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Anomalous magnetism of the frustrated compound GdInCu4
PHYSIGAI
Physica B 186-188 (1993) 633-635 North-Holland
Anomalous magnetism of the frustrated compound GdlnCu 4 H. Nakamura, K. Ito, H. Wada and M. Shiga Department of Metal Science and Technology, Kyoto University, Japan The magnetic susceptibility, high-field magnetization, low-temperature specific heat measurements for GdlnCu4 have revealed that its antiferromagnetic ordering is depressed down to a very low temperature of 6.9 K in spite of the large antiferromagnetic correlation between Gd moments, and magnetic correlation persists at temperatures higher than Ts. The temperature dependence of electrical resistivity has a broad maximum around 80 K as well as a sharp peak at TN. The concept of magnetic frustration is applied to explain such anomalous properties of GdlnCu4.
1. Introduction
2. Experimental procedures
The effect of frustration on magnetism has been extensively studied both experimentally and theoretically, especially for low-dimensional ionic crystals. It is well known that for three-dimensional systems, Heisenberg spins with three degrees of freedom may frustrate on certain crystal lattices. The face-centered cubic (FCC) lattice is the simplest example in which nearest-neighbor antiferromagnetic interaction frustrates. Among rare earth systems with FCC symmetry, we have found GdlnCu 4 a typical substance for investigating the effect of frustration. Since Gd has no orbital moment, the effect of frustration can be discussed without the complexity of the crystal field effect. Takegahara and Kasuya [1] calculated the band structure of LulnCu 4 and predicted that the substance is a compensated semimetal with a low carrier density. A similar electronic structure is thus expected for the RInCu 4 (R = rare earth) system. The low carrier density may cause the long-range RKKY interaction to be small and, as a result, some other short-range interactions to dominate the magnetism. In this paper we report on the magnetic susceptibility, high-field magnetization and low-temperature specific heat of GdlnCu 4 and discuss the role of frustration. The anomalous feature of electrical resistivity is also presented.
The sample of GdlnCu 4 was prepared by an argon arc-melting followed by annealing at 750°C in an evacuated quart tube for one week. The purity of Gd, In and Cu was 3N, 4N and 5N, respectively. X-ray powder diffraction measurements revealed that the crystal structure is C15b type and the lattice constant at room temperature is 7.235 ~ . The magnetic susceptibility was measured using a magnetic torsion balance between 4.2 and 300 K. The high-field magnetization was measured using a pulse magnet at ISSP, the University of Tokyo, up to 34 T at 4.2 K. The specific heat was measured by a heat-pulse method between 1.5 and 20 K. The electrical resistivity was measured using four-probe method between 2.8 and 1000 K.
Correspondence to: H. Nakamura, Department of Metal Science and Technology, Kyoto University, Kyoto 606-01, Japan.
3. Results and discussion Figure 1 shows the temperature dependence of magnetic susceptibility and inverse susceptibility of GdlnCu4 measured in a field of 8.28 kOe. It follows the Curie-Weiss law. The effective Bohr magneton number is estimated to be 8.15/xB/Gd, in reasonable agreement with the theoretical value for Gd 3 + ,7.9/z B. The paramagnetic Curie temperature, 0o, is estimated to be - 4 5 K, indicating a large antiferromagnetic interaction. These values agree well with those reported by Abe et al. [2]. Although a clear anomaly was not observed down to 4.2 K, the susceptibility deviates slightly from the Curie-Weiss behavior below 7 K, which will be shown below to be due to magnetic
0921-4526/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
634
H. Nakamura et al. / Anomalous magnetism o f GdlnCu 4
,
t 3°
GdlnCu4
a~
15
' ' f 1 ' ' ' 1 ' 1 ' 1 ' 1 1 1
°oo°°°°°° °°=°~°
ooloo .... .........i......
