合物の磁気的性質
を電子相聞という立場から理解するためには、電子相関を考慮していないバンド計算のl電子効果
と、電子相関の効果がそれぞれどれだけ実験で観測される物性に寄与しているかを知ることが重要
な鍵となる。それを解明することは容易ではないが、バンド計算の結果と電子状態を直接観測する
強力な方法である光電子分光の結果の比較はこの目的に最も適した手法で、近年この手法を用い
て電子相関の効果をかなり詳細に調べることが可能になってきた。本論文では、 B・20型の結品構
造を持つFexCOl_xSiおよび MnSi.C・15型立方晶ラーベス型結晶構造を持つ化合物 Yl・xScxMn2の遷
移金属間化合物の系を取り上げ、それらの電子状態と磁性との関係に関する知見を得ようとした。
FCxCOl_xSiは、特異な磁気的、電気的性質を示す。常磁性半導体であるFeSiは基底状態におい
ては非磁性であるが、その帯磁率は温度と共に急速に瑚大し、約 500K付近で幅広い極大を
示す。一方、半金属 CoSiは反磁性体である。 FexCol_xSiはこれらの合金であるにもかかわらず、
その中間濃度領域において弱い遍歴ヘリカル反強磁性を示す。この物質はMnSiと同じように、あ
る臨界磁場でヘリカノレ構造からコニカノレ構造に磁気転移を起こす。
MnSiはネール温度 30K以下、ゼロ磁場で長い周期のヘリカルスピン構造を持つ反強磁性化合
物であるが、わずか 6kOeの磁場で強磁性状態に転移する遍歴電子磁性体である。最近、この化合
物に対し高圧下における磁化測定が行われ、約 15kbarで常磁性に転移すること、磁気秩序状態に
磁場を加えるとヘリカル構造からコニカノレ構造を経て、磁化が飽和することが観測されている。ま
た、ヘリカルーコニカル転移磁場は圧力の増加と共に減少することも観測されている。
最後にラーベス相化合物は、複雑な化合物磁性の解明を進めるために有益な知見を与えてくれ
る典型的な磁性体の一つである。立方品ラーベス相化合物 YMn2は Mnのモーメントが 2.7μB、
ネール温度約 1∞Kの反強磁性体であり、その転移温度以下では体積が約 5%も増加する。 Yの3
%をScに置換したYO.97SC0.Q3Mn2は 150mJ/K2mo1という極めて大きな値を持つ常磁性体となる。
バンド計算から予想される電子比熱係数と比べて、実験値は約 23倍大きいことになる。これは 3
d遷移金属問化合物の中でも非常に大きく、電子の有効質量が 5f電子系と同程度に重い。
本論文では以上の磁性体について、光電子分光による電子状態の研究を行った。 FexCol_xSiにつ
いては光電子スベクトルにrigid-bandmodel及びFeSiとCoSiの重ね合わせモデルを適用し解析を行っ
た。 MnSiおよびYl・xScxMn2については、光電子スベクトルの解析にバンド計算に電子相関の効
果を考慮したモデル自己エネルギーを取り入れて行った。これらの方法は電子相闘を取り入れるた
めに有用な手段である。
2.実験方法
試料はYMn2，YO.97ScO.03Mn2， F，句.SC句 .2Si，FI句.5COO.sSiの多結品と MnSiとCoSiの単結晶を
用いた。測定はいずれの試料でも 10・I~orr前半の超高真空中で行い、試料の滑浮表面は測定槽内でのダイヤモンドやすりによるやすりがけによって得た。測定温度はそれぞれの試料について温度変
化をしながら行った。光源として MgKIα (1253.6eV)を用いたX線光電子分光(XPS)と、 He1 (hv=
21.2 eV)とHe11 (hv= 40.8 eV)を用いた紫外線光電子分光 (UPS)の測定を行った。
光電子分光は表面敏感な実験手段である。そこで本論文では、表面とバルク構造を区別するだめ
にm凶作合間 pa白とlatticeconstantを考慮した方法を用いてできるだけバルク成分を得ることをここ
ろみた。また、試料と電子エネルギ一分析器のなす光電子の脱出深さを変えることによりバルク成
分を分離することも試した。
3.結果
3.1. FexCol_xSi
FeO.SCOO.2SiとFeO.SCoO.SSiはそれぞれ UPSスベクトル (hv=21.2 eVとhv= 40.8 eV)を高
分解能で測定し、その電子状態を調べた。 Feo.sCOO.2SiとF句 .SCoO.SSiのスベクトルと、 CoSi，
FeSi，MnSiのスペクトルを比較をした。 CoSiで観測された -O.8eVでの構造が Feの置換に伴いフェ
ルミ準位の方に移動しながらその強度も小さくなることがわっかた。この結果は rigid-bandmodel
の予想で定性的に説明できることがわっかた。一方、 FeSiとCoSiの光電子スベクトノレの重ね合わ
r一、，
ヘーも.，
せとFeO.8 C<>().2 S iとFeO.5C句.5Siのスベクトルとの比較は rigid-bandmodelで予想した結果よりも
よい一致を見せた。すなわち、大きなエネルギースケルでは Co原子、 Fe原子の個性が電子状態に
反映されでいる。また、それぞれのスベクトルに対して温度変化を調べた。その結果、それぞれの
試料で温度の変化に伴うスベクトルの大きな変化は見られなかったが、 F句.8C句.2Siのフェルミ
準位近傍のスベクトノレ強度が低温でわずかに減少する現像が見られた。これは反強磁性転移点以下
の電気抵抗の上昇と符合している。
3.2. MnSi
低温で測定したスペクトルをバンド計算と比較した。電子相関の効果を取り入れたモデル自
己エネルギーの導入によって実験結果との再現性がよくなることがわかった。 これにより、 MnSi
の電子構造がバンド構造に電子相関の効果を取り入れることによって、よく説明されることがわかっ
た。 UPSスペクトルの温度変化を謂ベた結果、反強磁性から常磁性の転移に伴うごくわずかのス
ベクトルの変化があり、定性的には反強磁性秩序によるMn3dバンドの分裂として説明できる。
3.3. Yl・xScxMn2
YMn2とYO.97S句.03Mn2に対して、それぞれ UPSスベクトルを高分解能で測定し、その電子
状態を調ベた。また、YMn2はよりパノレク敏感な XPSも測定した。 Sc置換による反強磁性から常
磁性への転移に伴い、スペクトルがごくわずかに変化することが観測された。これは Sc置換によっ
てMn3dバンドが狭くなることを反映しているものと恩われる。また、低温で測定したYMn2と
YO.97Sco.03Mn2のスペクトルをバンド計算と比較した。比較の際、バンド計算に電子相関の効果
を取り入れるためにモデル自己エネルギーを導入し、実験結果をもっともよく再現するようにモデ
ル自己エネルギーのパラメーターを決定した。 YO.97SCO.03Mn2の場合はこれによって実験結果と
よく一致する結果を得ることができ、バンド計算では取り入られていない電子相聞の効果が重要で
あることがわかった。この際、大きな電子比熱とコンシステントに、フェルミ準位近傍の自己エネ
ルギーの強い温度依存性を入れる必要があった。この自己エネルギーはフラストレーションによる
スピンゆらぎを反映しているものと思われる。一方、温度による UPS、XPSスベクトルの変化は、
FexCOl_xSi， MnSiに比べても小さいことがわかった。
4.結論
本論文ではFexCOl_xSiおよびMnSiのUPS，ラーベス相化合物YMn2のXPS及び UPSと
YO.97ScO.03Mn2のUPSを測定した。 FexCOl-xSiでは組成によるスペクトルの変化が観測され、
rigid-band modelで予想した結果とよく会うことがわっかた。 MnSiおよび Yl・xScxMn2の UPSスペ
クトルは、モデル自己エネルギーにより電子相関の効果を取り入れてバンド計算とを比較、解析し
た。 これら物質についてバンド構造と電子相闘の効果がともに重要であることがわかった。また、
それぞれ物質の温度による変化はバンド理論で予想されるものと定性的に一致したが、その変化は
フェルミ準位近傍に限られ、バンド理論よりも小さいことがわかった。しかしなから、フェルミ準
位近傍のみ弱い温度変化が観測されることは、磁気相転移の温度スケールとも矛盾しない。
Contents
αlapter 1. In町付uction..................................................... ................. ............. 1
Chapter 2. Photoemission spec町'Oscopy..・H ・H ・H ・....・ H ・.....・ H ・....・ H ・.....・ H ・..7
2.1 General Principles ...・ H ・.....・ H ・..…...・ H ・..…...・ H ・.....・ H ・..… 7
2.2 Experimental …...・ H ・.....・ H ・.....・ H ・..…...・ H ・.....・ H ・H ・H ・..…・・ 7
2.3 Single-particle spec甘alfunction and
effect of self-energy correction .… H ・H ・.....・ H ・..…...・ H ・..18
Chapter 3. Photoemission study of the itinerant helimagnet
民向去位................................................................................25
3.1 Introduction …...・ H ・..…...・ H ・.....・ H ・..…...・ H ・.....・ H ・.....・...25
3.2 Experiment ...・ H ・H ・H ・.....・ H ・..…...・ H ・..…...・ H ・.....・ H ・..…・・ 33
3.3 Results and discussion ...・ H ・.....・ H ・.....・ H ・H ・H ・.....・ H ・..… 33
3.4 Summary …・.............................................................44
Chapter 4. Photoemission study of the itinerant helimagnetic
民生lSi・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・…47
4.1 Introduction …...・ H ・.....・ H ・H ・H ・.....・ H ・.....・ H ・.....・ H ・H ・H ・...47
4.2 Experiment ...・ H ・.....・ H ・.....・ H ・.....・ H ・.....・ H ・.....・ H ・.....・ H ・.56
4.3 Results and discussion ...・ H ・H ・H ・..…… H ・H ・...・ H ・.....・ H ・..56
4.4 Summary .…….日….日.…….日…….日…….日…….日…….日…….日…….日…….日…….日…….日…….日…….日…….日…….日…….口…….日…….日…….日…….日…….日…….日…….日…….日…….日…….日…….日…….日…….日…….“…….日…….日…….日…….日…….日…….日…….日…….日…….日…….日…….日…….日…….. …….日…….日…….日…….日…….日…….日…….日…….日…….日…….日…….日…….日…….日…….日…….日…….日…….日….日.6ω8
Chapter 5. Photoemission study of the Laves-phase compounds
Y乱~andYo.抑~.æ~................................................... 71
5.1 Introduction …...・ H ・.....・H ・..…...・H ・..…...・ H ・.....・ H ・H ・H ・...71
5.2 Experiment …...・ H ・.....・ H ・..…...・ H ・..…...・H ・.....・ H ・.....・ H ・.79
5.3 Resu1ts and discussion ...・H ・..…...・H ・.....・ H ・H ・H ・.....・ H ・...79
5.4 Summary .…….日….日.…….日…….日…….日…….日…….日…….日…….日…….日…….日…….日…….“…….日…….日…….日…….日…….日…….日…….“…….日…….日…….日…….日…….日…….日…….日…….日…….日…….リ……..…….日……..…….日…….日…….日…….日…….日…….日…….日…….日…….日……..…….日…….日……..…….日…….日…….日…….日…….日…….日…….日…….日…….日…….日…….日…….日…….日…….日…….日….日.8ω9
αlapter 6. Summary and Conc1usion …...・H ・.....・H ・....・H ・....・H ・-……........93
API光ndix………........…H ・H ・-………………………………………….........…..98
Acknowledgmerr包......................................................................................103
Chapter 1.
Introduction
I曲lerant・electr'Onm伊 e也m加蹴m蹴必c∞mpoun也 hasbeen由esut脚 t'Of
great副館詑st企ombo血血叙>reti伺 1and e珂刻erimentalpoints 'Of view. Studies 'Of
magnetism in these c'Ompounds have been perf'Onned for a l'Ong time. Am'Ong them， 3d
佐ansiti'Onme凶sand血位 c'Ompoundssh'Ow a wide variety 'Of el即位'Onicand magnetic
properties， including an姐ferromagnetism，伽T'Om相 letism， parョma伊 e伽n，
metama伊 etiSInand spin fluctuati'Ons. In order to unders阻nd血.eirmagnetic戸'0戸柑鎚， it
is important旬 clarifythe el民的 国.cs旬飽S 'Of these c'Ompounds thω，retically and
e却erimen凶 y.As is we1l kn'Own， ell民館m ∞if1'elati'Onis generally str'Ong f'Or 3d
el配位四1sand play an加portantrole泊血位electro凶.cand magnetic prope凶.es.
τbemagnetic組 delectronic properties 'Of 3d住姐siti'Onme旬1m'On'Osilicides wi白血e
cubic B-20 structure have been血esubject 'Of numerous investigati'Ons.官邸 isbecause
血eye.油ibitdifferent types 'Of magnetic and el即位羽田 pr'Opertiesdepending 'On血e
c'Onstituent elements [1.1]. Among血悶， we have chosen tw'O kinds 'Of mon'Osilicides，
MnSi and Fe.玄C'Ol_XSi.MnSi h邸血.ehelical spin struc加rewith a l'Ong period 'Of 180 A
bel'Ow 30 K [1.2].百 lem碍ne也ati'Onat l'Ow胞mpera旬開sincreases alm'Ost linearly wi血
increasing field up t'O 6 kOe [1.3] and then saturates at 0.4 JlB per Mn at'Om， which is
substantial1y smal1er血m血ee百ectivemagnetic m'Oment 'Of 1.4μB estimated合om血e
Curie-Weiss behavior 'Of the magnetic susceptibility at high tempera伽res[1.4].官1es関e
facts sh'Ow由a紙tMnS割ii匂sc10se ωa旬pl伺 1weak iti也ne悶e白r溜1t-屯elect佐r'Onfi島er汀romagnet.A1so， the
rnixed system betw民:ntwo m'On'Osilicides FeSi and C'OSi has been studied. FeSi is an
unusua1 paramagnetic narrow-gap sernic'Onduct'Or.百1esusceptibilityχ(T)血C陀鎚es
rapidl y with tempera加reand takes a maximum at -5∞K [1.5，6]. C'OSi is a diamagnetic
sernimetal with a tempera加reindependent susceptibility [1.7]. The mixed system
FexC'Ol_xSi exhibits a magnetical1y 'Ordered phase in the concentrati'On range between x =
0.2 and x = 0.95 at l'Ow tempera仰向s[1.8，9]. Beillle et al .[1.10，11] have f'Ound血at
FexC'O'_xSi has a helical sp泊 structurewith a l'Ong period儲 inMnSi f'Or 0.3 ~ x ~ 0.9 by
small angle neutr'On scattering experiment.百lehelical spin struc仰向'OfFexC'O'_xSi is
transf'Ormed t'O a c'Oni，伺l'Onewhen血eapplied magnetic field is increased， and fmally t'O
組 inducedferr'Omagnetic structure above a critical field Hc・
On由e'Other hand，血emagnetic pr'Operties 'Of a class 'Of泊tenn銅山cc'Omp'Ounds
AB2' which f'Orm白 血.eLaves-phase struc佃re，have als'O been studied intensively.百leIr
crystal structures are generally complicated and there are several types
[1.12，13，14，15，16]. Am'Ong these c'Ompounds， ~ with the cubic Laves-phase (C-15)
S加 C卸reis an itinerant eL民 tronantiferr'Omagnet wi血血eNeel tem戸m加問'Of-100 K
[1.17，18].百le'Observed magnetic m'Oment 'On the Mn at'Om is 2.7 J.ls per at'Om. Upon Sc
substi仙ti'Onf'Or Y，血.esys飽m bec'Omes par百nagnetic.It has b倒 1f'Ound由atthe l'Ow
tempera加reelectr'O国.cs戸cificheat coe伍cient'y 'Of Y O.97ScO.03Mn2 is as large出 150
mJ / m'OleK2 [1.19]， which is about 23 times larger血m 血atexpected 合omthe bare band
density 'Of states [1.20].官邸白1hancementfactor is unusually large am'Ong 3d electr'On
systems and is s泊副E 加血atof 51 eL田岡nsys飽ms.On the 'Other hand， Yamada
[1.21，2勾andTerao and Shimizu [1.23] have perf'Ormed band-struc加.recalculati'Ons f'Or
parama，伊eticand antifen四 nagnetic~ using血etight-binding approximation and血e
recursi'On meth叫 respectively.By making use 'Of these calculati'Ons，血.eyexplained the
temperature dependenω 'Of the susceptibility above T N and showed白紙theDOSat血e
Fermi 1evel (1;，) is mainly due t'O由emin'Ority-spin d band and the main part of the
m吋'Ority叩 血bandlies below Ep・
百leaim of血is血esisis t'O e.河期加en阻llyclar均F 出.eelectr'Onic states 'Of th'Ose
3
in蜘蜘lliccomp'Ounds and the influence 'Of electron correlation 'On them by -， ph'Otoemissi'On spectr'Osc'Opy combined with model self-energy analyses. Photoemission
spectrosc'Opy is 'One 'Of the most direct experimental methods t'O study the electronic
states in solids， and we expect t'O reveal the electronic structure 'Of these compounds.
Previously， x-ray photoemission spectroscopy (XPS) studies of MnSi as well as FeSi
and CoSi were made by Beillle et al .[1.11] and血enby Speier et al .[1.24]. Kakizaki et
al .[1.25] have performed an叫travioletphotoemission spectroscopy (UPS) using
synchrotron radiation in the excitation energy range from 40 to 130 e V wi血血e
resolution of 0.2 -0.8 eV. They observed出atthe width of the main band decreases in
the order of MnSi， FeSi and CoSi. ln this work， in order t
2
detailed at vari'Ous tempera加res.The res'Oluti'On 'Of the UPS spectra was -0.03 eV. On
the 'Other hand， in a p陀 vi'Ousw'Ork [1.26]， we have perf'Orrned血.eph'Ot侃 rnissi'Onstudy 'Of
Y(C'Ot_xAlx)2' which c'Ompositi'Ons c'Over the paramagnetic， metamagnetic and
島町omagneticphases. We 'Observed血atph'Otoemissi'On spec回 wereshifted t'Oward
l'Ower binding energies with x.官leeヰ>erimenta1叩ectrawere c'Omp訂edwith band
structure calculati'On c'Orrected f'Or model self -energies，組d 血.eagreement was
c'Onsiderably improved 'Over the direct c'Omparis'On with the band density 'Of states. In血lS
血esis，in 'Order t'O further cl3I均， the electr'Onic structure 'Of 3d transiti'On metal
c'Omp'Ounds with the Laves phase structure systematically， we have studied the electr'Onic
structure 'Of Y t・xScxMn2wi出 x= 0.0 and 0.03 which c'Omp'Ositi'Ons cover the
antiferr'Omagnetic and paramagnetic phases by XPS and UPS. This thesis is 'Organized
ぉ f'Oll'Ows.In Chapter 2， we briefly describe the method 'Of ph'Otoernissi'On spectrosc'Opy
and the'Oretical analyses. Experimenta1 results 'On FexC'Ot_xSi， MnSi and Yt_xScxMn2， are
given in Chapters 3， 4 and 5， respectively. In Chapter 6， we summarize血epresent w'Ork
and give c'Onclusi'Ons ab'Out the electr'Onic structure 'Of these c'Omp'Ounds.
