Magnetic properties of Nd 1/3 Sr 2/3 FeO 3 by neutron and Mössbauer spectroscopyYoung Rang Uhm and Chul Sung Kim Citation: Journal of Applied Physics 89, 7344 (2001); doi: 10.1063/1.1361265 View online: http://dx.doi.org/10.1063/1.1361265 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/89/11?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Crystal structure and magnetic properties of Bi0.8A0.2FeO3 (A = La, Ca, Sr, Ba) multiferroics using neutrondiffraction and Mossbauer spectroscopy AIP Advances 4, 087121 (2014); 10.1063/1.4893241 Variable pressure-temperature neutron diffraction of wüstite ( Fe 1 − x O ) : Absence of long-range magneticorder to 20 GPa Appl. Phys. Lett. 86, 052505 (2005); 10.1063/1.1852075 Neutron diffraction and Mössbauer studies on Fe 1−x Cr 2 S 4 (x=0.0,0.04,0.08) J. Appl. Phys. 91, 7935 (2002); 10.1063/1.1451884 Charge ordering and Mössbauer studies of single crystal R 1/3 Sr 2/3 FeO 3 (R=Pr, Sm, and Nd) J. Appl. Phys. 87, 4873 (2000); 10.1063/1.373187 Spin dynamics of lightly doped La 1−x Sr x MnO 3 (abstract) J. Appl. Phys. 81, 5494 (1997); 10.1063/1.364577

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Magnetic properties of Nd 1Õ3Sr2Õ3FeO3 by neutron and Mo¨ ssbauerspectroscopy

Young Rang Uhm and Chul Sung Kima)

Department of Physics, Kookmin University, Seoul 136-702, Korea

Charge transfer type Nd1/3Sr2/3FeO3 was synthesized and its magnetic properties were investigatedby neutron diffraction and Mo¨ssbauer spectroscopy at various temperatures ranging from 4.2 K toroom temperature. The powder neutron diffraction shows magnetic ordering structures. TheNd1/3Sr2/3FeO3 with small lattice distortion underwent a charge ordering~CO! phase transition atand belowTCO5163 K corresponding to a sequence of Fe13Fe13Fe15Fe13Fe13Fe15 aligned alongthez direction of rhombohedral, as well as canted antiferromagnetic spin ordering. The spin rotationresulting from helical spin of Nd was observed at and below 25 K. At 4.2 K, the electric quadrupolesplittings for Fe31 and Fe51 are 0.06 and 0.15~mm/s! which were changed to20.06 and20.02~mm/s! at and above 20 K. It is evidence of spin rotation in Nd1/3Sr2/3FeO3. © 2001 AmericanInstitute of Physics.@DOI: 10.1063/1.1361265#

I. INTRODUCTION

The charge ordering effect in charge transfer typeR1/3Sr2/3FeO3 ~R5La, Pr, and Nd! was investigated over thelast few years.1,2 This charge ordering~CO! sequence ofFe31Fe31Fe51Fe31Fe31Fe51 exists along the@111# directionof the pseudocubic perovskite structure.3 In this case, twokinds of Fe ions with valence states Fe31 and Fe51 werefound, with ratio of 2:1 below a Ne´el temperature (TN) of160 K62%. The present authors investigated the variationof iron charge transition, Fe41, Fe31, and Fe51 inR1/3Sr2/3FeO3 by Mossbauer spectroscopy.4 The colossalmagnetoresistance~CMR! manganese has a CO sequence atthe area of the antiferromagnetic insulator.5 Recently, severalresearchers have insisted that the existence of two Ne´el tem-peratures results from Nd31 coupling with Mn31 inNd2/3Sr1/3MnO3 at low temperature.6 In comparison withNd2/3Sr2/3MnO3, we were interested in the magnetic orderingin Nd1/3Sr2/3FeO3, and several other effects below 25 K.

The results of magnetization, magnetic hyperfine fields,and electric quadrupole splitting indicate the existence ofspin rotation in Nd1/3Sr2/3FeO3. The quadrupole splitting ofMossbauer spectra is the best piece of evidence about spinreorientation or spin rotation.7 In this work, we present sev-eral pieces of evidence relating to spin rotation of iron below25 K.