1
10 x "
:
0
° /
o 5
0 0
100
200
0
300
Fig. 1. Temperature dependence of susceptibility (triangles) and its inverse (circles) for GdlnCu 4 measured in a field of 8.28 kOe. ordering below 6.9 K by specific heat and electrical resistivity measurements. The magnetization curve at 4.2 K measured using a pulse magnet is shown in fig. 2. With incresing field, the magnetization increases monotonously and gradually approaches 7/zB of the G d full moment. The low-temperature specific heat of GdlnCu 4 is presented in fig. 3. A clear A-type peak was observed at 6.9 K. Since there is no spontaneous magnetization at 4.2K, the transition is considered to be antiferromagnetic (in a broad sense). In order to analyze the magnetic contribution, it is necessary to subtract the lattice term from the experimental result. Preliminary calculations using the data of the isostructural compound LuAgCu 4 [3] showed that the magnetic entropy reaches only about 60% of R ln(2J + 1) (J = ) just above TN. This indicates that the short-range magnetic correlation persists at temperatures higher than T N.
. . . .
6
I
. . . .
I
GdlnCu4 T = 4.2K
5
'
'
'
'
I
'
'
'
O 4
2
,
4
,
I
,
,
,
[
,
,
~
8 12 Temperature (K)
I
,
J
16
20
Fig. 3. Temperature dependence of specific heat for GdlnCu 4. These data can be explained in terms of magnetic frustration. The effect of frustration in antiferromagnets is roughly measured by the ratio IOo]/TN, which rarely exceeds 10 even for geometrically frustrated insulators such as triangular lattices [4]. For metallic compounds containing Gd, the largest value known is 5.3 for GdzZn17 (=53/10 K) and GdAI 3 (=90/17 K) in 50 , ....
420 0 ....
1O0 , ....
150 , ....
200
410
400 %
390
GdlnCu4
"%"
38O
E
~
370 420
170 IO0
N"
10
,
,
,
~
i
I
=
i
i
i
I .
~ .
, .
,
,
. . . .
I
.
220
.~
0
I
270
.e"
1
,
320
/,~,-"
/
3
,
Q" 370
'
fF
"13
,
\ ,...~.,j'
0
Temperature (K)
v
GdlnCu4
P
/*
~10
°"°~,
''1
20
30
40
Fig. 2. Magnetization curve of GdlnCu 4 at 4.2 K.
,
,
Temperature (K)
i
J
,
,
1000
Fig. 4. Temperature dependence of electrical restivity for GdlnCu 4. (a) Low-temperature part between 2.8 and 2130K; (b) high-temperature part between 100 and 1000 K (semi-log scale).
H. Nakamura et al. / Anomalous magnetism o f G d l n C u 4
which the frustration of antiferromagnetic interaction was suggested [5,6]. The ratio for GdInCu4 is estimated to be 6.5, exceeding those of Gd2Zn17 and G d A i 3. Hence GdInCu 4 is a typical substance in which the frustration dominates the magnetic properties, i.e., the frustration depresses the static magnetic ordering down to a very low temperature in spite of the large antiferromagnetic interaction between Gd spins. As a result, dynamic correlation persists up to high temperatures, as was shown by the analysis of specific heat. Although we have no information on the magnetic structure, a complicated structure may be realized due to the frustration. Figure 4 shows the temperature dependence of electrical resistivity of GdInCu4. The sharp peak observed at 6 . 9 K indicates clearly the magnetic transition. Striking features are a broad maximum around 80 K and the negative temperature dependence above 80 K. It should be noted that above 200 K it follows almost - l n T temperature dependence up to the highest temperature. A t present, even the origin of the broad maximum is not yet clear. A feasible explanation is that it may be ascribed to the temperature-dependent conduction character originating in specific band struc-
635
ture near the Fermi level as the nature of compensated semimetals. However, further information on the conduction band structure is necessary before we can discuss the transport properties. A t present, the study of a pseudo-binary system ( G d - L u ) I n C u 4 and other RInCu 4 compounds are in progress. The authors thank Professor T. Goto and Dr. H.A. Katori for their kind cooperation in the high-field magnetization measurements.
References
[1] K. Takegahara and T. Kasuya, J. Phys. Soc. Jpn. 59 (1990) 3299. [2] S. Abe, Y. Atsumi, T. l~aneko and H. Yoshida, J. Magn. Magn. Mater. 104-107 (1992) 1397. [3] M.J. Besnus, P. Haen, N. Hamdaoui, A. Herr and A. Meyer, Physica B 163 (1990) 571. [4] A.P. Ramirez, J. Appl. Phys. 70 (1991) 5952. [5] R.H. Taylor and B.R. Coles, J. Phys. F 5 (1975) 121. [6] B.R. Coles, S. Oseroff and Z. Fisk, J. Phys. F 17 (1987) L169.