3
References
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(1976).
[1.3] D. Bloch， J. Voiron， V. Jaccarino and 1. H. Wemick， Phys. Lett. 51A， 259 (1975).
[1.4] H. Yasuoka， V. Jaccarino， R. C. Sherwood and J. H. Wernick， 1. Phys. Soc. Japan
44，842 (1978).
[1.5] M. G. Foex， J. de Physique 8，37 (1953).
[1.6] H. Watanabe， H. Yamamoto and K. Ito， J. Phys. Soc. Japan 18，995 (1963).
[1.7] H. J. Williams， J. H. Wemick， R. C. Sherwood and G. K. Wertheim， 1. App1.
Phys.37， 1256 (1966).
[1.8] J. H. Wernick， G. K. Wertheim and R. C. Sherwood， Mater. Res. Bul1. 7 143
(1972).
[1.9] D. Shinoda， Phys. Status Solid; A 11，129 (1972).
[1.10] 1. Be出e，1. Voiron， F. Towfiq、M.Roth and Z. Y. Zhang， J. Phys. F 11， 2153
u立己1).
[1.11] J. Be出e，J. Voiron and M. Roth， Solid State Commun. 47， 399 (1983).
[1.12] F. Laves， Naturwissenschaften 27， 65 (1939).
[1.13] K. N. R. Taylor， Adv. Phys. 20，551 (1971).
[1.14] K. H. J. Buschow， Rep. Prog. Phys. 40， 1179 (1977).
[1.15] H. R. Kirchmayr and C. A. Poldy， J. Magn. Magn. Mat. 8， 1 (1979).
[1.16] H. R. Kirchmayr組 dC. A. Poldy， in Hand book on the Physics and Chemistry
of Rare Earth ， vol 2， edited by K. A. Gshneidner Jr and L. Eyring (No口h-
Holland， Amsterdam， 1979) p. 55.
[1.17] R. Ballou， J. Deportes， R. Lemaire， Y. Nakamura and B. Ouladdiaf， J. Magn.
Magn. Mat. 70，129 (1987).
[1.18] Y. Nakamura， M. Shiga and S. Kawano， Physica B 120‘212 (1983).
[1.19J H. Wada， M. Shiga and Y. I¥akamura， Physica B 161‘ 197 (1989)
[1.20] H. Yamada and K. Terao， unpublised.
4
日.21]H. Yamada， J. Inoue， K. Terao， S. Kanda and M. Shimizu， J. Phys. F: Met. Phys.
14， 1943 (1984).
[1.22] H. Yamada釦 dM. Shimizu， J. Phys. F: Met. Phys.17， 2249 (1987).
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H. J. Buschow， Phys. Rev. B. 39，6∞8 (1989).
[1.25] A. K紘izaki，H. Sugawara， 1. Naga1cura， Y. Ishikawa， T. Komatsubara and T. Ishii，
J. Phys. Soc. Japan 51， 2597 (1982).
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コ
Chapter 2.
Photoemission spectroscopy
2.1 General Principles
h血ischapter， we describe the basic principles of photoemission spectroscopy and
血emodel田町・energyar叫ysis，which is used in Chapters 4組 d5. Photoemission
SJ蹴加scopy(PES) is a power負uω01ωdir民 tlyinvesti伊低血eelectronic struc旬開 of
solid. The bII叫ingenergy of血e泊itialstate is ob阻ined前
E帥 =hv-φ ・EB' (2.1)
where Ekjn is the kinetic energy of phot侃:Ull阪del，民的ns，hv is the incident phot，∞
energy，φis the work function of the sample and Es is the binding energy. The obtained
spe他国ncan be comparedω 血eone-electron energy distribution function血血esolid.
Figure 2.1 shows the schematic命ョwingof photoernission pr'∞ess [2.1].
2.2 Experimental
In this section， we describe白eexperimental methods and apparatus which were
used in this study. Photoemission spectroscopy (PES) was performed in an ultrahigh
vacuum chamber equipped wi血anelectron energy analyzer and an excitation light
7
Ekin Spectrum
EF Valence Bond
1ミω
E t Sampte
一一切cuumAe貯i-' i ト N(El
E" I Core Level B
"一ニヒ::~~m~ E~~gy
H(E)
nω
Fig. 2.1. Schematic picture on the principles of photoemission spectroscopy [2.1].
8
3
source. In the pre詑 ntx-ray photoemission spectroscopy (XPS) and ul回 violet
photoemission s戸C釘oscopy(UPS)， we have used an x-ray source with a Mg Kαline
(hv = 1253.6 eV)， a VG He discharge lamp (He 1 = 21.2 eV and He 11 = 40.8 eV)， a
VSW CLASS・150hemisp恥ricalana1yzer and釦 OmicronEA・125analyzer.百le
schematic figure of the measurement system is shown in Fig. 2.2.
Hemispherical analyzer
b 一一channeltron
sample
Light source
Fig. 2.2. Schematic description of the photoemission measurement system.
To interpret photoemission spectra， it is important to notice由atdifferent atomic
subshel1s have di百e詑 ntphotoionization cross sections， which are dependent on the
photon energy.百lesubshell atomic photoionization cross sections for Y t_xScxMn2 (x = 0， 0.03)， MnSi and FexCot_xSi (x = 0.5， 0.8) for various photon energies are given in
Table 2.1 [2.2]. Figure 2.3 shows the va1ence-band He 1 and He 11 UPS spectra for
~・ According to血ephotoionization cross sections， the He 1 spectra consist of Mn
3d and Y 4d states for YMn2. For He 11，血.ephotoionization cross section of Mn 3d is
larger than血atofY4d.
9
ヘぐ 21.2 40.8 1253.6
(He 1) (He 11) (MgKα)
Mn3d 5.344 8.354 0.26 x 10・2
Fe 3d 4.833 8.751 0.45 x 10-2
Co 3d 4.356 8.738 0.67 x 10-2
Si 3p 0.3269 0.3286 0.35 x 10-3
o 2p 10.67 6.816 0.50 x 10・3
Y 4d 4.795 0.5147 0.72 x 10・3
Sc 3d 3.134 1.922 0.10 x 10・3
Table 2.1. Atomic subshell photoionization cross section (in Mb) [2.2].
YMn2 116 KI
htωcoHC
〆、.
-2.0 -1.5 ー1.0 ー0.5 。Energy relative to EF (eV)
Fig. 2.3. Valence-band He 1 and He II UPS spectra for Y恥1n2・
10
In genera1， phot侃 missionspectra a隠 a百回胞dby surface e偽 ctsdue to the short mean-
free pa血sofphot侃:lectrons，and it is not位ivia1to d回 ompo関山S戸 山ainto血esurface
and bulk components. As路 enfrom Fig. 2.4， the electron e釦apedepth is only...3 -20 A
for hv -20・15∞eV. Therefore， in PES measurements， to ob阻血c1eansurfaces has
been an important issue. In血iss加dy，血esamples were釦 m戸d加situwi由 adiamond
file to obtain c1ean surfaces. In the case of UPS，出equality of the surface were checked
30
.<: -o s -c 。-0 3 c • ミ10
Fig. 2.4. Mean企eepa血ofelectrons in solids [2.3].
MnSi hv = 21.2 eV 50 K
hH
一ωco-c
-10 ・8 -6 ・4 ・2 0
Energy relative to EF (eV)
Fig. 2.5. He 1 UPS spectra for MnSi before and after c1e国ung.
11
by monitoring photoemission signals from the 0 2p signals， which appe訂 at--4・ー10
eV. Figure 2.5 shows the He 1 UPS spectra for MnSi before and after cleaning.
In the pre詑 ntwork. in order to decompose the spec加 担tothe surface and bulk
contributions， we have made measurements by changing the angle between血esample
surface and the analyzer a∞ep阻nce thereby changing the escape depth of
photoemissions according to F = Acosφ， where λIS血.eelectron mean-free path. Figure
2.6 shows the geometry of the detection of photoelectrons. Such an angle-dependent
measurement has difficulty in obtaining bulk-sensitive spectra precisely because of血e
inherent roughness of the sample surface prepared by scraping.
n:
づ:町町
Fig. 2.6. G切 me町 ofthe de悦 tionof photoelectrons and血.esurface of由esample，
where n is the sample surface normal.
As an altemative method to decompose the spectra into the surface and bulk
contributions， we have also subtracted the He II spectra from the He 1 spectra. Assuming
出atthe surface contributions come合omthe outermost atomic layer because in many
metallic systems， only the outermost atomic layer is strongly a百ectedby the surface and
that the second layer has the electronic structure similar to the bulk [2.4]， and血at血e
photoelectron current is exponentia1ly attenuated from the surface. Contributions from
12
the suIfaωand bulk electronic s加 C加resto the叩ectramay be estimated as follows.
Here.血.edet舵旬dphotl侃 :lectron血.tensity1 is de街路dby
1 = i-CTω (2.2)
whel叫 (z)is the dis凶bution伽，ctionof the detec凶 phot侃 lectrons部 afunction of the
distance z from the swface. Therefore. the mean dep血ofsignal ED， namely the escape
depth is defmedぉ
ED= i-z中伽/わ(z)ゐ(2.3)
Here; we邸 su脱出eprobab胸 中(z)削除 electrontravels the dis加句 zwithout
collision is given by，
中(z)=cexp(ーzlλ)， (2.4)
whereλis血，einel郎氏m伺 nfree pa血.then
ED=λ (2.5)
Then， we can estimate the∞n凶butionof the suIface layer :
x f:ct(z) dz f:切(-~)dz = 1一切(-~). わ(z)dz i-ex
(2.6)
where x is the thickness of the surface layer.
Such an energy dependence of the photoelectron mean-f冗epath can be utilized to
perform such a decomposition if the photoionization cross sections of both the He 1 and
He II spectra are dominated by血emetal d partial density of states (DOS) while other
contributions are negligible [2.2]， which means血atthe bulk and surface spectral
lineshapes do not change appreciably with photon energy and血atphoton-energy
dependence is largely due to the different degrees of surface and bulk contributions for
He 1 and He II. This is indeed the case for MnSi and FexCo'_xSi， where the Si sp cross-
sections are negligible compared to the 3d cross-section. However， in the case of the
13
~， the He 1 spectra consist of Mn 3d and Y 4d states， if whereas He 11血e
photoionization cross section of Mn 3d state is larger出m 血atof Y 4d (Table 2.1).
Therefore， the same method is difficult to deduce the bulk-sensitive s戸ctrafrom the He 1
and He II UPS spectra. It is well known白紙 XPSis more bulk-sensitive血anHe 1 and
He II UPS measurements because phot∞lec汀onswi血 higherkinetic energies well
reflect血eel民 tro凶cs加 C仰向血 thebulk [2.1]. Therefore， in the case of the ~， we
have perfonned XPS measurements in order to detect more bulk-sensitive spectra.
The background has been determined by integrating the measured spectrum仕om
the lowest binding energy. This tota1 background gives a good description of the
inelastic background when photoelectron energies are low as in UPS [2.5]. Figure 2.7
shows the UPS spectra from Cr203 for hv = 40 eV， where the long dashed line is the
to阻1background and the short dashed line is the integral background. In some cases the
integral background function白紙 isgenerally applied to XPS and Auger spectra has
been used， but it gives a 伊 orfit for low phot∞lectron energies，ぉ shownin Fig. 2.7. In
血eintegra1 background model， it is assumed血ateach electron血血esignal has some
kinetic energy-independent probability of being inelastically scattered， and thus of
con凶.butingto血.ebac勾round， but fu巾 erinel路島 scatte由19of those secondary
electrons is not considered [2.6]. That is
、、d
N(E)
15
hv = 40 eV
、、、、、、、、、、¥
¥ 、¥
10
Binding Energy (eV】
CrzOl (10I2)
--. ..由g・IbIC:qJ凹""-一 T制加依抑制
‘ 、、千
→-~ I!p -0
Fig. 2.7. Angle-integrated UPS spectra from Cr203 for hv = 40 eV (solid line); total background (long dashed line) and integ凶 background(short line) [2.5].
14
J
明 αLEmo.1，(E')ι (2.7)
where 1，(E} is血eintensity of the actual signal above background and Emu. is血.ehighest
kinetic energy of the signal. Since由em回 suredspectrum 1，0，( E} is the sum of the
background and the signal
I佃，(E)= Is(E) + lb(E) (2.8)
釦 d1，(E) is not known a p巾 ri，1 b(E) must be found by an iterative procedure [2ム7，8].
In theω凶 backgroundmodel， the simplest way of incorporating組血elastic民atte出19
of all of血eelectrons is to assume血atthe background at any kinetic energy is
propo凶∞alto the in低g叫 of血.etotal叩ec回 ml佃，(E) at all higher kinetic energies and
not justω 血eintegrョ1of the signal above background.百lefrrst s胞pin genera由理由e
旬凶 backgroundis de白nitionof the energy range over which the signal∞curs.百us
req凶resthe determination of two energies as follows; (1)血.eenergy Emax above which
血ereis no signal， (2) the energy below which the measured electron叩印刷mconsists
only of the background For悦 UPSs戸ctra泊 Fig.2.7，血.eactual photoemission
S戸C回 lfi伺加問S紅 ebelieved to 0∞町並 theregion above about 13 eV， so below 13 eV
the data presumably consists only of血e泊el邸 ticallyscattered background. For
∞m抑制onalp田posesit is∞nvenient to deftne
ら(E)= llO/(E)ー1/O，(Emu.) (2.9)
The background function 1 b(E) is then
明=仙aJ+AiEmax
仰 )dE(2.10)
whe陀 fル(E')dE'is d問問的lewi出outiteration. The constant A is
detennined合om
I b(Emin) =ル(Emin) (2.11 )
15
since the spectrum there consists entirely of the血el鎚 ticaIlyscattered background. Thus
A=II加t(ιJーら(E]'max)] ruo，(E)dE (2.12)
百lerefo回血.eto凶 backgroundfunction c組 bewritten simply邸:~ y
4
一a
E
-
E
rEA同国
，
zk
F
d
-
2
1
h
:
一'叩由
，ssh
一，EEJE
× m
E
，， ••
、佃，t
ι
加FE
F----L +
E
師，t 一一司，，a
(2.13)
As seen from Fig. 2.7，血eω凶 backgroundfunction (long dashed line) gives a much
better fit to the ac佃albackground in也e問 gionbelow the 0 2p band血andωseither the
in低gra1background (short dashed line) or出es回 ightline. As discussed above， bo血
也記gra1and total backgrounds have been used to fit XPS spectra. Figure 2.8 shows a Ti
2p XPS spec回 m for Ti02 taken using Al KαX-rays， and the background functions
obtained using both approach.τ'he to凶 backgroundalways lies slight1y below the
integral one but the di百erenceis smaIl. In this曲目is，we have adopted the total
background for the analysis of the UPS He 1 and He 11 spectra for FexCo1_XSi， MnSi and
y l_xSCx~' which is appropriate for UPS spectra. Figure 2.9 shows the valence-band
He 1 UPS spectra for YM~ and its background.
")
16
τ百02(110)--. In陣&nlb抵kCr四 M
- - T，回albaclcCIO細川
hu = 1482.6 eV
Ti 2P1l2 N(E)
-司、、Jv
、
、、、、、‘、
-、
455 460
Binding Energy (eV) 465 ‘70
。
Fig. 2.8. Al KαXPS spectrum of the Ti 2p co問 levelsin single-crystal Ti02 (solid curve)
total background (long伽shedcurve); and血tegralbackground (short伽shedcurve) [2.5].
YMn2 hv = 21.2 eV
16K
-5 ・4 ・3 ・2 ・1 0
Energy relative to EF (e V)
会ωcsc
Fig. 2.9. Valence-band He 1 UPS spec回 (solidcurve) for YM~釦d its background
(白shedcurve).
17
2.3 Single-particle spectral function and the effect of
self-energy correction [2.9，10，11，12]
Thes矧 e-p訓 clespectra1 function p (k，ω) gives the spectra1 weight for adding or
removing a single electron wi血 energyωand momen加mk. At T = 0， the spectral
function is defmed by
p(い)三千I(ゆj(N-1)1 Ck l<Pg(N)) ro(ω+μ+Ej(N-l)-E山))
+子(中j(N+ 1)
whereμis the chemical pot印刷，も(N)，供(N-1) and県(N+ 1) are the wavefi削 ionsof
血eN -electron ground state，血e(N -1)-electron excited s阻teand血.e(N + 1)-electron
excited state， re甲田tively， ι(N) ， E~N-l) and E，μ+ 1)脱出eirenergies and C k and cl
are the annihilation andα-eation operators of a Bl∞h electron with momentum k.
Equation (2.1のforω<Oand ω>0伺 nbem伺 suredby phot侃 missionand inverse-
phoほ missions戸沈os∞py，respectively. Here，ωis related to血ekinetic energy el:;in of
血.eemit凶 (absぽbed)el即位on血ro曙hω=el:;jn - hv， where hv is血eenergy of the
absorbed (er凶配d)pho畑1.
p(k，ω) is血eFo凶 ertransform of血eim柳町 P制 ofthe single-p副 cleGreen
function and therefore， contains the full information about the temporal and spatial
evolution of a single electron or a single hole in血.einteracting many-electron system.
