MgB2超伝導体の...
Transcript of MgB2超伝導体の...
MgB2超伝導体の
ピンニングセンター
鹿児島大学 土井俊哉
物質・材料研究機構 北口仁
九州大学 波多聰,ハリニ・ソシアティ
A part of this work is supported by Research Promotion Bureau, Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan, under the contract No.16-554, 17-213.
本研究のサマリー
MgB2超伝導体において、
(1) 結晶粒界、特に柱状に成長したMgB2結晶の粒界
(2) Mg欠損組成に起因して結晶内に導入された何らかの欠陥、
即ち、Mg欠損もしくはMgサイトへのB置換
(3) MgO
(4) 層状に挿入したNi層
は有効なピンニングセンターとして働く。
Sample preparations
Source(B)
Thickness sensor
Lamp heater
Source(Mg)
Substratec-Al2O3
Ultra high vacuum chamber
Gas inletT. P.
Background pressure 1.0 x 10-7 Pa
Substrate Sapphire (0001)
SourceMg : 99.9 %
B : 99.5 %
Substrate temperature 180-240
Evaporation rateMg : 1.0-2.0 nm/sB : 0.3-1.2 nm/s
MgB2 Growth rate 1-2 nm/s
Typical deposition time 5 minutes
Thickness 280-400 nm
Electron beam gun
R. P
First experiments
A MgB2 thin film preparedwithout substrate spinning.
Mg B
xy
z
+15°-15°
Columnar grain structureTEM: z-x plane view
15°
Results of TEM observations
x
yz
z
x
y
Boron fluxMagnesium flux
Boronflux
Magnesiumflux
y-z plane
BMg
Columnar grain structure reflecting the direction of boron flux
Specimen for Jc-θ measurement
z
x
y Specimen (a)
Cut out z
x
y
(b)(a)
zx
y
Specimen (b)
Effects of substrate spinning (1)In order to cancel (neutralize) the effect of tilted evaporation flux, we prepared another sample with spinning the substrate during the evaporation.
Boron flux
Substrate x
z
Magnesium flux
+15o-15o
3.25 rpm
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
-30 0 30 60 90 120
Rotation in z-x plane
θ [Magnetic field angle] (deg.)
4.2K(Liq. He), 5T
Normalized Jc vs. rotation angle
z
x
y
Current
zx
y
Current
c軸と磁場のなす角はどちらも15°↓
規格化Jcはどちらも1.7
z
x
y
Current
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
-30 0 30 60 90 120
Prepared with spinning the substrate
θ [Magnetic field angle] (deg.)
4.2K(Liq. He), 5T
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
-30 0 30 60 90 120
Rotation in y-z plane
θ (deg.)
4.2K(Liq. He), 5T
c軸と磁場は,どちらも平行↓
規格化Jcはどちらも2.1
Summery for the second experiment
MgB2薄膜の磁場中Jcは,磁場が柱状晶に平行に印加された
とき時,最も高くなる。
MgB2において,結晶粒界がピンニングセンターとして
有効に働いている
Current
Magnetic field Magnetic field Magnetic field
Properties of the MgB2 thin films prepared
at the same substrate temperatures (180, 210 )
with changing the composition.(Mg : B = 1 : 0.9 ~ 1 : 2.6)
Second experiments
Mg:B=1:2.6
1:2.2
1:1.5
1:2.11:1.8
0
50
100
150
200
250
300
20 25 30 35 40
Res
istiv
ity (μΩ・
cm)
Temperature (K)
Ts=210
Fig. 5 Temperature dependences of the resistivities of the MgB2 thin films prepared at 210 with the composition of Mg:B =1:1.5, 1:1.8, 1:2.1, 1:2.2, 1:2.6.
48 49 50 51 52 53 54 55 56
Inte
nsity
(a. u
.)
2θ (degrees)
1:1.81:1.5
1:2.21:2.1
1:2.6
Mg:B
(0002) from JCPDS
Fig. 6 X-ray diffraction spectra of the MgB2 thin films prepared at 210with the composition of Mg:B =1:1.5, 1:1.8, 1:2.1, 1:2.2, 1:2.6.
-6 -4 -2 0 2 4 6
Mg:B=1:1.5 (Tc=30 K)1:1.8
1:2.11:2.2
1:2.6 (Tc=24K)
-10
M (T
)15
-15
-5
10
5
0
-20
20
µ0H (T)
Ts : 210
Fig. 7 The magnetization hysteresis curves measured at 4.2 K for the MgB2 thin films prepared at 210 with the composition of Mg:B =1:1.5, 1:1.8, 1:2.1, 1:2.2, 1:2.6. The magnetic fields were applied perpendicular to the film surfaces.