Physica B 186-188 (1993) 633-635 North-Holland
Anomalous magnetism of the frustrated compound GdlnCu 4 H. Nakamura, K. Ito, H. Wada and M. Shiga Department of Metal Science and Technology, Kyoto University, Japan The magnetic susceptibility, high-field magnetization, low-temperature specific heat measurements for GdlnCu4 have revealed that its antiferromagnetic ordering is depressed down to a very low temperature of 6.9 K in spite of the large antiferromagnetic correlation between Gd moments, and magnetic correlation persists at temperatures higher than Ts. The temperature dependence of electrical resistivity has a broad maximum around 80 K as well as a sharp peak at TN. The concept of magnetic frustration is applied to explain such anomalous properties of GdlnCu4.
1. Introduction
2. Experimental procedures
The effect of frustration on magnetism has been extensively studied both experimentally and theoretically, especially for low-dimensional ionic crystals. It is well known that for three-dimensional systems, Heisenberg spins with three degrees of freedom may frustrate on certain crystal lattices. The face-centered cubic (FCC) lattice is the simplest example in which nearest-neighbor antiferromagnetic interaction frustrates. Among rare earth systems with FCC symmetry, we have found GdlnCu 4 a typical substance for investigating the effect of frustration. Since Gd has no orbital moment, the effect of frustration can be discussed without the complexity of the crystal field effect. Takegahara and Kasuya [1] calculated the band structure of LulnCu 4 and predicted that the substance is a compensated semimetal with a low carrier density. A similar electronic structure is thus expected for the RInCu 4 (R = rare earth) system. The low carrier density may cause the long-range RKKY interaction to be small and, as a result, some other short-range interactions to dominate the magnetism. In this paper we report on the magnetic susceptibility, high-field magnetization and low-temperature specific heat of GdlnCu 4 and discuss the role of frustration. The anomalous feature of electrical resistivity is also presented.
The sample of GdlnCu 4 was prepared by an argon arc-melting followed by annealing at 750°C in an evacuated quart tube for one week. The purity of Gd, In and Cu was 3N, 4N and 5N, respectively. X-ray powder diffraction measurements revealed that the crystal structure is C15b type and the lattice constant at room temperature is 7.235 ~ . The magnetic susceptibility was measured using a magnetic torsion balance between 4.2 and 300 K. The high-field magnetization was measured using a pulse magnet at ISSP, the University of Tokyo, up to 34 T at 4.2 K. The specific heat was measured by a heat-pulse method between 1.5 and 20 K. The electrical resistivity was measured using four-probe method between 2.8 and 1000 K.
Correspondence to: H. Nakamura, Department of Metal Science and Technology, Kyoto University, Kyoto 606-01, Japan.
3. Results and discussion Figure 1 shows the temperature dependence of magnetic susceptibility and inverse susceptibility of GdlnCu4 measured in a field of 8.28 kOe. It follows the Curie-Weiss law. The effective Bohr magneton number is estimated to be 8.15/xB/Gd, in reasonable agreement with the theoretical value for Gd 3 + ,7.9/z B. The paramagnetic Curie temperature, 0o, is estimated to be - 4 5 K, indicating a large antiferromagnetic interaction. These values agree well with those reported by Abe et al. [2]. Although a clear anomaly was not observed down to 4.2 K, the susceptibility deviates slightly from the Curie-Weiss behavior below 7 K, which will be shown below to be due to magnetic
0921-4526/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
634
H. Nakamura et al. / Anomalous magnetism o f GdlnCu 4
,
t 3°
GdlnCu4
a~
15
' ' f 1 ' ' ' 1 ' 1 ' 1 ' 1 1 1
°oo°°°°°° °°=°~°
ooloo .... .........i......
1
10 x "
:
0
° /
o 5
0 0
100
200
0
300
Fig. 1. Temperature dependence of susceptibility (triangles) and its inverse (circles) for GdlnCu 4 measured in a field of 8.28 kOe. ordering below 6.9 K by specific heat and electrical resistivity measurements. The magnetization curve at 4.2 K measured using a pulse magnet is shown in fig. 2. With incresing field, the magnetization increases monotonously and gradually approaches 7/zB of the G d full moment. The low-temperature specific heat of GdlnCu 4 is presented in fig. 3. A clear A-type peak was observed at 6.9 K. Since there is no spontaneous magnetization at 4.2K, the transition is considered to be antiferromagnetic (in a broad sense). In order to analyze the magnetic contribution, it is necessary to subtract the lattice term from the experimental result. Preliminary calculations using the data of the isostructural compound LuAgCu 4 [3] showed that the magnetic entropy reaches only about 60% of R ln(2J + 1) (J = ) just above TN. This indicates that the short-range magnetic correlation persists at temperatures higher than T N.