Using批 Fouriertransform G(k，ω) ofthe reta耐 dGreen function
G(r-r¥t) = -iS(t)(gl {帆(/)，"，:->Ig)， (2.15)
or
G(k， t) =一iS(t)(gl( ck(t)， C:.} Ig)， (2.16)
the spectral function is given by
18
J
p(k，ω)=一会加G(k，ω). (2.17)
He陀 ，"'r叩 d"':脱出eannihilation and creation operators of an electron at r，
(A，B}三AB+ BA， A(t) == eiH1Ae-iH1 and Ig)三|悔い)).This di削 lyfollows from
G(い)=ーや(gl{ c.(t)， ck} Ig)ekD1
E1EE，，，e
oo
11E，IIIllit
--胆l
l
+
凡
ら
一
H
一H
一一
一
+
一
ES
-Es--+
1
且一一一nu
一
泊
一
+
一
+
一
ω
一ω
ら
ペ
liq-
-BEBEE--III-
FFIr--+
=手1(i(N -1)1 cklg) 12[川 +Eふ一侃o(ω+μ+兵(N-1) -Eg)]
+子1(i( N + 1) 1 c~ Ig) 12 [ω+μ-EP -
ーベω+μ一民(N+ 1) +民)](2.18)
τbe Green function of an interacting electron system G is related to出atof the non-
inte即 tingsy蜘 nGo(k，ω) = 1/[ω-E~] 伽ough 出e Dyson equati
G(k，ωr1 = Go(k，ω)一1-E(k，ω)， (2.19)
or
G(k，ω)=一一一 1ω-E~-E(k ， ω) ， (2.20)
where E~ is the energy of a non-interacting Bloch electron and E( k，ω) is called self勘
energy. 1n the non-interacting system， sinceI:(k，ω)三 0，the spec仕alfunction is writ町 1as
a d-function at ω= E~ as shown in Fig. 2.10 (a)， which is Koopmans' theo陀mitself. 1n an
19
interacting elωon system， E(k，ω) is not zero and the pho蜘 nissionspectrum is given
by
似k，ω)=-去ImG(k，ω)
1 1mE(k，ω)
π(吋-ReE (い)Y+ (山，ω)y(2.21)
Figure 2.10 (b) and (c) show the spectral function wi由 theself-energy and the real part
and imaginary parts of the self-energy， respectively. Therefo民 ReE(k，ω) and
IrnE (k，ω1) give， res戸ctively，the energy shift and the broadening of血eone-electron
eigenvalueω= ~ due to interaction. The Fermi energy is located atω= O. The real p剖
of the pole of G(k，ω1)，ω=ιis determined by the叩組on
E;-d-Rez(k，E;)=O， (2.22)
and the residue of the pole Zk is given by
ペlaRt3A)|岨 E:)1(2.23)
Nearω= E:， we can expand Eq. (2.22) as fol1ows，
、、EE
，，J*Lκ
E
ω
'''EE
‘、1
一7h~-ω
'κ γ-e
R
凶勺ω
(2.24)
Therefore， Eq. (2.21) is written as
ヌ Z，lmE (k，ω) p(k，ω)=-7f， J
Lω-E~) + ( Zk1mE (k，ω))ニ
= ZkN (k，ω) ， (2.25)
20
(a)
(b)
(c)
p(k，ω)
p(k，ω)
ReI(k，ω)
ImI(k，ω)
function Fig. 2.10. Spectral
applicable and (b) when the
O εk
K
εo k
EF Energy
coherent pa口(quasi-partical peak)
221cIml:(k，ω)
Energy
Energy
ReL(k，ω)
ImL(k，ω)
exacuy
and
(a) when the one-electron approximationis
electron correlation is considered. (c) Real part
imaginary part of the self-energy L( k，ω).
21
Here，Z"く 1ロ.9]and N* (k，ω) is出equasi-particle density. As shown in Fig. 2.10 (b)，
the peak position of the qua吋 artical，which is叫 ledcoherent part ， is locatedω=Zkd
with spectral weight Z k・Theremaining spectral weight is distributed in the incoherent
part away from ~.
Now we describe the e百ectof L (k，ω) on血ee百ectivemass of a quasi-particle
[2.10，11，12]. Near the Ferrni level， we expand ~ (k，ω) around (k F' 0)出 follows，
ð~ (k，ω1) I __. ð~ (k，ω) I L (k， ω)三(い)+寸z-lJ+寸~~II日 F(k一川，
(2.26)
whe児 ，k F denotes the Ferrni wave vector.
Since the e旺ectivemass m * on the Ferrni surface is defined as
m 三(dl)l(2.27)
we can expressιnear the Fermi level from (2.22) as
、‘lJ
F
LM晶L
此，，EEk
b，
ω一
k-ぬ
工一、。一+
・kE 曲
ω一
丸一的
工一「dv-+
FE --
一.，
ικ E
(1.18)
Here， we assume L (k F' 0) = O. Assuming 出剖 L(k，ω) L(k，ω) =L(ω) ，we obtain出efollowing fonnula，
is k -independent， i.e.，
(ぽ)Ih'F ，，2.(0)'((出I.J(2.29)
Zk (0) is called the陀 nonnalizationfactor. 1/ Zk (0) gives the mass enhancement factor at the Ferrni level
m . dReL (ω) I mb Zk(O) 拘 IW=Q (2.30)
22
In this thesis， we have adopted the model self-energy for the analysis of the DOS of
MnS釘iand Y叱1ト_.Sc
energy s訓a剖矧tiωi均白白eKrarr訂m附ers-長Kronigrl的dぬla矧Uω削肌O∞肌n札. For a Fenni liq凶d，Re E(ω)区一 ωand
ImL(ω)民 -OY， near the Fermi level. We assume that the self-energy included two <.t>-
dependent terms，
+i-2gh"(hW -(w+首y
、BB
，，今百一VJ
ニ詩
句
MJ-+
rq一'W
。。一圃、‘，，J
ω一・叫
h-+
oo-一ω-，，t
‘、z
(2.31)
ζ=-g{市サ記号+ベ53亨
and
(2.32)
where gh' "(h and g" "(1 are町eatedas adjustable par富田ters.Near the Fenni level (ω..， 0)，
(2.25) and (2.26) cand be written as
乙..， _ gh ffi _ i2gh ffil n ii. -- t
and
L..， -g，ωーigtd t 甘 す
(2.33)
respectively. Hence the mass enhancement factors of the two cases have the same
formula，
(2.34)
=1+三ず
叫 =0
1 ‘ aReL(ω)
z(o) ω
band
narrowing of the coherent part and spectral weight transfer from ne訂 EF(ω:::: 0) to away from ~・
The Lhi<h terrn gives rise to incoherent features away from ~ while出eL，ow terrn contributes extra
broadening 10 the incoherent features.
23
the and cause The u)-dependent terrns the L(ω) represent local and dynarnical effects
References
[2.1] S. Hufner， Photoelectron Spectroscopy (Springer-Ver1ag， Berlin， 1995).
[2.2] J. J. Yeh and 1. Lindau， At. Data and Nucl. Data Tables 32， 1 (1985).
[2.3] D. A. Shirley， in Photoemission in Solids !， edited by M. Cardona and L. Ley
(Springer-Ver1ag， Berlin， 1978) p. 165.
[2.4] A. Koma， K. Y;噂， M. Tsukada and M. Aono， Hyomen-Kagaku Nyumon (in
Japanese)， p.89 -91 (1994).
[2.5] X. Li， Z. Zhang and V E. Henrich， J. Electron Spectrosc. Relat. Phenom. 63， 253
(1993).
[2.6] D. A. Shirley， Phys. Rev. B. 5， 4709 (1972).
[2.7] A. Proctor and P. M. A. ShelWood， Anal. Chem.， 54， 13 (1982).
[2.8] A. Proctor and D. M. Hercules， Appl. Spectrosc.， 38， 505 (1984).
[2.9] N. Tsuda， K. Nasu， A. Fujimori and K. Shiratori， Electronic Conduction in Oxides
(Shokabo， Tokyo， 1993).
[2.10] C. W. Greef， H. R. Glyde and B. E. Clements， Phys. Rev. B 45， 7951 (1992).
[2.11] I.H. Inoue， I. Hase， Y. Aiura， A. Fujimori， Y. Haryuama， T. Maruyama and Y.
Nishihara， Phys. Rev. Lett. 74， 2539 (1995).
[2.12] T. Saitob， A. Sekiyama， T. Mizokawa， A. F吋imori，K. Ito， H. Nakamura and M.
Shiga， Solid State Commun. 95， 307 (1995).
24
Chapter 3.
Photoemission study of the
itinerant helimagnet FexCo1_xS i
3.1 Introduction
百les'Olid s'Oluti'On 'Of tw'O m'On'Osilicides FeSi and C'OSi， namely， FexCol_xSi， h出
a血'aCtedinterest because 'Of its u凶quemagnetic and el，民凶calprope凶es[3.1，2.3，4].
while FeSi is an unusua1 narrow-gap semiconduct'Or and CoSi is a diamagnetic
民m泊leta1[3.5]， FexC'Ol_xSi exhibits a magnetica11y ordered phase at l'Ow tempera佃 問S泊
血econcentration range 'Of 0.2 S x S 0.95 [3.5，6]. Th'Ose comp'Ounds have a common
αysta1 structure of the cubic B20 tYl児assh'Own in Fig. 3.1 [3.7]. Figure 3.2 shows the
variati'On 'Of the lattice par富田町部afuncti'On'Ofx[3.1，6]. X-ray di飴 actionanalysis 'Of
FexCo1_xSi shows血atthe alloys are single phase and the lattice constsnt a varies
linearly with composition x. According t'O sma11-angle neu仕onscattering experiment
[3.8，9]， FexCol_xSi with 0.3 $ x三0.9has a helical spin structure wi血 along period as
in the case of MnSi. Under magnetic field， the helical spin structure of FexCo1_XSi is
transf'Onned to a conica1 spin structure and血ento組 inducedferromagnetic structure
above a critical field Hc・Figure3.3 shows the variation of the scattered neutron average
intensity for di百erentmagnetic fields as a function of the scattering vector q in the
directi'On of the magnetic field for FeoιCoo.2Si. The integrated intensity of the satellite
decreases with magnetic field and completely disappears at 2 kOe. From the position of
the satellite， the helix period for x = 0.8 is found to be 295 A and x = 0.5 is 900 A.
Figures 3.4 and 3.5 show the magnetization curves for Feo.SCoO.2Si and Feo.sCoosi.
25
respectively [3.10，11]. In the low magnetic field，血emganetization inc陀儲eslinearly
Wl血 a飽m戸m加陀U1dependentsusceptib出ty.In the ca岱 ofFeo.SCoO.2Si，血eαitiωl
field at which the magnetization almost saturates suddenly about l.9ωe at 4.2 K.
Above H c' the magnetization slight1y increases with increasing magnetic field.百le
magnetization process of Feo.SCoO.2Si and Feo.sCoo.sSi are qualitatively similar to白紙 of
FeO.9CoO.¥Si [3.8]. Figures 3.6釦 d3.7 show the An汀rroひO倒ttplots for Fe匂o.SよCo句0.2Siand
Fe句o.sCo句o.sS剖i，陀芭'esp戸ectively[β3.10叫].These plots show a slight deviation 合om血es住aight
lines， which are smilar to those observed in MnSi [3.11]. From the extrapolation of血e
straight p制 toH = 0，血.eCurie記mperatureT c is ob凶 edto be 44.5 and 44 K for
Feo.SCoO•2Si and Feo.sCoo.sSi， respectively. Beil1e et al .[3.9] performed the resistivity
measuremets on FexCo¥_xSi between 4.2 and 300 K. In Fig. 3.8， the values of
R(T)IR(3ωK) are plotted as a function of tempera刷re.官leresistivities of FexCo¥_xSi ~:
show an up佃m below Tc' Figure 3.9 shows the low tempera旬respecific heats of
FexCo¥_xSi [3.12]. While in FeSi， CoSi and FeO•7Cooßi 血.e specific heats linearly
decreases down to the lowest temperatures， those for the Feo・¥Coo.9Siand Feo.sCoo.sSi
show an up佃matlow胞mperaω.res.Very recent1y， Susaki et al .[3.13] have performed
a detailed phot侃 missionstudy of Fe¥_xCoXSi， (x = 0.05 and 0.1) and found血atthe gap
泊 FeSiis fi.lled when a sma1l amount ( x -0.05) of Co is substituted for Fe.
In this study， U10rder to obta泊 experimenta1information about the electronic
S加 C加reof FeXCo¥_xSi wi血 x=0.5 and 0.8 and CoSi， we have studied de阻iledat
various temperatures by ul回 violetphotoemission of FexCo¥_xSi and CoSi.
26
‘ --
Fig. 3.1. Crystal structure of FexCo1_xSi [3.7].
4.4Q
4.44 >-<
. 4.47
u
u凶ト凶す
2dda凶
u-トト〈J
4.40
1.0 0.0 0.-4 0.0
COMP031TI01< x
0.2
4.45
..44 D
Fig. 3.2. Lattice parameter a versus composition x for FexCo'_xSi [3.1，6].
27
Fe， (01-.< Si
ご一22552ι
o ∞ q (A)-1
Fig. 3.3. Scattered neutrons mean intensity under different magnetic fields for
FeO.gCo02Si: H ニ o(A)， 950 Oe (B)， 1420 Oe (C)， 1600 Oe (D) and 2000 Oe (E) [3.8].
28
匂 7K
10
IS
20
2S
30
~一一←
匂5
50
Feo.acoo.2SI
12
10
8
6
2
【
O¥コEUW)
zo一ト『N
「巳
54E
10 8 6 2
。。(kQeI
Fig. 3.4. Magnetization versus magnetic field for Feo.SCoO.2Si at various tempera佃res
from 4.2 to 50 K.百learrow indicates the critical field Hc at 4.2 K [3.10].
(JI
~ 3
EIO
‘' )
Z O ト-q ト、J
トー
'i 5 t.:> 《
主
同AG旺TIC FIELD
.K( •
三4lUO 50
FIELD (kOt-)
Fig. 3.5. Magnetization versus magnetic field for FeO.SCo05Si at various temperatures
from 4.2 to 200 K [3.11].
29
APPlIED
150
Feo.acoo.2SI
50 K
45 K
4.2 K
10 K
15 K
20 K
25 K
30 K
35 K
向oK
1∞ 仁3崎、、E l1J
《、、aまこ
50
5 向3 2 。。
to 50 K
(koe/日oo/g)
Fig. 3.6. M2 versus H/M for Feo.SCoO.2Si at various temperatures from 4.2
[3.10].
H/M
」
FeO.5CoO.5S!
ιo ，
150
100
50
N (
0> 、、コE l1J
)
N
三=
6 5 H 2 O O
(kOel創刊J/g)
M2 versus Hル1for FeosCoosSi at various temperatures from 4.2 to 60 K
30
HI門
7 3
m，
-EEA
ou-
-11J
F
A
V
し
3
FExeat-XSi
//一¥. A
E!?L〆 rJ
__ c ・・三、'"'--同
1三士-L.JJ JLムぷ以
Lマ
(
〉
-00門)
邑
¥
(
」F
)
色
ユ~200
T (K) 100
Fig. 3.8. Normalized resistivity R(T)尻(300K) as a function of tempera加refor FexCo，.
xSi at H = 0 and under magnetic field H = 8 kOe (o): x = 0.95 (A)， x = 0.9 (B)， x = 0.8
(C)， x = 0.65 (0)， x = 0.5 (E)， x = 0.4 (F)， x = 0.3 (G) and x = 0.2 (H). Arrows indicate
the transition temperature Tc The magnetic contribution at 4.2 K， Pw is indicated on the
31
curve (B) [3.9].
炉、eX
325 ¥
o ι』
y Feo.sCoosSi
Aーす4喝---..
J70↑
¥Fe07Co03Si
x
ト
~♀
υ
15
/.Feo1coo.gSi
〆/COSi
2 4 6 8 10 12 14 16 18 20
T2 (K2)
Fig. 3.9. Low temperature specific heat of FeXCo1_XSi [3.12].
32
3.2 Experiment
Poly，αystalline samples of FeXCo1_xSi wi血 x= 0.5 and 0.8， and a single crystal of
CoSi were supplied by Prof. Kanomata (Fac叫tyof Engin白血19，Tohoku-Gakuin
University) and Dr. Note (Institute for Ma削alsResearch， Tohoku University).
Samples of FexCo1_xSi， x=0.8 and x=0.5， were p問paredfrom 99.99 % pu時 Fe，99.9
% pure Co and 99.999 % pure Si by arc mel也19血 anargon atmosphere. The CoSi
single crystal was grown by the Cz∞h叫skitechnique.百leprepared samples were
checked by Laue x-ray di飴action.
m凶吋.oletphotoemission spectroscopy (UPS) measurements wereωrried out
using a VSW CLASS-150 hemispherical anal戸er，組 omiα'OnEA-125 analyzer and a
He discharge lamp但e1: hv = 21.2 eV， He ll: hv = 40.8 eV).百leFermi edge of Au
evaporated on血e回mplesurf舵 ew邸 m伺 S町edafter伺 chseries of m伺 surements血
orderto a∞町蹴:lyωdetennine血eFenni level但p)and血e血S飢lffien阻1resolution.官le
問 solutionof the He 1 and He 11 UPS measurements we問-34 meV組 d-80 meV for
Feo.SCoO•2Si, .... 22 meV and -70 meV for Feo.sCoo.sSi and 22 meVand -80 meV for
C.oSi， res戸ctively.1n.order to ob組担 CL伺 nsurface， the sample was re戸a胞dlyscra戸d
in situ with a diamond file.
3.3 Results and discussion
Figure 3.10 shows the valence-band He 1 UPS spec位um(solid curve) of
Feo.SCoO.2Si and its background (dashed curve) subtraction. Here， the background is
assumed proportional to血eintegrated measu陀 dspectrum. This background subtraction
pr'∞edure has been successfully employed to describe the inelastic background when
photoelectron energies are as low as in He 1 UPS [3.14].
33
Figure 3.11 shows He 1 UPS s戸C回 forMnSi， FeSi [3.13]， Feo.SCoO.2Si，
Feo.sCoo.sSi and CoSi in血.eentire valence-band region (a) and n釦Ep(b). In general，
photoemission spec回 areaffected by surface e百ectsdue to the short mean-free pa血sof
photoelectrons. and it is not出via1to decompose the s戸ctrainto血esurface and b叫k
components. In the present case，血ephoton energy dependence of血ephotoelectron
mean-台関 pa血 can be utilized to perform such a decomposition because the
photoionization cross sections of bo白血eHe 1 and He 11 s戸 C回 aredominated by出e
metal d partia1 density of states (OOS) and Si 3sp con住ibutionsare negligible [3.15]，
meaning制御 bulkand surface spec凶 lineshapesdo not change叩preci油lywi血
photon energy and血atphoton-energy dependence is largely due to血edi旺erentdegrees
of surface and bulk con凶butionsfor He 1 and He 11. Therefore， we subtracted そS4手
b一ωCOHC
Feo.aCOo.2Si
hv = 21.2 eV
23K
_:.J
-4 -3 ・2 ・1 0
Energy relative to EF( e V)
Fig.3.1O. Va1ence-band He 1 UPS spectra (solid curves) for Feo.SCoO.2Si and its
background (dashed curve).
34
、AH一ωCOHC -
(a) hv = 21.2 eV
FeSi
圃 4 圃 3 -2 ・1 0 -2.0・1.5 -1.0 -0.5 0
Energy relative to EF (eV)
Fig. 3.11. Comparison of He 1 UPS spec位混血血eentire va1ence-band region (a)
and near Ep (b) of MnSi， FeSi [3.13]， FeO•8CoO.2Si, Feo.sCoosi and CoSi. The
spec回 weretaken at 23・28K.The spectra has b回 nnormalized to血eto旬1area.
35
the He 11 sp切闘合omthe He 1 spectra to ob阻ined‘b叫k'spectra shown in Fig. 3.12.