II. EXPERIMENTAL TECHNIQUE

The apparatus used for the crystal growth ofNd1/3Sr2/3FeO3 was a floating zone furnace.4 Magnetizationdata were taken using a commercial superconducting quan-tum interference device~SQUID! magnetometer~QuantumDesign Inc.!. A Mossbauer spectrometer of electromechani-cal type7 was used in the constant-acceleration mode.

III. RESULTS AND DISCUSSION

X-ray and neutron diffraction patterns of the samplesindicate a lattice constant with a rhombohedrally distortedperovskite structure with lattice constant ofa055.457 Å anda560.122°. Refinement was carried out in the space group

R3c by Rietvald analysis. Figures 1~a! and 1~b! show thetemperature dependence of the zero field cooled~ZFC! andfield cooled~FC! magnetization for Nd1/3Sr2/3FeO3 under ap-plied field of 100 Oe. The magnetization at 100 Oe showsvery different phases between ZFC and FC underTCO. Wededuced the spontaneous magnetization ordered state in aslightly distorted perovskite structure with an antiferromag-netic order of Fe31 – O–Fe31 and a ferromagnetic order ofFe31 – O–Fe51. The perovskite Nd1/3Sr2/3FeO3 system un-derwent a phase change from the paramagnetic state to theantiferromagnetic CO state atTCO(5TN). In the chargedriven transition,TN should be lower thanTCO. However, inthe charge–spin coupling driven and spin driven transition,TN should be same asTCO.8,9 The magnetization goes downto 0 emu/g below 25 K. Kajimotoet al. reported that thedecreases of magnetization reflection belowT;25 K comesfrom ordering of the spin of Nd31 ions in Nd1/3Sr2/3FeO3.

8

We detected the magnetic phase at low temperature byneutron high resolution powder diffraction. The arrows indi-cate a magnetic ordering phase. Neutron diffraction experi-ments were performed using a wavelength ofl51.8346 Åin KAERI, Korea. The magnetic structure has modulationvectors ofk1/65(1/6,1/6,1/6),k1/35(1/3,1/3,1/3), andk1/2

5(1/2,1/2,1/2).8 The modulation vector ofk1/3 is definedwell by electron diffraction.10,11 The modulation vectorsobey theQ dependence of the magnetic form factor of Feions.8,11

Using a least-squares computer program, two sets of sixLorentzian lines due to Fe31 and Fe51 were fitted to theMossbauer spectra of Nd1/3Sr2/3FeO3 from 4.2 to 90 K. Thereis a centered line of Fe41 from 105 to 150 K. The isomershiftsd of two subspectra revealed that one main componentwas Fe51 and the other Fe31. The one broad centered line

a!Author to whom correspondence should be addressed; electronic mail:cskim@phys.kookmin.ac.kr

JOURNAL OF APPLIED PHYSICS VOLUME 89, NUMBER 11 1 JUNE 2001

73440021-8979/2001/89(11)/7344/3/$18.00 © 2001 American Institute of Physics

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around 0 mm/s can be assigned to the tetravalent iron, whichreveals a low spin~LS! state.12 This is one piece of evidencethat the tetravalent iron coexists in Nd1/3Sr2/3FeO3 at andabove 105 K. The ratio of tetravalent iron increases from13% to 66% as the temperature rises from 105 K to the Ne´eltemperature. The centered line has a characteristic of inter-atomic d–d interaction, nominally Fe311Fe51

→Fe411Fe41, to which the hybridized O 2p states also con-tribute.

Mossbauer spectra are composed of two Lorentzians atand aboveTN ~Fig.2!. The isomer shifts,d, of the subspectraindicate that one is tetravalent and the other trivalent. Mo¨ss-bauer parameters and the ratio of three kinds of iron are

exhibited in Table I. The magnetic hyperfine fields ofNd1/3Sr2/3FeO3 have slowly varying values below the Ne´eltemperature; however, they suddenly disappeared at the Ne´eltemperature, as shown in Fig. 3~a!. This is one of the bestpieces of evidence for charge ordering and magnetic phasetransfer from an antiferromagnetic insulator to a paramag-netic metal.4 The magnetic hyperfine fields have lower valueat 4.2 K than those detected at 18 K. The magnetic hyperfinefield of 57Fe reflects the spin reorientation of iron. It is inde-pendent of the ordering of the spin of Nd31. The magnetichyperfine field at 4.2 K is evidence for spin rotation of ironresulting in the canting effect of coupling with Nd31.