Mg:B=1:0.91:1.8
1:1.91:2.2
1:2.4
-10
M (T
)15
-15
-5
10
5
0
-20
20
-6 -4 -2 0 2 4 6
Ts=180
µ0H (T)
Fig. 8 The magnetization hysteresis curves measured at 4.2 K for the MgB2 thin films prepared at 180 with the composition of Mg:B =1:0.9, 1:1.8, 1:1.9, 1:2.2, 1:2.4. The magnetic fields were applied perpendicular to the film surfaces.
Properties of the MgB2 thin films prepared in the existence of O2 gas.
Third experiments
After O2 Gas was introduced, the deposition started.basal pressure < 10-7 Pa
BMg
O2O2 Gas Ts=240
Mg B
0
20
40
60
80
100
20 25 30 35 40Temperature (K)
Res
istiv
ity (µΩ・cm
)
PO2= 1.3×10-5 Pa
3.5×10-5 Pa
1.0×10-7 Pa
Fig. 9 Temperature dependences of the resistivities of the MgB2 thin films prepared in the different oxygen partial pressure of < 1.0×10-7, 1.3×10-5 and 3.5×10-5 Pa.
Ts : 240
2
1.0×10-7 Pa
1.3×10-5 Pa
3.5×10-5 Pa
-6 -4 -2 0 4 6µ0H (T)
0
10
20
-10
-20
M(T
)
Fig. 12 The magnetization hysteresis curves measured at 4.2 K for the MgB2 thin films prepared in the different oxygen partial pressure of < 1.0×10-7, 1.3×10-5 and 3.5×10-5 Pa. The magnetic fields were applied perpendicular to the film surfaces.
Summery for the third experiment
The MgB2 film prepared in the appropriate PO2
have larger ∆M in the M-H curve.
Magnesium oxide may work as pinning centers
in MgB2 thin films
15nmMgB2
MgB2に Ni層を挿入した膜の作製方法
Al2O3基板
B Mg
同軸型真空アーク蒸着源
シャッター
Al2O3基板
メインシャッター
フィラメントフィラメント
基板加熱
(1) 15nm厚のMgB2層を形成(2) 0.3nm厚のNi層を形成
・・・・・以下,13回繰り返す。
0.3nmNi
200nm
15nmMgB2
作製した積層薄膜のX線回折測定結果
48 50 52 54 56
2θ[degree]
Intensity[arb. units]
Inte
nsity
[arb
. uni
ts]
2θ [degree]
48 50 52 54 56
MgB2(0002)
MgB2層+B層
MgB2薄膜
MgB2層+Ni層
作製した薄膜の抵抗率の温度依存性
MgB2層+B層
MgB2層のみ
MgB2層+Ni層
0
50
100
150
200
250
300
20 25 30 35 40
Temperature (K)
Resistivity (μΩ・cm)
103
104
105
106
0 2 4 6 8 10 12
MgB2/Ni/MgB2
Typical MgB2 EB-film
J c / A
cm-2
B / T
4.2K (Liq. He), B // Film Surface
Ni層を挿入したMgB2薄膜のJcの磁場依存性
連続的に作製したMgB2薄膜
要素ピン力の磁場依存性
0
1
2
3
0 2 4 6 8 10 12
MgB2/Ni/MgB
2
Typical MgB2 EB-film
F p / G
Nm
-3
B / T
4.2K (Liq. He), B // Film Surface
The peak position (9T) in the Fp-B relation of the MgB2/Ni alternately layered film corresponds to the flux lattice spacing of 14.1nm. It is very close to the designed Ni-layer spacing of 15nm.
Sapphire substrate
MgB2 layer15 nm thick
Flux line spacing matches Ni layer interval
14 nm
Flux line lattice in a superconductor when the magnetic field of 9T is applied parallel to the film surface
Ni layer0.3 nm thick
まとめ
(1) MgB2において,結晶粒界は有効なピンニングセンターとして働いている。
(2) Mg欠損組成がもたらす何らかの欠陥は,有効なピンニングセンターとして働いている。この欠陥は,Mgサイトの空孔,もしくはMgサイトへのB置換であると推測できる。
(3) 酸素ガス導入雰囲気で作製したMgB2薄膜中には,結晶粒界以外の等方的なピンニングセンターが存在している。このピンニングセンターは,MgOであると予想される。
(4) MgB2薄膜に挿入したNi層はピンニングセンターとして有効に働いている。