Here， we assumed血.at血esurface contribution∞mes from the outennost atomic layer.
Thus the surfa，偲 con凶butionto血es附加恥comes1・exp十dlλ)，whereλis血e
electron mean~官民 pa:血 andd is血.ethickness of由.eoutermost atonnc layer. Here， we
have assumed血atd = a /2， where a is血，ecubic 1attice cons阻ntof CoSi/FeSi b切 au舘
each cube contains two CoSi/FeSi molec叫.es.The‘b叫k'spec回 thusobtained show
伽 t血es加 C加rearound Ep becomes s加時erand sh釘perex印 ptfor CoSi. In orderω
check the consistency of the above decomposition procedure， we have a1so made
measurements by changing the angle between the sample surface and anal戸町
ぉceptanαtherebychanging the escape depth of photoelectrons according ω F=
λcosφ.百leHe 1 UPS spec凶 forang1es 00 and 450 between血ephotoelectron
momentum and血esurfaαnorma1 are shown in Figs. 3.12 (b)組 d(c) toge血，erwith a
‘b叫k's戸ctrumob旬並.edby subtrac曲19450 spectrum from血e00 s戸 凶umafter an
appropriate intensity norma1ization. The result aga:in shows血at血.estructwち aroundEp
is enhanced， qua1itatively∞nsistent wi血血e‘b叫k'spectrum血 (a)，however， the peak
around Ep somewhat weaker也anin (的.官邸 q凶 ntitativediscrepancy between the two
‘b叫k'spec回 wo叫dbea脚ibutedωthesimplified assumption of the mean-白'eepa:血
m似た:1employed here and the inherent roughness of the sample surfaces p悶P紅 edby
scraping.官lUSwe can conc叫de白紙偽也edifferefiiωbetw民 n血e‘b叫k'叩ectraand
He 1 UPS s戸ctrais白首tebut not remarkable，組dconclusions from血.efollowing
discussions will not be altered by the surface e百ects，but we will use the ‘bulk' spec釘a
whenever possible.
Now， let us examine the alloying e圧倒 onthe electronic structure of FexCo1_xSi
based∞血ephotoemission spec回.As shown in Fig. 3.11 (a)，血.estructure at -・0.8
、》
e V in CoSi is shifted toward lower b仙 genergy wi血 increasingFe concer削 .tion. 議According to band-structure ca1culations，血.eFen凶 levelin MnSi is located wi白血血e
band below the gap [3.16]， it is located within the gap in FeSi， and it is located wi白血
the band above the gap in CoSi [3.17].百lIsmea:ns th剖， as the number of 3d electrons
increases，侃 Fennilevel is shifted upw訂dand passes across血eenergy gap. lndeed，
the UPS s酔C汀aof CoSi show a dip structure at --0.5 e V， where the band-structure
ca1culation predicts the band gap [3.17]. Therefore， we have attempted to simulate the
x -dependence of出espectra of Fe.Co1..Si within the simplest rigid-band model. To do
this， we have utilized the fact由at出ephotoionization cross sections of both the He 1
and He 11 spectra are
36
(a) Feo.aCOO.2Si (b)
hv = 21.2 eV
、AH一ωcoHC
Feo.aCOO.2Si
-1.5 ・1.0 帽 0.5 0 -1.2 ・0.8 ・0.4 0
Energy relative to EF(eV)
Fig. 3.12. He 1 and He II UPS spectra for Feo.SCoO.2Si， Feo.sCoo.sSi and CoSi near ~
(a). The He 1 -He II di百erencespectra represent‘bulk' spectra. (b)組 d(c) show He 1
UPS spec回 forangles between the photoelectron momentum and the surface normal of
00 and 450• The 00
_ 450 difference spectra again represents a ‘bulk' spectra. Prior to
subtraction， the spectra has been normalized as described in the text.
37
、AH一ωcovc
(a) hv = 21.2 eV
(b)
、vpnJ
ー4 圃 3 -2 ・1 -1.5 -1.0・0.5 0 0.5 。Energy relative to EF (eV) J
Fig. 3.13. Comparison of the measured ‘bulk' spectra [Fig. 3.12(a)] with the
simulation of the rigid-band model. Dotted curves show the simulated spectra using
the spectra of CoSi. The spectra has been normalized as described in the text.
38
ヘ
assumed伽.tnd = 5， 6， 6.2， 6.5， 7 for MnSi， FeSi， Feo.aCo0.2Si， Feo.sCoo.sSi and CoSi，
respectively， the same錨 thed -electron numbers of宣明 atoms.官leresult is shown in
Fig. 3.13， where the inte伊.tedarea of the m儲 sureds戸C回 arealso nonnalized to the
a加，vend's. The figure shows that the simulated叩ec国紙面qualitative相 eeme凶 wi血
血eme部 U陀dspec佐aal出ough由.emeお町eds戸お佐alintensities are distributed over a
much a wider energy ranger血m 血es泊lUlatedspectra. We have extended血is
simulation to MnSiωshown in the same figure and found白紙 thediscrepancy between
the rigid-band simulation and the measured s戸C凶 isfurther担crl伺 sed.
As an altemative way to see出ee百ectof alloying泊 FexCot_xSi，we have simula.ted
血espectra for FexCot_xSi by superposing血es戸ctraof FeSi and CoSi. In由lS
sim叫ation，也 S戸C加 m of FexCot_xSi， Px(ω)， is given by [xp陥 j(ω)σd(Fe)+ (1・
X)PCoSl(ω)σd(CO)] I [xσdσ'e) + (1・x)σ'd(CO)]，where， P附 (ω，)and PCoSj(ω) are the
me錨 uredsp民 国ofFeSi組 dCoSiand σdσe)佃 dσd(CO)are the atomic cross sections
of Fe 3d and Co 3d， respectively. As shown in Fig. 3.14， agreement betw田 nthe
g路舗uredspectra and血.es泊lulatedones is rather g'α刈.Es戸cially，血e喝I田 mentin由e
wide range spectra [Fig. 3.14 (a)] is much more satisfacω，ry血組曲erigid-b組dmodel
(Fig. 3.13). Near the Fermi level [Fig. 3.14 (b)]， there are some disα-epancies between
血etwo叩ec回， probably ret1ec也19interaction (hybridization) between the Fe 3d and
Co3do由i阻lsin the alloys.
Figure 3.15 shows血etempera旬開 dependen叩 ofHe 1 UPS S.戸C図面白een歯e
valence-band region (a) and fli回 r~ (b) for Feo.aCoo.2Si， Feo.sCoo.sSi and CoSi. In島
en也'evalefli偲 banι 血eme部 U詑 dspec伽 donot change with舵mpera加reon血is
en町 'gyscale. In order to de胤 tsubt1e changes泊出eooS near EF' it is desirable旬
remove血ee町民tof the tempera旬 開 dependentFerr世主泊"8.cdistribution function.
官官陀fo問， we have divided the photoemission spc::お佐aby the Ferr凶-Diracdis凶bution
function (convoluted with a Gaussian corresponding to血.einstrumental resolution) at
each tempera旬開. As shown in Fig. 3.16， 也.espectral OOS血usobtained for
Feo.sCoosi and Feo.sCoo.sSi show small changes血血.espectral OOS血血evicinity of
EF・Thiswould correspond to血e泊crea記 of由eelec凶伺1resistivity below TN [3.9].
百時Insetshows enlarged spectra ooS near the Ferrni level. The change in the OOS
with tempera知町 forFeo.sCoosi at血eFerr凶 levelis larger由釦由atof the Feo.sCoo.sS i
and CoSi， which is also consistent with the resistivity data [3.9]， which show血at血e
changes in Feo.SCoO.2Si is larger由組曲atin Feo.sCoo.sSi. Figure 3.17 show an error
bars for Fig. 3.16. In the case of FeSi， Susaki et al [3.13] obtained the s
39
出.evicinity of~ which slight1y decreases'with decreasing tempera何回.百lespec回 of
CoSi， which d侃 snot show corresponding magnetic transition， ind民 dshow no
temperature dependence around ~.
〉、4・d
ω c O 4・..It.'
hv = 21.2 eV' (a) 1 11 (b)
f: I Feo.sCoo.sSi jも
-4 ・3 ・2 ・1 0
. . ... experiment - simulation
-1.5 -1.0 -0.5 0
Energy relative to EF (eV)
Fig. 3.14. Comparison of the measured ‘bulk' spectra of Feo.gCoO.2Si and
FeO.5CoO.5Si with the superposing of the spectra of FeSi and CoSi.
40
l ~) .y
't d
(a) hv = 21.2 eV
b
一ωCOHC
:"~、
Fe0.8Coo.2Si/. .，・
300K _.:'- I ~ 4 戸 J
J.."，..""，..~ . /
225k rjf ，/‘ ，
a' ， ，
，
，
a' ，
，
，
KJ
nu
，
7''
'‘"
Feo.sCoo.sSi
bl. 向
ωI c: 2170Kre--e・
-6 ・5 ・4 ・3 ・2 ・1 0・2.0 ・1.5 ・1.0 ・0.5 0
、cacgc
CoSi
100K ".../ノ f、:
-4 ・3 -2 ・1 0 ・2.0 ぺ.5 -1.0 ・0.5 0
Energy relative to EF (eV)
Fig. 3.15. He 1 UPS spectra of Feo.SCoO.2Si. FeO.5CoO.5Si and CoSi in the entire valence-
band region (a) and near EF (b) at various temperatures. The spectra have been
normalized to the total area.
41
義込…
"，..画、 8:Q ω 4・d
-ー 0.5 c コ
0.4 ---70K{ AC E .
D L圃 一一23K ~0.50 CtS 0.3 コ0.45、-〆
告$00.3405 cf) 0.2
80030 o O 0.1 -0.10
Energy relative to E~ e V) 。0.0
0.4
0.6
0.4
0.2
0.3
0.2
0.1
FeO.sCoO.2Si
v
e
oz同・nM
ATt e
v
&冒・‘a
叩
陪
仏
川
町，
••
mm川却絹
Mmmmm
nununununununuFE
(ωtcコ.2』伺)∞
OQ
knknknkn
0503
nunJ』ヲ
'nJ』
qunJ』
込37・
宣O刈'20.35 ヨ0.30!:; 0.25 ";;"0.20 CQ 0.15 百ー0.10 0
Energy relative to EF¥eV)
-合
zaf
0.0 ・0.20 ・0.15 0.00
Energy relative to EF (eV)
Fig. 3.16. Temperature-dependent spectral DOS of Feo.SCoO.2Si， Feo.sCoo.sSi and CoSi
obtained by dividing the He 1 UPS spectra by the Fenni-Dirac distribution function
convoluted with a Gaussian. The inset shows enlarged spectra DOS near the Ferrni level.
42
0.20
0.15
0.10
-100K - 28K
-40 -20 O
Energy relative to EF(meV)
Fig. 3.17. Error bars for the temperature-dependent spectra1 DOS of Feo.gCoO.2Si.
Feo.sCoo.sSi and CoSi obtained by dividing the He 1 UPS spectra by the FerrnI-Dirac
distribution function convoluted with a Gaussian.
43
3.4 Summary
We have studied the electronic structure of FexCot_xSi with x = 0.5 and 0.8 by
means of UPS. As we compared血.es戸ctrafor MnSi， FeSi [3.13]， FeoιCoo.2Si，
Feo.sCoo.sSi and CoSi，血estruc加reat --0.8 eV血 CoSiis shifted toward lower
binding energies. Such a shift of the spec仕a1features is qua1itatively consistent with the
shift of the Fenni level according to血erigid-band mode1. However， a supe中ositionof
血espectra of FeSi and CoSi better describes the x-dependent behavior白血血erigid-
band model. In Feo.SCoO.2Si and Feo.sCoosi，出espectra1 weight near ~ decreases with
decreasing temperature， which is consistent wi血由etemperature dependence of the
resistivity caused by the magnetic ordering.
44
References
[3.1] S. Asanabe， D. Shinoda and Y. Sasaki， Phys. Rev. A774， 134 (1964).
[3.2] V. Jaccarino， G. K. Wertheim， J. H. Wemick， L. R. Walker and Sigurds Arajs，
Phys. Rev. 160， 476 (1967).
[3.3] H. Yasuoka， R. C. Sherwood， J. H. Wemick組 dG. K. Wertheim， Mater. Res.
B叫1.9， 223 (1974).
[3.4] G. K. Wertheim， J. H. Wemick and D. N. E. Buchanan， J. App1. Phys. 37，
3333 (1964).
[3.5] J. H. Wemick， G. K. Wertheim and R. C. Sherwood， Mater. Res. Bull. 7， 1431
(1972).
[3.6] D. Shinoda， Phys. Status Solid; All， 129 (1972).
[3.7] O. N紘鉱山hi，A.Y組出eandM.Ka阻.oka，in Electron Correlation and Magnetism
in Narrow-Band Systems， edited by T. Moriya. (Springer Series in Solid-State
Sciences 29， Springer-Verlag， Heidelberg， 1981) p.126.
[3.8] J. Beille， J. Voiron， F. Towfiq， M. Roth and Z. Y. Zhang， J. Phys. F 11， 2153
(1981).
[3.9] J. Beille， J. Voiron and M. Roth， Solid State Commun. 47， 399 (1983).
[3.10] H. Watanabe， Y. Tazuke and H. Nakajima， J. Phys. S∞. Japan 54， 3978
(1985).
[3.11] D. Bl∞h， J. Voiron， V. Jaccarino and J. H. Wemick， Phys. Lett. 51A， 362
(1975).
[3.12] S. Kawarazaki， H. Yasuoka and Y. Nakamura， J. Phys. Soc. Japan 41， 1171
(1976).
[3.13] T. Sus紘i，T. Mizokawa， A. Fujimori， A. Ohno， T. Tonogai and H. Takagi，
Phys. Rev. B. 58， 1197 (1998).
[3.14] X. Li， Z. Zhang and V E. Henrich， J. Electron Spectrosc. Relat. Phenom， 63，
253 (1993)
[3.15] J. J. Yeh and 1. Lindau， At. Data and Nucl. Data Tables， 32， 1 (1985).
45
[3.16] O. Nakanishi， A. Yanase and M. Kataoka， in Electron Correlation and
Magnetism in Narrow-Band Systems， edited by T. Moriya. (Springer Series泊
Solid-State Sciences 29， Springer-Ver1ag， Heidelberg， 1981) p.126.
[3.17] H. Yamada， K. Terao， H. Ohta， T. Arioka and E. Kulatov， Proc. 4th Int.
Symposium on Advanced Physical Fields p.151， (Tsukuba， 1999).
46
~ < ~
j
Chapter 4.
Photoemission study of the itinernant
helimagnetic MnSi
4.1 Introduction
3d凶 lsitionmeta1 monosilicides with血ecubic B20 type struc加re，MnSi， FeSi and
CoSi， have at回 C飽d∞nsiderableattention becau記血eyshow a variety of et民的nicand
magnetic properties. Among them， MnSi is well known to show a helical sp白血JC佃m
Wl血along period of 180 A below TN::= 30 K [4.1，2].百lemagnetization curves of MnSi
at various tempera佃resfrom 4.2 K to 196 K are shown in Fig. 4.1 [4.3]. It is clear血at
血em相 le也ationat low tempera旬 開S即時おesa1most linearly wi血 magneticfield upω
6.2 kOe， whe問 itabruptly組制rates.This behavior is inte中reted部 dueto血eheliα1
→conical →ferromagnetic回 nsitionsas a function of magnetic field.百lehelical spin
s加 C制reof MnSi has been discussed based on田:veralmodels [4.4，5，6]. It has been
suggest血at血.ehelimagnetic s阻.tein MnSi originates from a spin-lattice interaction
arising from the spin-orbit interaction as in the case of the Dzyaloshinsky-Moriya
interaction [4.6]， because the cubic B20 structure has no inversion symmetry. Figure 4.2
shows the Arrott plot for MnSi in the temperature range from 4.2 K to 196 K [4.3].百le
induced magnetic monent 0.4μs/Mn at 6.2 kOe， however， is much smaller than 1.4
μs/Mn estimated from the Curie constant in the paramagnetic phase [4.7]. Figure 4.3
show the crystal structure of MnSi [4.8]. A unit cell contains four Mn atoms and four Si
atoms shown by large and small circles， respectively. The electronic specific heat zero
magnetic field is as large as -35.56 mJ/ moleK2 (Fig. 4.4) [4.9]. The thermal expansion
47
作品
coefficientαdecreases linearly wi血 decr:伺singtempera加re，p街路sthrough a sharp
negative戸akat T N = 30 K， and血.enincreases below it (Fig. 4.5) [4.10]. Such a
tempera旬間 dependence has been widely observed 血 itinerant毛lectron w回k
ferromagnets and invar alloys [4.11，12，13，14]. These facts indicate白紙MnSiis close to
a typical itinerant -electron w回 kferromagnet and may be described by self-consistent
renormalization血eoryfor spin fluctuations [4.5，15]. Thessieu et al .μ.16] performed
血emagnetoresistance experiments for MnSi under hydrostatic pressure. Figure 4.6
shows the temperature dependence of the resistivity at di首erentpressures. As shown in
Fig.4ム血eresistivity monotonica1ly decreases with decreasing temperaωre and shows a
more pronouαdd民間sebelow Tc' which is defined by a maximum血 dp/af.The
transition飽mpemMet也creaseswith hydroststic pressure. Figure 4.7 shows the
m 伊 eticphase diagram血血etemperaMe-pmssure plme.Stoner14171has given a 3 simple model for the pressure dependence of血.eCurie却 mpera制rein蜘 nsof Tc
民 (p
-P c ) 1(2. However， this model does not include the e百ectsof spin fluctuations. A more
detailed model gives a different pressure dependence Tc民 (p-Pc)3μ[4.18，19，20，21]. It
shows血at血.eexperimenta1 data are in good agreement wi血血isform at pressures upω
10kb紅白 shownin Fig. 4.8， but above血ispress町 'ea deviation be何回n血.etheo問tical
and the experlmental data is observed. These results are in good agreement wi血血e
previous resu1ts [4.22]. As shown in Fig. 4.8， the inset shows Tc oc (p -Pc)山田町Pc.
Recently， Koyama et al . [4.23] observed a metamagnetic transition around B = 0.1 T
with hysteresis below 6 K.