. . . .
6
I
. . . .
I
GdlnCu4 T = 4.2K
5
'
'
'
'
I
'
'
'
O 4
2
,
4
,
I
,
,
,
[
,
,
~
8 12 Temperature (K)
I
,
J
16
20
Fig. 3. Temperature dependence of specific heat for GdlnCu 4. These data can be explained in terms of magnetic frustration. The effect of frustration in antiferromagnets is roughly measured by the ratio IOo]/TN, which rarely exceeds 10 even for geometrically frustrated insulators such as triangular lattices [4]. For metallic compounds containing Gd, the largest value known is 5.3 for GdzZn17 (=53/10 K) and GdAI 3 (=90/17 K) in 50 , ....
420 0 ....
1O0 , ....
150 , ....
200
410
400 %
390
GdlnCu4
"%"
38O
E
~
370 420
170 IO0
N"
10
,
,
,
~
i
I
=
i
i
i
I .
~ .
, .
,
,
. . . .
I
.
220
.~
0
I
270
.e"
1
,
320
/,~,-"
/
3
,
Q" 370
'
fF
"13
,
\ ,...~.,j'
0
Temperature (K)
v
GdlnCu4
P
/*
~10
°"°~,
''1
20
30
40
Fig. 2. Magnetization curve of GdlnCu 4 at 4.2 K.
,
,
Temperature (K)
i
J
,
,
1000
Fig. 4. Temperature dependence of electrical restivity for GdlnCu 4. (a) Low-temperature part between 2.8 and 2130K; (b) high-temperature part between 100 and 1000 K (semi-log scale).
H. Nakamura et al. / Anomalous magnetism o f G d l n C u 4
which the frustration of antiferromagnetic interaction was suggested [5,6]. The ratio for GdInCu4 is estimated to be 6.5, exceeding those of Gd2Zn17 and G d A i 3. Hence GdInCu 4 is a typical substance in which the frustration dominates the magnetic properties, i.e., the frustration depresses the static magnetic ordering down to a very low temperature in spite of the large antiferromagnetic interaction between Gd spins. As a result, dynamic correlation persists up to high temperatures, as was shown by the analysis of specific heat. Although we have no information on the magnetic structure, a complicated structure may be realized due to the frustration. Figure 4 shows the temperature dependence of electrical resistivity of GdInCu4. The sharp peak observed at 6 . 9 K indicates clearly the magnetic transition. Striking features are a broad maximum around 80 K and the negative temperature dependence above 80 K. It should be noted that above 200 K it follows almost - l n T temperature dependence up to the highest temperature. A t present, even the origin of the broad maximum is not yet clear. A feasible explanation is that it may be ascribed to the temperature-dependent conduction character originating in specific band struc-
635
ture near the Fermi level as the nature of compensated semimetals. However, further information on the conduction band structure is necessary before we can discuss the transport properties. A t present, the study of a pseudo-binary system ( G d - L u ) I n C u 4 and other RInCu 4 compounds are in progress. The authors thank Professor T. Goto and Dr. H.A. Katori for their kind cooperation in the high-field magnetization measurements.
References
[1] K. Takegahara and T. Kasuya, J. Phys. Soc. Jpn. 59 (1990) 3299. [2] S. Abe, Y. Atsumi, T. l~aneko and H. Yoshida, J. Magn. Magn. Mater. 104-107 (1992) 1397. [3] M.J. Besnus, P. Haen, N. Hamdaoui, A. Herr and A. Meyer, Physica B 163 (1990) 571. [4] A.P. Ramirez, J. Appl. Phys. 70 (1991) 5952. [5] R.H. Taylor and B.R. Coles, J. Phys. F 5 (1975) 121. [6] B.R. Coles, S. Oseroff and Z. Fisk, J. Phys. F 17 (1987) L169.