The unusual effect exists in electric quadrupole splittingand magnetization measured by the SQUID magnetometer.The quadrupole splitting is given by

DEQ5 12e

2qQ~11 13h

2!1/2. ~1!

Here,h is the asymmetry parameter of the electric field gra-dient ~EFG! and eq is the electric field gradient along themaximum principal axis of EFG.Q is the nuclear quadrupolemoment of the first excited state of57Fe. The electric quad-rupole splitting reflects variation of the charge state of ironresulting from intersite Coulomb repulsion. The CO phase

FIG. 1. Temperature dependence of~a! the magnetization of zero fieldcooled and field cooled magnetization for Nd1/3Sr2/3FeO3 under applied fieldof 100 Oe and~b! neutron high resolution powder diffraction.

FIG. 2. Mossbauer spectra for Nd1/3Sr2/3FeO3 from 4.2 K to the Ne´el tem-perature.

7345J. Appl. Phys., Vol. 89, No. 11, 1 June 2001 Y. R. Uhm and C. S. Kim

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was detected below the Ne´el temperature for R5Nd. Theelectric quadrupole splitting with 0.06, 0.15 mm/s at 4.2 Kand with20.06 and20.02 mm/s at 20 K as so that this large

difference in electric quadrupole splitting is evidence forspin rotation of irons resulting in the canting effect of cou-pling with Nd31 below 25 K as shown in Fig. 3~b!. Theelectric quadrupole splitting is changed again at 120 K; how-ever it does not mean spin rotation. We detected similar ef-fects in other slightly distorted perovskites of R1/3Sr2/3FeO3 ~R5Pr and Sm!.4 These varying values indicate a charge tran-sition state corresponding to the creation of tetravalent ironwith a LS state (3T1g).

Ordering of the spin of Nd31 ions should be one originof the decreases of magnetization below 20 K. However, it isnot enough to explain the decrease of the magnetic hyperfinefield of 57Fe at 4.2 K. The magnetic hyperfine field of57Fe isindependent of the ordering of the spin of Nd31. The de-creases of magnetic hyperfine field of57Fe at 4.2 K should beexplained by a change in direction of the spin of iron. In thisstudy, several pieces of evidence of spin rotation are shownby analysis of Mo¨ssbauer spectra. In future studies, we willclarify spin coupling of Nd31 and iron ions below 25 K.

ACKNOWLEDGMENTS

This work was supported by the Korea Science and En-gineering Foundation~97-0702-0401-5! and by the BrainKorea 21 Program of the Ministry of Education.

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TABLE I. Mossbauer parameters for Nd1/3Sr2/3FeO3, whereHhf ~kOe! is the magnetic hyperfine field,DEQ

~mm/s! is the electric quadrupole split andd ~mm/s! is the isomer shift.

T~K!

Hhf ~kOe! DEQ ~mm/s! d ~mm/s! Ratio ~%!

Fe31 Fe51 Fe31 Fe51 Fe31 Fe51 Fe41 Fe31 Fe51 Fe41

4.2 467 243 0.06 0.15 0.19 20.03 ¯ 67 33 ¯

20 468 267 20.06 20.02 0.24 20.12 ¯ 66 34 ¯

60 445 258 20.07 20.02 0.26 20.10 ¯ 65 35 ¯

105 423 246 20.05 20.03 0.25 20.10 0.06 62 29 9120 410 235 20.02 20.04 0.25 20.10 0.08 61 24 15150 385 236 20.03 20.03 0.22 20.11 20.02 33 15 52160 0.31 ¯ 20.05 33 ¯ 67295 0.30 ¯ 20.06 34 ¯ 66

FIG. 3. ~a! Temperature dependence of magnetic hyperfine fields from 4.2K to the Neel temperature and~b! electric quadrupole splitting (DEQ) from4.2 to 150 K for Nd1/3Sr2/3FeO3.

7346 J. Appl. Phys., Vol. 89, No. 11, 1 June 2001 Y. R. Uhm and C. S. Kim

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