In order to understand血.eunusual magnetic prop削 esof MnSi， it is import倒的
have information about its electronic struc旬re.AIωo吋泊gto band-struc加recalcu1ation
using the augmen低d-plane-wave(APW) method by Nakanishi et al.， [4.8] in血e
p訂佃lagnetics阻 低 血.eFermi level ~) is 1，∞ated at a sharp peak泊 thedensity of states
(DOS) and an instability towards ferromagnetism is suggested (Fig. 4.9). Recently，
Yamada and Terao have performed band structure calculations for bo白血eferromagnetic
and paranlagnetic states using the linear-muffin-tin-orbital-一色tomic叩 here-
approximation (LMTO-ASA) method [4.24]. The DOS at the Fermi level is shown to be
high and consists mainly of the Mn 3d states. The calculation predicts metamagnetic
behaviour at high pressure. Experimentally， x-ray photoemission spectroscopy (XPS)
studies of MnSi as well as FeSi and CoSi were made by Beill
48
主
eV excitation. Figure 4.11 [4.27] shows a comparision among the photoemission spectra
of MnSi， FeSi and CoSi， at the excitation energy of 40 e V. The s戸ctra1profiles of血e
energy distribution curves in Fig. 4.11 are in good agreement with the XPS data [4.26].
In the present work， we have perfonned a UPS study of MnSi wi血muchimproved
energy resolution and tempera組問 dependence，in order to obtain more detailed
infonnation about the electronic s汀uc加reof MnSi. In order to comp紅色 theresu1t wi血
血.eband-structure ca1cu1ations， we have applied self-energy corrections to血eband-
structure DOS. We have also studied the tempera旬resdependence of the photoemission
spectra below and above T N・
49
， 1 ̂ .， .、ζ l> .
(' u ... )'κ
)< "
G <0侭
H ~O t(
" . ".
眠、ι4"
K 時‘.
I MnS， I
30
10
-o~コEe}{ト.工)芝
150
Fig. 4.1. Magnetization curves for MnSi at various temperatures from 4.2 to 196 K
[4.3].
800戸ー
100
H (k~ )
50 O
戸ノ~---
/;:;
_4./' I 5
包ヨ4
4
10-1 H I H(liT)
6
∞
{
何
-03E&一一円亡工)主
O
:~門.ul
Fig_ 4.2. M2 versus H/M plot for MnSi at various temperatures from斗.2to 196 K [4.3].
50
Fig. 4.3. Crystal structure of MnSi [4.8].
-J /
/ /
/ 〆
/ 〆
/
/ 〆
〆/
/ /
/ 〆
〆〆
0.20
0.25
0.30
015
(TM」OENK
・J4U)
ト
¥U
1600 1200 800
TZ (OKz)
throU2:h curve
400
Fig. 4.4. Specific heat of MnSi under zero magnetic
experimental points) and under a magnetic field of 9 kOe (dotted curve) [4.9].
51
(solid field
O
Thermol Expo ns.on Coef f 民 lenlMnS. α1(.10b)
!K-'I
10
s
/ lSO
Temperoture ! K I
160 1[.0 120 100 80 印
20
時、・ ι。喝、
O
-5
•.
‘
••
、.ー10
Fig. 4.5. Thermal expansion coe仔icientαofMnSi measured at zero magnetic field
52
plotted against temperature [4.10].
50
ε u
α ::1.
2、25
>
ロヨUM凶
o o ¥0 20
Temperature[K] 30
Fig. 4.6. Temperature dependence of the resistivity at di百erentextemal hydroststic
pressures [4.16].
30
T (O)=29.1K
20 Non・ι1agnetlc
nv
一凶
-E三嶋
』
LWιEω』‘
Magn<lIc
。。 10
Pr<ssurtlkbarl I~ 20
Fig. 4.7. Magnetic phase diagram of MnSi as a function ofpressure [4.16].
53
100
I~ 13.11 l"Tt園田{kbuJ
t∞
目
" ￥ ..-b・0 IU
75
A
U
ζ
J
〈J
ヲ&
{肉、，凶]肉、ヘド
• O
16
Fig. 4.8. Power-law dependence of the Curie-temperature as a function of pressure
[4.16].
12 Pre鎚ふ[kbar]
4 O
70
J
60 ~ u
ロニ50 ~
Vラ
Z
40 0 Eニト-
'-"
30 ~ い」
jloi
300
1.00 0.60 0.80
ENERGYCRYDJ 0.40
刊NSJ
0.20
200
O O.
100
(JJωυoト広¥の
ω↑ι↑ω}日U
↑Eト的
LO
」「↑一
ωzuo
Fig. 4.9. Density of states of MnSi calculated by the APW method. The number of
54
electrons/ cell is also shown [4.81.
〉ト一的
ZωトZ一
。凶
N
コd2goz
Fig. 4.10. XPS and BIS spectra for the 3d transition metal monosilicides comp訂edwi由
the theoretical s戸ctra(band DOS). including matrix elements. Note血athere血.es戸ctra
have been nonnalized with the aid of the出eo陀 ticalspec回.The metal d con釘ibutionto
出eto凶 intensityis shaded [4.26].
A
7ミω=40eY
CoSi
¥¥ーMnSi
(凶ヒ
56gzヒ凶zu↑z一Z25udpo正
10 0
B けWING ENERGY(~V)
Fig. 4.11. Comparison among the photoelectron spectra of MnSi， FeSi and CoSi at thc
excitation energy of 40 eV [4.27].
55
20
4.2 Experiment
A single crystal of MnSi was supplied by Prof. Kanomata (Fac叫tyof Eng血eering，
Tohoku-Gakuin University) and Dr. Note (Institute for Materials Research， Tohoku
University). The single crystal was grown by the Czochralski method. As starting
ma低rials，99.99 % pure Mn and 99.999 % pure Si we回 arc-meltedin an argon
atmosphere. The synthesized crystal was checked by血eLaue x-ray di飴 action.百le
resistivity of a MnSi crystal 合omthe same batch as血atused in血isstudy is shown in
Fig. 4.12.百leresidual resistivity p (T→0) wぉ 2J.LO.cm and出e児 sistivityratio R三
P (T = 290) / P (T→O)w出 130.The transition tempera加reis 28.5K.
Ultraviolet photoemission spectroscopy (UPS) measurements were carried out
using a VSW CLASS-150 hemispherical analyzer， an Omicron EA-125 analyzer and a
He discharge加np(He 1: hv = 21.2 eV， He II: hv = 40.8 eV). After each series of
measurements， the Fermi edge of Au evaporated on the sample surface was measured to
detennine the Fermi level (1;，) and悦 instrumentalresolution. The energy resolution of
the He 1 and He II UPS measurements was ~ 29 meV and ~ 70 meV， respectively. In
order to obtain clean surface， the sample was repeatedly scraped in situ with a diamond
file.
4.3 Results and Discussion
Figure 4.13 shows the valence-band He 1 UPS spectra for MnSi and its background
subtraction. Here， the background has been assumed proportional to the integrated
measured spectrum and has been subtracted from the raw data. This background
subtraction procedure gives a good description of the inelastic background when
photoelectron energies are low as in He 1 UPS [4.29]. Figure 4.14 shows the valence-
band He 1 and He II UPS spectra for MnSi below T N・ Accordingto the atomic
photoionization cross sections [4.30]， the He 1 and He II UPS spectra of MnSi are
56
b-ωCOHC
250
p(2釦向=翻lJ..nan
200
3150 1
a.
MnSI
RRR-130
100
50
8 o 50 100 150 200 "250 300
TOく}Fig.4.12.回目肘伺1resistivity of MnSi [4.28].
MnSi
hv = 21.2 eV 23K
Energy relative to EF (eV)
Fig. 4.13. Valence-band He 1 UPS s戸ctra(solid curves) for MnSi and its backgrounds
(dashed curves).
57
most1y derived from the Mn 3d states and contributions from血.eMn 4s， Si 3s and 3p
s阻tesare neg1igibly small. Both白eHe 1 and He 11 UPS s戸C回 monoω凶.callyincre邸 es
ωwardEp.悶uting泊由epeak at ~. Also， there exisωaWI拙， broad struc仰向 around...
-2eV.
In order to decompose the spectra into suIface and bulk contributions， we have
sub回 .ctedthe He 11 spec回仕omthe He 1 spec回 asshown by the difference spectra in
Figs. 4.14 (a)組 d(b). Assuming血剖出esuIface con凶butionscome from the outennost
atomic layer and血at血ephotoelectron current is exponentially attenuated 針。m 血e
suIface， we consider the suIface contribution to be 1 -exp( -d/A)， where λis血.eelectron
mean-free pa血 andd is the thickness of the outennost atomic layer. Here， we have
assumed血atd = a.β， where a is the cubic lattice constant of MnSi because each cubic
unit cell contains 8 MnSi molec叫es，ム twoMnSi molecules along the cubic edge.
Con凶butions合omthe fITSt atomic layer for He 1 and He 11血MnSiis given in Table 1，
which shows白紙 theHe 11 spectra contain a large ( ... 40%) amount of suIface ~
contributions whi1e the He 1 spc詑回 containa sma11er ( -20%) amount.百m‘bulk'
spectra血usdeduced do not show the weak broad structure around -2 e V and show more
血lenseand sh紅perpeak at Ep.百leweak peak at -・6eV血 theHe 1 and‘b叫k's戸ctra
remains appreciable and is probably due to oxygen contamination or chemisorbed
oxygens which ∞uld not be totally removed by the prior cleaning pr:α沼dure.A1so，
配 cord泊gto Yeh and Lindau [4.30]， the atomic photoionization cross sections of Mn 3d
states for the He 11 UPS spec紅ヨ arelarger血組曲atfor the He 1 and 0 2p states in由e
He 11 UPS叩 即 位aare smaller than白紙forthe He 1， resulting in血estruc加民 at-・6eV
in the He 1 s戸ctra回 mains.In order to check the consistency of血.eabove
decomposition prl∞edure， we have also made a measurement by chan.伊19the angle
between the sample suIface and the analyzer ac回 p凶 αtherebychanging血.ees回 pe
depth of photod民 tronsa∞o吋血gto F =λcos <p. The He 1 UPS s戸C回 forangles 00
and 450 between the photoelectron momentum and the suIface nonnal are shown in Fig.
4.14 (c) toge血erwith a ‘bulk' spectrum obtained by subtracting血e450 spectrum from
the 00 spectrum af飽ran appropriate intensity nonnalization. The resu1t again shows血at
曲目加C知re訂 oundEp is e出anced，qualitatively consistent with the‘bulk' spec加 min
(b)， however， the peak at Ep somewhat weaker出anin (b). This quantitative discrepancy
between the twO ‘bulk' spectra would be at出butedto血esimplified assumption of the
mean-free pa出 modelemployed here and the inherent roughness of the sample suIface
prep訂 edby scraping. In the angle-dependent measurements unfortunately， we could not
obt氾nthe‘bulk' spec凶 inthe entire valence-band region because changing the angle
changed the background lineshape so strongly血ata large ambiguity remained in the
background subtraction.
58
〉
(b) MnSi (a)
〉、4・d
23K
Hel (hv = 21.2 eV)
一ωcovc-聞E
He 11 (hv = 40.8 eV)
(c) φ=0。
φ= 450
He 1 -He 11 (bulk) _e : <p....， 1 .・・・...
-8 -6 -4 ー2 O -1.2・0.8・0.4 0
Energy relative to EF (e V)
Fig. 4.14. He 1 and He n UPS spectra for MnSi in the entire valence-band region (a) and near EF (b). The He 1 -He n difference spectrum in panel (b) represent a ‘bulk' spectra.
(c) He 1 UPS spectra for angles between the photoelectron momentum and the surface
normal of 00 and 45 0• The 00
- 450 difference spectrum again represents a ‘bulk'
spectrum. Prior to subtraction， the spectra has been normalized as described in the text.
59
守秘
. '
、亀
、剛
r. ， ，:
p
J'" AI バ打
1/"- r ~ J ，.... /"
// / ，J
J、、・;....・_.1・ソ
ム，...:. _/"，../.・、't，".:11 f..
f'~吋， ~，. .
. 、"A_".~.、，・・6:-，.......，.，.，.. .. ~
MnSi
hv = 21.2 eV
一一 100K. 50K 23K
、C窃COHC
j ‘，・
，_.. --.
-リ
Es--E，・・"..
d
-
-
-
-
-
d
・
J
J
・-，
.」Feaarr
・--
.F噌
fJ--r
.
J
&
v
，
.• -
a
e
J
a
-
-
・2
・'f
d
r
・-
J
-
1
・・far--l
I
l
-
-
d
'
'
d
・・・
ノ
ff
・''
r
s
J
・・
-
-
-
e
宇
a''
・・・
.・1
・
J
f
，
〆
〆
J
J
J
F
'
a
'
"
へ
円
九
戸
‘‘--
ノ
%
MnSi
hv = 40.8 eV
ー -.100K….50K -23K
、AH
一ωcovp』
O
EF (eV)
Fig. 4.15_ Valence-band He 1 and He II UPS spectra for MnSi at various temperatures.
60
-2
to
ー4
relative
-6
Energy
-8
注、-ーCJ) c Q) 4・d
c 一
4〉・d、-ーCJ) c 4q・34
c 一
MnSi (a)
""'""̂ 100K
50K
4 ・ ..-叫....，.句吋仇P
7K
hv = 21.2 eV
MnSi (b)
100K
50K
23K ー小作市平J『・、句剛・
7K
hv = 40.8 eV
-1.2 -0.8 四 0.4 0
Energy relative to EF (eV)
Fig. 4.16. He 1 (a) and He 11 (b) UPS spectra near the Fermi level for MnSi at
different tempera旬res.The spectra have been nonnalized to the total area.
61
light source mean free pa出 surface layer contribution
HeI (21.2eV) ーlOA -20%
He II (40.8 eV) -5A -37%
Table 1. Contributions from the surface layer to the He 1 and He II UPS spectra of
MnSi. Here， the cubic lattice constant 2.3 A has been used.
Figures 4.15 and 4.16 show the tempera印redependence of the He 1 and He II UPS
spectra in the entire valence-band region and near ~ for MnSi， respectively.百lefigures
show血at由espectra do not change appreciably with tempera佃reon this energy scale
except for the temperature-dependent broadening of the Fermi edge. In order to detect
subtle tempera加re-d叩endentchanges in the DOS near Ep， it is desirable to remove出e
temperature-dependent e百'ectof the FermI-Dirac distribution function.百lerefore，we
have divided the photoemission spectra by the FermI-Dirac distribution function
convolllted with aη判明iancorrespondin9: to the instmmenta1 resolution at each
tempera印re[4.31]. The spectral DOS thus obtained are shown in Fig. 4.17. We fmd
that the spectral DOS at Ep slightly decreases with decreasing tempera加re.Presumably，
the magnetic ordering below TN and possible short-range order somewhat above TN
cause the splitting of the d bands and hence the reduction of the DOS at Ep. It should be
noted血atthe weakness of the temperature dependence may part1y be due to
contributions to the He 1 spectra from the surface layer， which probably does not show
the phase transition as the bulk one at the same tempera加re.Nevertheless one can
conclude that出etempera印redependence of the ‘bulk' spectra is weak because -80%
of the He 1 spectra reflect the bulk electronic structure according to Table 1. The DOS
for MnSi in the paramagnetic and ferromagnetic states have been calculated by Yamada
and Terao using the linear-muffin-tin-orbitaト-atomic-sphere-approximation(LMTO-
ASA) method.百lecalculated DOS for the paramagnetic and ferromagnetic states in
MnSi are shown in Figs. 4.18 and 4.19 [4.24].
Now， we make comparison between the calculated band DOS [4.24] and the
experimental ‘bulk' spectra. Figure 4.20 show comparison between the band-structure
DOS [4.24] (dot-dashed curve) and the experimental spectrum (dot curve) in
62
(a)
100K _. -50K
23K 7K
MnSi 1.6
1.4
1.30 皇 1.25c z 120
ま1.15
2110
1.05
1.0
0.6
0.4
-40 ・20 0 Energy relative to EF(meV) 0.2
0 ・0.20 O -0.05
Energy relative to EF (eV) ー0.10ー0.15
(b)
1.0
置
1e
，F F
ー-100K ---50K
23K 7K
MnSi p 、1.3ω +-'
c コ
211
CJ)
O Q 0.9
-40 ・20 0
Energy relative to EF (meV)
3E1.2降J
C コ..c ~
ま0.8
O Q
0.8
Fig. 4.17. Temperature-dependent spec汀alDOS (a) for MnSi obtained by dividing the
He 1 UPS spectra by the Fenni-Dirac distribution function convoluted with a Gaussian
and血.e(b) shows an error bars. The inset shows an enlarged spectra DOS near the
Ferrni level.
63
3.5 MnSi Total
、I
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Mn3d 2.0
(ωεo芯.〉OB25ω)
1.5
Si3p
1.0
0.0
0.6
0.4
0.2
0.0
0.5
c/)
O 。
5 。Energy (eV)
-5 -10
Fig. 4.18. Calculated DOS curves for MnSi in the paramagnetic state by the linear-
muffin-tin-orbital-atornic-sphere-approximation (LMTO-ASA) method. The top
panel shows the total DOS， the rniddle and bottom panels show the Mn 3d and Si 3p
partial DOS， respectively [4.24].
64
MnSi Total
3.0
2.5
2.0
1.5
1.0
0.5
Mn3d
Si3p
1.0
0.5
0.0
-0.5
0.4
-1.0
0.2
0.0
(ωεoH6.〉OB25ω)c/)
O O
ー0.2
ー0.45 O
Energy (eV) 副 5-10
Fig. 4.19. Calculated DOS curves for MnSi in the ferromagnetic state by the linear-
muffin-tin-orbital-atomic-sphere-approximation (LMTO-ASA) method. The top
panel shows the total DOS， the middle and bottom panels show the Mn 3d and Si 3p
partial DOS， respectively [4.24].
65
出eentire valence-band region and near 1;，. Here，血eband DOS has been broadened
Wl出血.einstrumental resolution and lifetime. It should be noted血atwhile血eMnSi
sample was in the helimagnetic state at low記mpera加re，the theoretical DOS [4.24] is
血atof the ferromagnetic. However， the difference betw悶 1血eferromagnetic DOS and
血.ehelimagnetic one should be small becau田 ofthe long periodicity of the helical spin
struc仰向.There are discrepancies between the DOS and the experlmental spectra.百rat
is， the calculated band DOS shows two prominent peaks 1∞ated at 1;， and around --2
e V while there is no clear structure at -ー2e V in the measured spectra.
In order to improve the agreement between the measured spectra and the band DOS，
a self-energy correction wぉ appliedto the band DOS [4.24] as shown in the same
figures. We used a model self-energy L(ω) = Lhigh(ω) + L1ow(ω，)， where Lh喝h(ω)=
ghighω/(ω+ i 'Yhigb)2 and L1ow(ω) = -glow [1/(ω+ i 'Ylow) + i / 'Ylow]' which accounts for the
band narrowing near 1;， and血espectral weight transfer from low ωhigher binding
energies in the photoemission spe心加1m[4.32，33].官lemass enhancement factor is given
by mソmb= 1+ ghig/Yhigh + glow/Ylow' He民， ghigh' 'Ybigh' glow and 'Ylow are adjustable
p訂 ametersand were determined considering血especific heat result， namely， in the case
of血ehelimagnetic s阻teof mソmb-7.1 and血eparamagnetic state of mソmb-6.61.
Spec住alfunctions thus calculated using g high = 8.0 eV2， 'Yhigb = 3.0 eV， g low = 0.01 eV2
and 'Ylow= 0.044 eV for the helimagnetic state and ghigh = 8.0 eV2， 'Yhigh = 3.0 eV， g low =
0.02 e V2 and 'Y'ow = 0.065 e V for the paramagnetic state are shown by the solid curves in
Fig. 4.20. Here， the calculated mass enhancement factor is mソmb-7.1 and -6.6 for
血ehelimagnetic and paramagnetic states， respectively. The corresponding self-energies
[ReL(ω) and ImL(ω)] are shown in Fig. 4.20. When the self-energy co汀'ectionis
applied， the structure around --2 eV is shifted toward higher binding energies， and
decreases its intensity. Also， the structure around EF is narrowed， showing a sharp peak
at EF• Agreement between the theOIγand the experiment was considerably improved in
the entire valence-band spectrum as shown by the solid curve in Fig. 4.20， although
some discrepancies remain. As for the spectra near s.， the self-energy corrected DOS
shows too sharp a pe広 atEF• The k・dependenceof the self-energy may improve the
agreement between theory and experiment as in the case of Y(Col-xAlxh [4.34J. From
the
66
、ゎ
tωcoHC一
23K
MnSi (a) hv = 21.2 eV
~三ヴ4・、〆
.. ... exoeriment _. band DOS - self-energy corrected DOS
paramagnetic T 1- 50K ω c Q) 4・d
c
-6 ・4 ・2 0 -1.2 ・0.8 ・0.4 0
Energy relative to EF (eV)
Fig. 4.20. Comparison between the experiment UPS spectra in the entire valence-band
region (a) and ne訂 EF(b) (dots)， the band DOS (Ref.24) (dot-dashed c町 ves)and出e
band DOS corrected for a self-energies for MnSi. Best fit self-energies E<ω) are shown
below each spectrum.
67
4.4 Summary
We have studied血.eelectronic struc伽reof MnSi， which show a helical spin order
below 30 K， by means of ul回 violetphotoemission spectroscopy. In order to田p訂ate
血espec加担tosurface and bulk con凶butions，we have邸 sumed曲atthe surface
contribution comes合omthe outermost atomic layer and considered the electron m伺 n-
白-eepa血.百le‘b叫k'spectra血usdeduced are comp釘edwith the DOS derived合om血e
band-structure calculation corrected for the model self -energies. This indicates白紙
electron∞，rrelation and band・s加 C旬間 e旺ectsare bo血 importantin MnSi. In the
valence-band region， the spec回 showno appreciable tempera加redependence， whereas
血espec加nnn伺 rEp shows weak tempera組問 dependence.We fmd血at血.espec紅a1
DOS at Ep slightly decreases on cooling. We tentatively a町ibute血isto the exchange
splitting of the Mn 3d bands due to the ma，伊eticordering.
68
一正j
References
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19，529 (1976).
[4.2] Y. Ishikawa， K. T司ima， D. Bloch and M. Ro血， Solid State Commun. 19， 525
(1976).
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[4.4] T. Moriya， Solid State Commun. 20， 291 (1976).
[4.5] K. Makoshi and T. Moriya， J. Phys. Soc. Japan 44， 80 (1978).
[4.6] O. Nak山 shi，A. Yan邸 eand A hasegawa， J. Magn. Magn. Mat.15-18， 879
(1980).
[4.7] J. H. Wemick， G. K. Wertheim andR. C. Sherwood， Mater. Res. Bull. 7 143
(1972).
[4.8] O. N紘anishi，A. Yan部 eand M. Kataoka， in Electron Correlation and Magnetism
inNarrow-Bαnd Systems， edited by T. Moriya. (Springer Series in Solid-State
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[4.9] E. Fawcett， J. P. M出taand J. H. Wemick， Intem. J. Magnetism vol1， 29 (1970).
[4.10] M. Matsunaga， Y. Ishikawa and T. Nak可ima，J. Phys. Soc. Japan 51， 1153
(1982).
[4.11] S. Ogawa， Physica B. 119，68 (1983).
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[4.13] K. Suzuki and Y. Masuda， J. Phys. Soc. Japan 54， 630 (1985).
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Phys. Soc. Japan 46， 1767 (1979).
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[4.17] E. C. Stoner， Proc. Roy. Soc. A165， 371 (1938).
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[4.19] E. P. Wohlfarth， Phys. Lett. 28A， 569 (969).
[4.20] T. Moriya and A. Kawabata， 1. Phys. Soc. Japan 34， 639 (1972).
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[4.21] T. Moriya and A. Kawabata， J. Phys. Soc. Japan 35，669 (1973).
[4.22] C. Pfleiderer， G. J. Macmu11an and G. G. Lonzarich， Physica B. 206 & 207， 847
(1995).
[4.23] K. Koyama， T. Goto， T. Kanomata and R. Note， Phys. Rev. B. 62， 986 (2α泊).
[4.24] H. Yamada and K. Terao， Phys. Rev. B. 59，9342 (1999).
[4.25] J. Be出e，J. Voiron and M. Roth， Solid State Commun. 47， 399 (1983).
[4.26] W. Speier， E. v. Leuken， J. C. Fuggle， D. D. Sarma， L. Kum訂， B. Dauth and K.
H. J. Buschow， Phys. Rev. B. 39， 6008 (1989).
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J. Phys. Soc. Japan 51， 2597 (1982).
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[4.30] 1. 1. Yeh and 1. Lindau， At. Data and Nucl. Data Tables 32 1 (1985).
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Shiga， Solid State Commun. 95 307 (1995).
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Tsunekawa， T. Muro， T. Matsushita， S. Suga， H. Ishii， T. H組 yu，A. Kimura， H.
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Rev. B. 60 538 (1999).
70
Chapter 5.
Photoemission study of the Laves圃 phase
compounds YMn2 and YO.97ScO.03Mn2
5.1 Introduction
Intennetallic compounds of Yand 3d-住組sitionmetal M， YM2(M = Mn， Fe， Co or
Ni)， wi出血eLaves-phase structure show a variety of血te陀 stingmagnetic prope凶es.百le
田ritcell of血.eC15・typeLaves-phase contains two Yatoms and four Mn atoms. The Y
aωms make up the diamond s位uc旬reand泊血einterstices of the diamond Iattice of the
Yatoms， te回hedraof four Mn atoms are担問rted，as shown in Fig. 5.1(a) [5.1]. In Fig.
5.1(b)血isstructure is viewed企om血.e[111] di問 ctions.官leMn subla凶ceis identical to
血atof血e回nsition-metalatoms of the pyrochlore type， which have been known show
spin frus回 .tion[5.2]. Among these compounds， the cubic Laves-phase compounds
~ and Its substituted compound Y l_xSCx~ have attracted considerable interest
because of their unique magnetic and 血ermod戸四国c properties. Especially
p紅 amagneticY O.97ScO.03Mn2 shows a large electronic specific heat 'Y = 150 mJ/moleK2
comparableωanf-electron heavy Fermion compou叫
YM~ is an itinerant吃lectronantiferrom姥netWI血 along-range helical spin order
[5.3，4] wi白血eNeel tempera旬reof about 100 K.官lemagnetic struc印reis shown血
Fig. 5.2 and the observed magnetic moment on the Mn atom is 2.7 ~B. As shown in Fig.
5.2，血.eY atom site is nonmagnetic and the spin of the Mn atom orders in a complicated
configuration with a large magnetic moment [5.3]. Figure 5.3 shows the temperature
71
....
袋、，~
A
一二二
4111
o Mg.・CuA
MgCul (a)
o Mg.・Cu
J
(h) MgCu，
Fig. 5.1. Crysta1 struc同reofthe cubic Laves phase compound MgC~ [5.1].
Fig. 5.2. Magnetic structure of YMn2・Onlythe Mn sites are shown. Open and closed
circles represent up and down spins， respectively. Spin directions are not colinear with
each other and helically modulated with a periodicity of 400 A lattice constants [5.3].
72
dependence 'Of the magnetic susceptibility 'Of YM~・It is quite unusual above T N and血e
disc'Ontinuity in the susceptibility was 'Observed in cooling and heating釘oundTN [5.3].
In thennal expansi'On measurements， it als'O exhibits a disc'Ontinuity at nearly the same
胞mpera仰向s[5.5].官官民 factsindicate由ata first 'Order phぉetransiti'On takes place
ar'Ound 100 K.百lec'Oncentrati'On dependence 'Of血ela'凶偲 par百ne胞r'Of Y l_xScxM~ at
r'O'Om飽mpera加reis sh'Own in Fig. 5. 4 t'Oge出.erWl出 Vegard's law.百lela凶偲 par祖neter
decreases rapidly with increasing x [5.6].
The temperature dependence 'Of the lattice pararneter sh'Own in Fig. 5.5 indicates由at
YMI1Z exhibits a giant v'Olume change 'Of ab'Out 5 % at T N and血atthe v'Olume shrinkage
above T N may be a町ibutedt'O出ereducti'On of the Mn magnetic m'Oment [5.5，6].百le
strong temperature dependence 'Of the lattice pararneter above T N suggests a str'Ong
temperatu問 dependence 'Of血.emagnetic m'Oment due t'O spin fluctuati'On. The
antiferr'Omagnetism in ~ is easily suppressed by applying pressure 'Or substituting a
small am'Ount 'Of third element f'Or Y， which results in a c'Ompressive chemical pressure
because 'Ofthe small at'Omic radius 'Of Sc [5.7].百lelattice p訂 arneterssh'Own in Fig. 5.5
indicate出at血esubstituti'On 'Of a small arn'Ount 'Of Sc f'Or Y stabilizes由ep釘釘nagnetic
phぉed'Own t'O the l'Owest temperature. As sh'Own in Fig. 5.5， the expansi'On coefficient 'Of
G
7 申。o‘ 、、コ 6ε 申
(J)
O
>< 5
4
I
YMn2
100
H= 8.28 kOe
』 ，I200 300 400 500 600 700
TEMPERATURE (K)
Fig. 5.3. Tempera加redependence of the magnetic susceptibility of ~ for cooling
and heating [5.3].
73
7.75
~ 765 属工
。
1ω
o
l¥ ¥ ¥ ¥ ¥ ¥ ¥
0.1
¥ ¥
X
¥ ¥
0.2 0.3
Fig. 5.4. Concentrauon dependence of the la1Uce pru:百neterof Yl_xScxMn2 at room
tempera旬re_The broken line shows Vegard' s law for Y 1_xScxMn2 where the la1Uce
parameter of ScMn2 was esumated from the unit cell [5.6].
Yl.x SCx Mnl
7.70
x=o
7.65
.<1:
口
x:006
. /'
755 /
O 1∞ 2∞ 3∞ T (K)
Fig. 5.5. Temperature dependence of the lattice parameter for Yl_xScxMn2 [5.6].
74
31- 11101
! ! ロlSOK
d. 120K
o 8K
I 1 ¥ 2
{
同f，、.句回
，、.~ 1
v X 1/ t¥' 宣 ，、 可¥
J トO' o 2 3 4
Q (r1)
Fig. 5.6. Wave VI切旬rdependence of the paramagnetic neutron scaU切ng of
YO.97SCo.03~・An・ows indicate血eposition of magnetic Bragg peお of~[5.8].
1回ト -hV. 1 ・・-ー-'・.. | ・ 『凶ザ帽圃 ~.97 Seo.03Mn2 τJ句'Jv':;y
E 150r ・N x 、、ー、E l~O 亡+υ30
_0・・・・・・・・20ト-戸--YMn2
。!?LO 10
i
02
句
SMRt
‘，‘ マ』
nu 『，‘ ~O 50
Fig. 5.7. Low temperature specific heats of YI_.SC.Mn2 [5.9].
75
血eSc substi加tedcompounds above 100 K is still large， suggesting a large spin
fluctuation effect. In order ωdetect the spin fluctuations directly， Shiga et al. [5.8] have
petformed p創百nagneticneutron scattering e珂>erimentsfor Y O.97ScO.03Mn2 and observed
a 1訂geparamagnetic scattering around Q = 1.5 A-¥ at 8 K， indica出19giant spin
fluctuations with a strong antiferromagnetic correlations (Fig 5.6). Furthermore，血e
ampli加deof the scattering increases with increasing tempera加問s，suggesting血atspin
fluctuations are thermally excited， and therefore， a large enhancement of 'Y is expected白
血issystem. In fact， the low tempera印reelectro凶cspecific heat coe百Icient'Y of
YO・97ScO.03Mn2is出 largeas 150 mJ/moleK2， as shown in Fig. 5.7 which is about 23
由neslarger血m 血atex戸ctedfrom血.ebare band density of s旬tes[5.10]. This
enhancement f:蜘rof 23 is unus叫 Y1碍 eamong 3d 蜘 sition-me凶 compomds.〉Yamada et al. [5.11] have petformed a band-structure calcu1ation for the paramagnetic
YMnZ using the tight-binding approximation and shown白紙血.eFenni level (民)lies
near a sha中 minimumof由edensity of s阻tes(DOS). By making use of出iscalc叫ation
and taking into account the e百ectof spin fluctuations，血.eyexplained the unusual
胞mpera細胞 dependenceof the susceptib出tyabove TN・Onthe other hand， Terao組 d
Shimizu [5.12] and subsequently Yamada and Shimizu [5.13] calc山削除 band-
S加伽陀DOSfor the antiferromagnetic YMnZ us泊gthe recursion method組 dthe tight-
binding method， respectively.百leyhave shown that the DOS at Ep is mainly due ω血e
minority叩 ind band and the main part of the majority叩 inband lies below Ep・ Very
recently， the DOS of YMnZ血血.eparamagnetic and antiferromagnetic states have been
calcu1ated using血.elin伺 r-mu阻止血ーorbital-一台.tomic-sphere-approximation(LMTO-
ASA) me出odby Yamada and Terao [5.10]，出 shownin Figs. 5.8 and 5.9.百leyhave
also shown the similar result as Terao and Shimizu [5.12] or Yamada and Shimizu
[5.13].
h 血iss佃dy，in order to obtain experimental information about the el，即位oruc
S凶 C加reof Y1_xScxMnz， we have petformed the x-ray photoemission spectroscopy
(XPS) and high-resolution ultraviolet photoemission spectroscopy (UPS) measurements
of YMn2 and YO.97ScO.03Mn2' which are antiferromagnetic and paramagnetic， respectively.
The results are compared with the band-structure DOS. In order to examine the role of
electron correlation in these compounds， we have also applied model self -energy
12J
corrections to出eband-structures DOS.
76
YMn2 EF Total (paramagnet)
6
4
2
o 8
Mn3d
(ωεoH6.〉oho-Sω) 6
4
2
c/)
O 。
J円いJijjE4d Energy (eV)
Fig. 5.8. Calculated DOS curves for Y~ in血eparamagnetic state by the linear-
muffin-tin-orbitaト-atomic-sphere-approximation(LMTO-ASA) method.百letop panel
shows the tota1 DOS， the middle and bottom panels show the Mn 3d and Y 4d partial
DOS， respectively [5.10].
77
YMn2 E
Total (antiferro)
10
‘司』，
8
6
4
2
O 8
Mn3d 6
4
(ωεo芯.〉oho-Sω)ω 2
O 。Q
ー2
-4 3
Y4dJ いJ\~
Energy (eV)
Fig. 5.9. Calculated DOS curves for Y~ in the antiferromagnetic state by the linear-
muffin-tin -orbital---atomic-sphere-approximation (LMTO-ASA) method.百leωppanel
shows the total DOS， the middle and bottom panels show the Mn 3d and Y 4d partial
DOS， respectively [5.10].
78
5.2 Experiment
Polycrystalline s創nplesof ~ and Y 0.97ScO.03Mn2 were supplied by Prof. H.
Wada and Prof. M. Shiga of Kyoto University. They were prepared合om99.9 % pure
Y， Mn and Sc in an argon arc fumace followed by vacuum annealing at 8000 C for one
week. The prepared samples were checked to be single-phase by x -ray di飴action.τbe
lattice par酒田tersof ~ and YO・97Sc0.03Mn2used血血isstudy were 7.673 A and
7.643 A， respectively.官官記P訂ameterswere smaller血m血eprevious results (Fig. 5.4)
but the magnetic qua1ity were回me，YMnZ and YO.97ScO.03Mn2 were antiferromagnetic andp町田na伊 etic，res戸ctively.
X-ray phot侃 missionspectroscopy (XPS) measurements were performed using a
VSW hemispherical analyzer and a Mg Kαline (hv = 1253.6 eV). ul回 violet
photoernission spectroscopy (UPS) measurements were carried out using a VSW
hemispherical analyzer and a VG He discharge加np(He 1: hv = 21.2 eV， He 11: hv = 40.8eV).官leresolution of XPS， He 1 and He 11 UPS measurements 明記-0.9 eV，20
meV and -28 meV， respectively. The base pressure was about 3 x 10・10Torr. In order
toob阻担 cleansurfaces， samples were repeatedly scraped in situ with a diamond fIle. In
the measurements， the Fermi edge was referenced to the Fermi edge of Au evaporated on
the sample after each series of measurements.
5.3 Results and discussion
Figure 5.10 shows the valence-band XPS spectra for YMI1:!' According to the
calculated photoionization cross-sections of atornic orbital， valence-band XPS spectra
mainly consist of Mn 3d and Y 4d states [5.14]. Two prominent peaks are located
below the Ferrni level and separated from each other by -2 eV. A weak structure around‘
6 e V in the room temperature spectra may be due to the 0 2p state of oxygen
contamination because the surface of YMn2 is very easily oxidized at high temperatures
79
YMn2
hv = 1253.6 eV
・300K(para) 77K (antiferro) _.¥
¥AHBCGHC
"・. . . . • • • • -H
• • • 句-•
・
. . . ・..-・・.. . --. . .・..・'.. .. . . ・.. . . -. ..・.. . ... J句・.・ー.... ・. . --..・・
H
aF
句
.
-8 -6・4・2 0 2 Energy relative to EF (e V)
Energy relative to EF(eV)
Fig. 5.10. Valence-band XPS spectra for YMn2 in the paramagnetic (T = 300K) and
antiferromagnetic (T = 77K) states. The inset shows comparison of the calculated density of states between the antiferromagnetic and paramagnetic states [5.10]. The
calculated DOS have been broadened with the instrumental resolution and the lifetime
broadening. The weak shoulder at --6 eV in the 300K spectrum is due to oxygen
contammatlOn.
80
(most likely due to out-diffusion of oxygen atoms dissolved in the bulk).明記問sults
show血at， except for the ・6e V feature， the valence-band XPS spectra do not change
appreciably wi白隠mpera伽陀.The inset shows comparison between血.etheoretical DOS
[5.10] of the antiferromagnetic and paramagnetic states for ~.百e band DOS has
been broadened with the instrumental resolution (-0.5 e V FWHM) and the lifetime
width of the Mg Kαline( -0.36 eV FWHM). The theoretical DOS show白紙由e
antiferromagnetic and paramagnetic states are obviously di百erent.One can see白紙出e
theoretical DOS and the experiment agrees well with each other for the antiferromagnetic
state. This suggests血atthe strong magnetic fluctuation exists in the paramagnetic phase
and determine the globallineshape of the photoemission spectra.
It is well known血atXPS is more bulk-sensitive than He 1 and He 11 UPS because
the higher the photoelectron kinetic is， the longer the electron mean-free pa血 becomes
[5.15].On血eother hand，泊 manymeta1lic sys飽ms，only血eoutermost atomic layer is
strongly affected by血esurface and that the second layer has the el即位onicstructure
sim且arto the bulk [5.16，17].官lecontributions 合omthe outermost atomic layer for He L
HeITandXPS面白e~ using the 1・exp(-dt:入)are given in Table 5.1， which shows
血atthe He 11 spectra mainly consists of the surface electronic structure which血at血e
He 1 andXPS s戸C住amainly 回 flectthe bulk electronic s紅uc仰向 of血es組 lple.H町e，λ
is the electron me組・合eepa由 andd is the thickness of the outermost atomic layer，
which is阻kento be half of the cubic lattice constant of ~・古田問fore， we should
light source mean free pa出 surface layer contribution
He 1 (21.2 eV) -lOA ー32%
He II (40.8 eV) -5A -53%
XPS (1253.6 eV) -15A 四 22%
Table. 5.1. Contributions from the outermost surface layer to the He 1， He II UPS and
XPS spectra of Y~・ Here， the cubic lattice constant 3.8 A has been used.
81
detect血etempera仰向 dependentchanges using XPS if the DOS changes as p詑dic飽d
by the band-structure伺 lc叫ation.Therefo陀， we conclude白紙 thebulk DOS does not
change app陀ciablybetw田 n血.eparamagnetic and antiferromagnetic phases on血is
energy sca1e.
Figure 5.11 shows a typica1 va1ence-band He 1 UPS spc珂回mof ~， showing
how the background has been sub回 .cted.Here， the background has b田 ndetermined by
integrating the measured spectrum仕'omthe Fermi leve1.百usgives a good description of
血einelastic background when photoelectron energies are low as in He 1 UPS [5.18].
Figu陀 5.12show He 1 UPS spec回 ofthe entire va1ence band (a) and ne訂Epregion (b)
for YM~ and YO.97SCO，03Mn2 at various tempera知res.百leva1ence-band spectra near the
Fermi level (Ep) are derived mainly from the Mn 3d and Y 4d states [5.11，14].百le
emission near Ep is due ω血.eminority-spin band and血ataround ・2eV is due ω血e
majority-spin band [5.12，13]. Figure 5.12 shows血at血es戸ctrafor x = 0.0 and x = -i 0.03 a1so do not change appreciably wi白隠mpera卸rea1though UPS has much better
en町'gyresolution血.anXPS. Judging仕om血isresult， one can conclude由atthe absence
YMn2 hv = 21.2 eV 16K
hH
一ωCOHC
コ
-5 -4 ・3 ・2 ・1 0
Energy relative to EF (eV)
Fig. 5.11. Valence-band He 1 UPS spectra (solid curves) for ~ and its background
(dashed curves).
82
YMn2
hv = 21.2 eV
(a) YMn2 (b)
" ， ‘‘ •• --
h ，
，
〆，
b あ ，.ー、，C:: I 、，・
21130K/
*''''
4' -h
A
KJ
nu.r
ndAF
J'
J司
a-
a
d
- - 、‘・
YO.9-'sCo.03 M n2 J li Ydco訓 n2
hv = 21.2 eV / ~.、，4‘、
b ..
)，.
(f) 130K〆'，
c 〆"、，.，〆場。・4 ‘r ‘ 130K _ r
司、."c t' ，..，.，，-
〆J‘d、，..-'"
J r-I I
L...~
-4 -2 O -1.2 ・0.8・0.4 。Energy relative to EF (eV)
Fig. 5.12. He 1 UPS spectra of Y~ and YO.97ScO.03Mn2 in the entire valence-band
region (a) and near EF (b) at various tempera加res.百lespectra have been normalized to
the total area.
83
(ωZCコ.0」何
、同国〆
ω00
YMn2
130K 0.8ト…… 80K
16K
0.6
0.4 £コea 0.8 ω 00.7 0
0.6
ーー 130K80K
- 16K
0.2 -40 ・20 0 Energy relative to ~meV)
、，
P
0.0 1.6
YO.97SCO.03Mn2
ーー 130K- 16K 、、
ーも
130K - 16K
0.4
1.4
宣1.3
51.2
空 1.16
001.0
809 0.8
0.2 -40 ・20 0 Energy relative to E~meV)
ー0.20 ・0.15 ・0.10 -0.05 0.00
Energy relative to EF (eV)
Fig. 5. 13. Temperature-dependent spectral DOS of Y1_xScxMn2 obtained by dividing the
He 1 UPS spectra by the Ferrni-Dirac distribution function convoluted wi出 aGaussian.
The inset shows an error bars.
84
of appreciable慨 npera:加redependen<克也 theHe 1 and XPS sp民国a1sorefi.l削除
ab舘 R旬。fappreciable temper羽田edependence in the bulk eL民 tronicstruc旬開 of除
制 nple.In orderωdetect subt1e tempera:ture也pendentchanges in the∞S near Ep， it is
desirable旬開move血etempera:佃re-de戸ndentefJect of the Fermi-Dirac dis凶bution
function.官蹴efore.we have divided the photoemission s戸C凶 by血.eFen凶・D泊 c
dis凶butionfunction∞nvoluted wi血 aGaussian conesponding to the instrumen旬i
resolution at伺 .chtempera:卸re[5.18].百lespec釘a1DOS由usobtained are shown in Fig.
5.13. We fmd出at出.es戸C回 DOSdo not so change at di百erenttemperatures on his
energy sca1e.
Figure 5.14 shows change induced by Sc substitution near Ep in the He 1 UPS
明記回.In order ωemphasize the change， we have subtracted the spec回mof~
from血atof Y o.~cO.03~ お shown by血edifference叩ectrumin Fig. 5.14. The問 S叫t
shows由at， in going 針。m~ωYO.97SCo.03M~， the DOS in血evicinity of Ep
mα・easesand the OOS at・1.2ー -2.0eV slight1y decreases. This may be inte中間tedas
、C一ωcovc
Y1-xScxMn2
hv= 21.2eV
|ニロア|
(0.03)ー(0.0)difference x 2
-2.0 ・1.5 ・1.0 ・0.5 0
Energy relative to EF (eV)
Fig. 5.14. He 1 UPS spectra of Y.・xScxMn2near EF and the different spectra between x = 0.0 and x = 0.03.
85
c喜宮‘
a narrowing of the Mn 3d band induced by Sc substitution. This change may be
inte中Iretedぉ dueto血esplit白19of the Mn 3d band due to antifeπomagnetic ordering.
One can see that the s伊C回 near~ show small change with Sc substitution al由ough血e
S戸ctraof ~ and YO.97SCO.03Mn2 do not change appreciably wi由記mperatureon血IS
energy scale.
We have made a comparison between the theoretical DOS [5.10] and 血e
expe血nen凶 spectrain the entire valence band and near ~. The band DOS has been
broadened with the instrumental resolution and the lifetime broadening (FWHM = 0.07
Eo). Figure 5.15 shows comparison between血etheoretical DOS (dot -dashed curve) and
血eexperimental spectra (dots curve) for the antiferromagnetic and paramagnetic states，
nantely， for ~ and YO.97Sco.o3Mnia). 百le ex戸rimen旬1 spectrum of
antiferromagnetic Y加~白血e en曲芭 valence-bandregion is well reproduced by血e
calculated DOS. On the other hand， the spectrum of YO.97SCO.03~ is not reproduced by
the calculated DOS. In the near ~陀gion (b)， agreement betw田 n血ee却erimen旬I
spectra and血eband DOS is not satisfactory for bo白血eantiferroma伊eticand
paramagnetic states.
In order to take into account electron correlation e百ectswhich are not included in血e
band-struc旬間 calculation，a self-energy correction was made to the band DOS [5.10].
臨時， we have used a simple form of血emodel self -energy E(ω) = Ehigh(ω) +工low(ω)，
whe問 Lhigh(ω)= ghighω/(ω + i 'Yhigh)2 and E，ow(ω) = -glow [1/(ω + i "fJow) + i /"fJow]，
which accounts for血.eband narrowing nω~and 血e spec回 1weight仕組sferfrom low
to higher binding energies血血e photoemission spec仕a [5.20，21]. 百le mass
enhancement factor is given by mソmb= 1+ ghig/i¥ゅ+glow/ずlow'He陀， ghigh' 'Yhigh'
glow 組ld'Ylow were trea低d出 adjustablep訂却1侃 rsand we陀 determinedconsidering血e
specific heat result， namely， in the case of the paramagnetic state for Y O.97ScO.03Mn2 is
mソ~- 23 and血eantiferromagnetic state for ~ is mソmb- 1. S戸C回 1functions
出uscalculated using ghigh = 8.0 ey2， 'Yhゅ =1.8 eY， glow = 0.005 ey2組d'Y¥ow = 0.016 eY
for the paramagnetic state and ghigh = 7.2 ey2， 'Yhigh = 1.6 eY， g¥ow = 0.003 ey2 and 'Y¥ow =
0.052 eY for the antiferromagnetic state are shown by the solid curves in Fig. 5.15. The
corresponding self叩 ergies[ReE(ω) and ImE(ω)] are also shown in血esame figure.
百leinset shows an en1arged plot of血eself -energy E(ω)出血evicinity of Ep for
Y0.97ScO.03Mn2・Here，it should be noted白紙 theself-energy correction is strongly
varying near EF for the paramagnetic Y 0.97ScO.03Mn2' corresponding to the large mass
enhancement mソmb- 23. Due to this strong self-energy correction in the paramagnetic
state， the structure from --0.5 eY to --1.5 eY in the calculated DOS is shifted toward
86
弘
、
(b)
…. experiment _. band DOS - self-energy corrected DOS
YMn2 (a) YMn2 hv = 21.2 eV
会ωCOHC
(〉@)hAm』@C凶
KAH
一ωcgc一
0.0 -0.5
Energy relative to EF(eV)
-1.0 幽 1.5-2.0 。-2 -3 -4 -5
Fig. 5.15. Comparison between the experimental UPS spectra in the entire valence-band
region (a) and near EF (b) (dots)， the band DOS (Ref.9) (dot-dashed curves) and the
band DOS corrected for a self-energies for YMn2 and Y O.97ScO.03Mn2・Bestfit self-
energies L.(ω，) are shown below each spectrum. The inset shows an enlarged plot of the
self-energy [(ω) in the vicinity of EF for Y O.97Sco.o3Mn2・
87
lower binding energies and血estruc卸間町'oundF;: is narrowed. The struc組 問 around--
2.2 e V is shifted toward higher binding energies. Agreement between由e血eoryand血e
experiment was considerョblyimproved as shown by the solid CUIVe血Fig.5.15. In the
ca詑 ofantiferromagnetic state，出eagreement between the血.eoryand血.eexperiment wぉ
considerably improved白血eva1ence-band spectrum as shown by the solid CUIVe in Fig.
5.15 (a)， a1though some discrepancies remain. As for the spectra near ~ the self-energy
corrected DOS shows t∞sharp a peak at F;: as shown by由esolid CUIVe in Fig. 5.15.
Wenote血at，泊ameta1lic Kondo system， the position εF of血eKondo peak from EF is
related to T max through 1εF 1= 3 kB Tmax [5.22]. In the case of YO・97SCO・03Mn2'明。w - 0.02
e V has introduced a low energy scale出 demon抑 atedby the self-energy shown in Fig. )
5.15.明白 energysca1e falls駒山 samerange as 3九九回-0.02e V and may be
related to the energy sca1e of spin fluctuations in YO.97SCO.03Mnz・
-‘
88
5.4 Summary
We have studied血eel民 tro凶cs加仰向。f~ and Y O.97Sc0.03Mn2' which are an
unusual antiferromagnet and a strongly enhenced par百nagn民間S戸叫vely，using x-ray
photoemission spectroscopy (XPS) 組 dhigh resolution叫回:violetphot<侃 :mission
spectroscopy (UPS). In the entire valence-band region， the e.ヰ到enmen阻1spectra at
different tempera旬開sdo not show appreciable difference e~白pt for a small change血
血evicinity of Ep・Onthe other hand，血es戸ctrashow smal1 but appreciable change wi血
Sc substitution伽.tis consistent with the antiferromagnetic ordering in ~・The UPS
spec凶創-ecompared wi血 theDOS derived from the band-structure ca1c叫ationand
agreement between血.eoryande可児rimentwas improved after ha吋ngcorrected the DOS
for self-energies. For the paramagnetic s飽te，ind偲 d，the UPS spectrum n伺 rEp could
be well問 producedusing the self-energy analysis. For the antiferromagnetic s阻胞，血e
self-energy correction cannot improve the agreement between血e血ωryand血e
expe出nentnear Ep・ ForY O.97ScO.03Mn2'血eself-energy correction is found.. to be
particularly strong near Ep in order to explain血.ephotoemission spec回 and血e
electronic specific heat consistent1y.
89
References
[5.1] T. Nakam.ichi， in Non-Stoichiometric Intermetallic Compounds (in Japanese )，
edited by S. Takeuchi ( Maruzen， Tokyo， 1975 ) p. 227.
[5.2] R. M∞ssner and J. T. Chalker， Phys. Rev. Lett. 80 2929 (1998).
[5.3] Y. N紘am町a，M. Shiga and S. Kawano， Physica B 120， 212 (1983).
[5.4] R. Ballou， J. Deportes， R. Lemaire， Y. Nakam町 aandB.0叫addiaf，J. Magn.
Magn. Mat. 70， 129 (1987).
[5.5] M. Shiga， H. Wada and Y. Nakamura， J. Magn. Magn. Mat. 31・34，119 (1983).
[5.6] H. Nakamura， H. Wada， K. Yoshimura， M. Shiga， Y. N北創n町 aゅJ.Sakura and Y.
Komura， J. Phys. F: Met. Phys. 18，981 (1988).
[5.7] G. Oom.i， T. Terao， M. Shiga組 dY. Nakamura， J. Magn. Magn. M剖.70，137
(1987).
[5.8] M. Shiga， H. Wa白， Y. Nakamura， J. Deportes， B. Ouladdiaf and K. R. A. Ziebeck，
J. Phys. Soc. Japan 57，3141 (1988).
[5.9] H. Wada， M. Shiga and Y. Nakamura， Physica B 161， 197 (1989).
[5.10] H. Yamada釦 dK. Terao， unpublised.
[5.11] H. Yamada， J. Inoue， K. Terao， S. Kanda and M. Shimizu， J. Phys. F: Met. Phys.
14， 1943 (1984).
[5.12] K. Terao and M. Shirnizu， Phys. Lett. 104A， 113 (1984).
[5.13] H. Yamada and M. Shirnizu， J. Phys. F: Met. Phys. 17，2249 (1987).
[5.14] J. J. Yeh and 1. Lindau， At. Data and Nucl. Data Tables 32， 1 (1985).
[5.15] S. Hufner， Photoelectron Spectroscopy (Springer-Verlag， Berlin， 1995).
[5.16] A. Koma， K. Yagi， M. Tsukada and M. Aono， Hyomen-Kagaku Nyumon (in
Japanese)， p.89 -91 (1994).
[5.17] T. Takahashi， Solid State Physcis (in Japanese) 29， 25 (1994).
[5.18] X. Li， Z. Zhang and V E. Henrich， J. Electron Spectrosc. Relat. Phenom. 63，253
(1993).
90
f、
[5.19] T. Susaki， T. Mizokawa， A. Fujimori， A. Ohno， T. Tonogai and H. Takagi， Phys.
Rev. B. 58， 1197 (1998).
[5.20] T. Saitoh， A. Sekiyama， T. Mizokawa， A. Fujimori， K. Ito， H. Nakamura and M.
Shiga， Solid State Commun. 95 307 (1995).
[5.21] T. Susaki， A. Sekiyama， K. Kobayashi， T. Mizokawa， A. Fujimori， M.
Tsunekawa， T. Muro， T. Matsushita， S. Suga， H. Ishii， T. Hanyu， A. Kimura， H.
Namatame， M. Taniguchi， T. Miyahar孔F.Iga， M. Kasaya and H. Harima， Phys.
Rev. Lett. 774269 (1996).
[5.22] N. E. Bickers， D. L. Cox and J. W. Wi1kins， Phys. Rev. Lett. 54 230 (1985).
91
Chapter 6.
Summary and Conclusion
In this thesis， we have studied the electro凶.cs回 C加問。fin胞nnetalliccompounds
FexCot_xSi wi血x= 0.8 and 0.5， MnSi and Yt_xScx~ 訓由 x = 0.0 and 0.03， using x-
ray photoemission sp印刷copy(XPS) and high reぬlution叫岡山letphotoemission
spectroscopy (UPS).
In the伺 seof FexCot_xSi 8I叫MnSi，we have at臨叩陪dωseparate血.espec回血旬
surfa回 andbulk com阿 mωbyconsidering血eelectron m伺 n-freepa血 and血e
thickness of the outennost aω，mic layer. We compared出eHe 1 UPS spectra for MnSi，
FeSi [3.13]， Feo.sCoO.2Si， Feo.sCoo.sSi and CoSi. With substitution of Fe for Co， the
S町uctureat-・0.8eV血 CoSiis shifted toward lower binding energies， qualitatively
consistent with the prediction of the rigid~band model. However， a supe叩ositionof血e
S戸記traof FeSi and CoSi better describes血ex -dependent behavior血anthe rigid-band
model， al出ough血esupe中ositionf:剖Isto describe details. On the other hand， we found a
W伺 ktemper細胞 dependencearound血.eFermi level in血espectra of Feo.SCoO.2Si and
Feo.sCoo.sSi. This probably reflects the reduction of the density of states below由e
Neel tempera加re，consistent wi血由.etransport properties.
In the case of MnSi， the photoemission spectra were compared with the density of
states given by band-structure calculation corrected for model self-energies and
reasonable agreement was obtained. This indicates由atboth band-structure and electron
correlation effects we陀 importantto understand the physical properties of MnSi. The
spectra in the entire valence-band region do not show appreciable temperature
dependence， but the spectral weight in the vicinity of the Fermi level was slightly
decreased with decreasing tempera加re，which we tentatively anribute to the exchange
splitting of the Mn 3d bands.
93
吟畠
In the case 'Of YM~ and YO.97SCO.03~' 血.e XPS and UPS s戸C回 takena加veand
bel'OW T N d'O n'Ot sh'OW appr配 iablediffe問nω，indica白19血剖由echange in血.eSpec回 I
DOS is negligibly srnall between出eantifeπ'Omagnetic and paramagnetic ph邸 es'Of the
same sample. H'Owever， the ph'Ot<侃rnissi'On 叩ec凶m 'Of~n伺r Ep sh'OW srnall
change by Sc substi加ti'On.百leph'Otoernissi'On s戸C回 werec'Ompared wi由也.es伊仙沼l
functi'Ons c'Orrected f'Or a mαlel self-energy. It is f'Ound血atel即 tr'Onc'Onちlati'Onin血e
Mn 3d states 'Of YM~ and Y O.97Sco.o3Mn2 plays important roles and es戸ciallyn伺 rEp，
it reflects the ge'Ometrical仕ustrati'One百ects'On spin fluctuati'Ons.
In 'Order t'O explain血.eelectro国cs加 C佃陀'OfMnSi， ~ and YO.97SCO.03M~， we
have applied model self-energy c'Orrecti'Ons t'O the band DOS. If electr'On c'Orrelati'On is
n'Ot negligible， 'Y 'Obtained fr'Om血eelectr'O凶cspecific heat is different fr'Om血atestima低d
合omthe band DOS 'Yb'百lerati'O between 'Y and 'Yb yields the mass enhancement fact'Or
mソ~・ h 血is self -energy analysis， we c'Onsidering the mass enhancement fact'Or which WTu，
is given by mソmb= 1+ ghig/仇ゅ +gIOW/Ylow' Here， ghigh' 'Yhigh' glow and 'Ylow were
trl回 tedas adjustable p訂ame胞rsand were deterrnined c'Onsidering the electr'Onic s戸cific
heat result. Such self-energy analysis discussed as sh'OW in Appendix.
It is c'Omm'On1y 'Observed由at，血 FexC'OI_XSi，MnSi and Yl_xScx~也~，出e en曲e
valence-band spectra f'Or the different magnetic phases d'O n'Ot sh'OW app児ciable
di百erencealth'Ough the spectra str'Ong1y change with c'Ompositi'On. In fact， such behavi'Ors
have indeed been widely 'Observed in仕組siti'On-me阻1c'Omp'Ounds. NiS exhibits a frrst-
'Order n'Onmetal-metal蜘 siti'On叫-260 K;血emetallic phase 伽 veTt sh'OWS Pauli
par百nagnetismand血.en'Onmetal phase bel'OW Tt sh'OWS antifeηomagnetism. Fujim'Ori et
al. [1] (Fig. 1) and Nakamura et al. [2] (Fig. 2) have studied NiS using XPS and UPS
and 'Observed血at血eexperimen阻lspec仕af'Orthe di百erentmagnetic phases d'O n'Ot sh'OW
appreciable di百erences'On釦 energyscale 'Of e V alth'Ough the band-structure calculati'Ons
sh'OW n'Otable differences between the different magnetic phases. Indeed， Fig. 2 sh'OWS
由atin the 300 K spectrum 'Of NiS，血eEF is located at出ecenter 'Of the leading edge. At
l'OW tempera佃res，h'Owever， the edge bec'Omes steeper and is shifted t'Owards higher
binding energies by -10 me V as sh'Own in Fig. 2， signaling the 'Opening 'Of a gap below
T t • ln the case 'Of C'OS2' which is a ferromagnetic metal bel'OW T c = 120 K and a
paramagnetic metal above it， and C'OS2_xSex'出etempera加redependent changes are even
smaller as sh'Own in Figs. 3 and 4 [3，4]， where the He II UPS s戸ctra'Of C'OS2 and血e
He 1 UPS spectra of C'O(SO.99SeO.0l)2 tak:en at LNT and RT， i.怠ムe
Curie t除emp戸erah佃1陀 assh'Own. Alth'Ough C'OS2 and C'O(SO.99SeO.Ol)2 are ferr'Omagnetic at
LNT and paramagnetic at RT，出eexperimental spectra for the di百erentmagnetic phases
d'O O'Ot sh'OW detectable differences 'On the energy scale of the ph'Otoemissi'On spectrum.
94
Als'O， 'One can see血atc'Omparis'On 'Of the spec釘ataken with di百erentlight s'Ources do n'Ot
sh'Ow any appreciable differences even if血emean-台関 pathchanges n'Otably betw田n
出.em.In the ca記'Of刊1rtz， f'Or example， the Neel tempera加問'Of1∞K c'Orresp'Onds t'O -
8 meV 'Or -3 kBTN c'Orresp'Onds t'O - 25 meV while the res'Oluti'On 'Of the He 1 UPS
measurements w出向 20meV at 16 K.官1eref'Ore，the fact白紙血.eintensity slightly
changes 'Only at ~ acr'Oss the magnetic phase回 nsiti'Onis fully c'Ompatible wi白血e
thermodynamic pr'Operties 'Of ~ even th'Ough it does n'Ot agree wi血血eband-
struc旬開 calculati'Ons.On the 'Other hand，血 general，ph'Otoemissi'On experiment is a
surface sensitive technique in its nature because 'Of the re1atively sh'Ort mean仕'eepath 'Of
ph'Otoelectr'Ons as sh'Own in Fig. 2.4. In血isthesis， in 'Order t'O dec'Omp'Ose the spectra
int'O the surface and b叫kc'On凶buti'Ons，we have凶edseveral methods: changing the
angle between the sample surface and the anal戸er釦cep凶 lce，sub回 C也19the He II
spectra fr'Om the He 1 s戸氾紅ヨ andusing XPS. Nevertheless the result again sh'Ows白紙
血.etempera佃redependence 'Of the spec回 areweak. RI氏自1tly，the discussi'On 'On the
surface and bulk sensitive ph'Otoemissi'On related t'O由euniversal curve has b民 nactively
陀吋ved[5]. It has been als'O seri'Ously asked about the validity 'Of血.eexis也19analysis.
τ'he weakness 'Of the temperature-dependence may be partly viewed as due t'O
c'Ontributi'Ons t'O the He 1 spec釘a合om血.esurface layer， but we c'Onclude出atthe weak
temperature-dependence 'Of the measured spectra is企omthe bulk 柑 uc加rebecau詑 ~
70% 'Of the He 1 spectra reflect the buIk elec甘onics釘uc加問.
Infu加re，it is highly desirable t'O carry 'Out m'Ore detailed temperature-dependent
ph'Otoemissi'On measurements by using better energy res'Oluti'On and in the angle-res'Olved
m'Ode of single crystal samples. Also， the phen'Omen'Ol'Ogical c'Onsistency between由e
由.ermodynar凶c，仕組sport，magnetic組 dspec住osc'Opicproperties presen旬dhere sh'Ould
be confirmed by microsc'Opic calculati'Ons.
References
[1] A. Fujim'Ori， K. Terakura， M. Taniguchi， S. Ogawa， S. Suga， M. Mat'Oba and S.
Anzai， Phys. Rev. B. 37，3109 (1988).
[2] M. Nakamura， A. Sekiyama， H. Namatame， H. Kin'O， A. Fujim'Ori， A. Misu， H.
Ikoma， M. Mat'Oba and S. Anzai， Phys. Rev. Lett. 73， 2891 (1994).
[3] K. Mamiya， Ph. D Thesis (University 'OfT'Oky'O， 1996)， Chap. 3.
[4] K. Mamiya， private c'Ommunication.
[5] A. Sekiyama， T. Iwas北i，K. Matsuda， Y. Sait'Oh， Y. Onuki and S. Suga， Nature. 430，
396 (2000).
95
H . . . . . . .'、. . ・'、 E
. ' . . ・..・・.・.・、・.・. . -・・.、.・.・.・1・'、，、. . . 、.‘伊・
1'11.1 64.V
BANO THEORY
NiS
PAAA
o 10 8 6 4 2
BINDING ENERGY (eV) 12
宣言
.est-ω音量
zogTao-oE
and
出e
Fig. 1. Valence-band photoemission Spectra of NiS in the paramagnetic
antiferromagnetic phases compared wi血血，etheoretical spectra calculated from
energy-band densities of states [1].
(ωZCコ.2」の)と一
ωC2c」
30K
ー0.5
21.2 e V
2.0 1.5 1.0 0.5 0.0
Binding Energy (eV)
ν
'h川
2.5
Fig. 2. Photoemission spectra of NiS [2].
96
CoS2 hv=40.8eV
(a) experiment 〆-、怠 | ロ 3 ∞ K (p仰ara削 ma凶a勾gn附e創叩li:5 I • ア川ferroma叫a勾申申帆仰gr伊仰n附附1沼削el叫t“i.D l-o
〈7工二 I (ωb防)LD臥A叫 ω叫 la創仙仰11ωlωO
..... 凶
ロ4) ‘・8
』
ロ paramagnelic・ferromagnelic
fA%。
/三
5 4 3 2
Bindig Energy (eV)
¥ 、
O
Fig. 3. (a) He 11 UPS spectra of ferromagnetic and paramagnetic CoS2・ (b)Spectra
derived from the LDA calculations for the ferromagnetic and paramagnetic states [3].
D a ロ
2
ヨb
12 10 8
CO(SO.99SeO.01h
hv=21.2eV
6 4
Binding Energy (eV)
2 O
Fig. 4. He 1 UPS spectra of ferromagnetic and paramagnetic CO(SO.99SeO.OI)2 [4].
97
Appendix.
In this thesis， the UPS spectra of MnSi， ~ and Y O.97SC0.Q3Mn2 were compared
with the DOS derived from the band-structure calculation but agreement between the
calculation and the experimental spectra is not satisfactory， sugges白19血at3d electrons
are strongly correlated. In order to explain the difference between the measured spectra
and the band DOS， spectral functions were calculated by applying a self-energy
correction to the band DOS and are comp訂edwi由 themeasured spectra. If we use a
model se恥 nergyof the form L.high(ω) = gωI (ω+ iy)2， which accounts for由eband
narrowing， the spectra1 weight transfers from low to higher binding energies白血e
photoemission spectra and the strong lifetime broadening occurs away from 1;，. Here， g
and y were住'eatedas adjustable parameters. Using this self-energy， the agreement
between血eoryand experiment were considerably improved for the spectra as shown
by the dot-dashed curves in Fig. A.l， except for YMn2・However，this self-energy
L.high( (β) has on1y small imaginary part near 1;， and thus cannot well reproduce the
experimental data as shown in Fig. A.l. Furthermore， the mass enhancement factor， for
example Y 0.97ScO.03Mn2' m牢1mb= 1 + ghig/Yhigh 2 -3.47， is found to be much smaller
血組曲eexperiment value m*/mb
_ 23 deduced from the elec汀onicspecific heat
measurement. Therefore， we employed a model self-energy which is a sum of a term
having a high-energy scale L.high(ω) and that having a low-energy scale L.¥ow(ffi) in order
to cause an additional broadening of出eDOS. In the case of M凶， I Imr.¥ow(ω) I
increases from _ 0 to _ー0.1e V in going from the band edge to _ -0.1 e V. Introduction
of such ImL¥ow(ω) necessarily cause a steep rise in ReL¥ow(ω) from the band edge to _ー
0.1 eV below it owing to出eKramers-Kronig relation， which leads to a strong mass
enhancement near the band edge. Here， we used L.(ω) = Lhigh(ω) + L¥ow(ω)， where
L.h噌tμ1】)was taken to have the same form as above
98
[L(ω) = gω/ (ω + i'Y)2] and L1ow(ω) = -glow[ 1/(ω+i'Ylow) + i/'Ylow]'百lemass
enhancement factor is thus given by m*/mb=l+gbi"lrhigh+gloJYlow' Here ghigb' 'Yhゆ'
glow and 'Ylow were悦 atedas adjustable p訂 ameters.ghigh and 'Yhigh were fixed at va1ues
obtained from the high-energy sca1e. glow and "f..ow were血endetermined so asωsatisちr
血.especific heat result. The spec回 1functions ca1culated using glow and 'Ylow創-eshown
by the solid curves in Fig. A.l. The above ReL1ow(ω) cause the sharpening of the edge，
and leading to a better agreement betw田n血eory.andexperiment as .shown by the solid
curves in Fig. A.l， a1though agr田 mentare still not perfect. U sing the self -energy
parameters ar芭 given血TableA.1. Such self-energy ana1ysis method has previously
been applied successfu11y to the FeSi as shown in Fig. A.2 [1].
calculated MEF* measured
self-energy parameter (us旬gparameter) MEF*
FeSi gh = 13 1h= 5 mりmb-1.52(a)
gl=O.∞お 1，=0.025 mりInb-5.52(b) m*/mb -6
恥fnSi gh = 8 1h = 3 m*/mb -1.88
(helimagnetic) gl = 0.01 11 = 0.044 m*/mb -7.1 m*/mb -7.1
MnSi &h= 8 γ，， =3 m*/mb -1.88
(paramag) gl = 0.02 11 =0.065 m‘1mb町 6.6 m*/mb -6.61
YO.97Sco.03Mn2 gh = 8.0 1h = 1.8 m*/mb -3.47
(para) gl = 0.005 11 = 0.016 m事1mb-23 m*/mb-23
Y勘色12 gh = 7.2 1h = 1.6 m*/mb -3.81
(antiferro) gl = 0.003 11 = 0.052 m*/mb -4.9 m*/mb -1
* MEF : mass enhancement factors
Table A.1. Self-energy p:町出neters:(a) Self-energy on the high energy sca1e L.high(ω)，
(b) Self叩 .ergyincluding two terms for血ehigh and low energy sca1es Lhigh(ω) +
L1ow(ω). Here， (a) and (b) correspond each samples in the third column.
99
antiferromagnetic
。 ー1.2 -0.8 同 0.4 。1.0
0.5 H.eL ¥、J }
/
-0.5
-1.0 0.2 -0.4 -0.2 。 0.2
kC
一ωcgc
helimagnetic
-1.2 -0.8 ・0.4 。
-0.2 。
KAH
一ωCOHZ
paramagnetic
-1.2 幽 0.8 ・0.4
-0.2 0.0
Y O.97SCO.03Mn2
(c)
paramagnetic
-1.2 -0.8 ・0.4 。
、TF。 0.2
YMn2
(d)
Energy relative to EF( e V)
Fig. A.1. Comparison of the UPS spectra (dots curves) for MnSi， YMn2 and
Yo・97SCO・03Mn2with the band DOS modified by the self-energy correction: used self-
energles are白紙 forthe high energy scale (dot-dashed curves)， and由atincluding two
tenns for the high and low energy scales (solid curves). Best fit self-energies are shown
below each spectrum.
1∞
FeSi
(a)
(b)
ドト同∞Z同ト
z- (c) (XO.l)
(d)
S ω
h
~ 0.0 E・0.1
凶 0.4 0.2 0.0 ・0.2BINDING ENERGY (eV)
Iml司ω)
0.2
0.1
Fig. A.2. Cornparison of the UPS spectrurn (dots curve) for FeSi with variously
broadened DOS (solid lines); (a): spectral function p(ω，) calculated using :E(ω) shown in
the lower panel， (b): p(ω，) calculated using on1y :Ehigh(ω)， (c): Fe partial DOS (dashed
(d): curve)， (solid curve) and its Gaussian (39 rneV FWHM) broadened curve
quasiparticle DOS.
Reference
[1] T. Saitoh， A. Sekiyarna， T. Mizokawa， A. Fujirnori， K. Ito， H. Nakarnura and M.
Shiga， Solid State Cornmun. 95， 307 (1995).
101
Acknowledgments
1 would like to take血isopportunity to express my sincere gratitude to my
supervisor， Prof. A. Fujimori for his continuous and invaluable guidance，
encouragement and kind help throughout the study. 1 am particularly indebted to
Prof. T. Mizokawa for his va1uable advice， suggestions， enthusiasm and kind help
during this course of study.
1 would like to thank Prof. T. Kanomata (Tohoku-Gakuin University) and Dr.
R. Note (Institute for Materia1s Research， Tohoku University) for preparing the
MnSi and FexCo1_xSi samples for this study and for many valuable comments and
discussions. 1 a1so would like to thank Prof. H. Wada and Prof. M. Shiga (Kyoto
University) for providing me with the s創nplesof the Y l_xScxMn2 compounds and
helpful advice. Also 1 would like to thank Prof. H. Yamada and Prof. K. Terao
(Shinshu University) for providing me with the results of the band司 structure
ca1culations on YMn2 and MnSi.
1 wish to thank former members of Fujimori group: Prof. H. Namatame
(Hiroshima University)， Dr. S， Nohara， Dr. K. Yamaguchi (Saitama University)，
Dr. M. Nakamura (Nara University of Education)， Dr. A. E. Bocquet， Dr. O. Radar
(BESSY， Germany)， Dr. K. Morikawa， Dr. K. Shimada (Hiroshima University)，
Mr. 1. Hase， Dr. A. Sekiyama (Osaka University)， Dr. K. Mamiya (Hiroshima
University)， Mr. K. Fujioka， Mr. T. Tsujioka，陥.M. Satake， Mr. Y. Ishikawa and
Prof. M. Okusawa (Gunma University) for their help and encouragements. 1 am
grateful to Dr. T. Saitoh (Photon Factory)， he was my tutor， for his va1uable advice
and suggestion during the study. 1 would like to thank Dr. T. Konishi (Chiba
University) for fruitful discussions especia11y on itinerant magnetism. 1 am a1so
grateful to Dr. K. Kobayashi (ISSP， University of Tokyo) and Dr. A. Ino (Spring-8，
JAERI)， both of my best friends since the entrance to University of Tokyo and a1so
members of Fujimori Group， for support， invaluable discussions and
encouragement ~bout not only physics but a1so genera1 topics and subjects.
Also 1 would like to thank由epresent members of Fujimori group: Mr. J.
Okabayashi， Mr. H. Ishii， Mr. T. Nambu， Mr. N. Harima， Miss H. Wakazono， K.
Tanaka， S. Nawai組 dY. Hitsuda for their support and encouragements.
103
• ，
1 acknowledge Dr. T. Susaki (RIKEN)， Dr. J. Okamoto and Mr. J. Matsuno for
enlightening discussions and teaching and helping during the data analysis of this
work. 1 thank Mr. T. Yoshida and Mr. K. Okazaki for their support and help during
the UPS measurements. 1 also thanks Dr. J. D. Lee for his valuable advice and
encouragement.
1 would like to thank: my family: my wife Mi-Jeong Kim， my daughters Hei-In
and Hei-Jin， my brother Bu-Y oung and my sister Eun-Y oung for their support and
their love.
Last but not least， 1血ankmy late p訂 ents:Y oung-Suk Son and Ok-E Sin.
104
