- Email: [email protected]
Effect of substitution on magnetic properties of CuGeO3
Journal of Magnetismand MagneticMaterials 140-144 (1995) 1691-1692
~
Journalof magnetism and magnetic materials
ELSEVIER
Effect of substitution on magnetic properties of CuGeO 3 M. Hase a,*, y . Sasago a, K. Uchinokura a, G. Kido b,c, T. Hamamoto b a Department of Applied Physics, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan b Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai-shi, Miyagi 980, Japan c National Research Institute for Metals, Tsukuba-shi, Ibaraki 305, Japan
Abstract We measured the magnetization of CUl_xMgxGeO3. A spin-Peierls-transitiontemperature and a critical field associated with a transition to a magnetic phase reduce up to x = 0.06. A spin-glass-like transition was not seen, although it was observed in Cu I_~ZnxGeO3. The effects of substitution of Mg are weaker than those of Zn.
A spin-Peierls (SP) transition was discovered in an inorganic compound CuGeO 3 [1]. Besides, effects of impurities in antiferromagnetic (AF) chains were studied in the SP system for the first time [2]. In CUl_xZnxGeO3, the value of the SP-transition temperature (Tsp) decreases rapidly with doping up to x = 0.02, and a spin-glass-like (SG-like) phase transition appears in the samples with 0.02 < x < 0.08 below 5 K. It is thought that the reduction of Tsp is due to disorder in AF chains, and that the destruction of the SP order leads to the occurrence of the SG-like transition. In this paper, we will report further studies of influence of substitution in the SP system. We made polycrystalline CUl_xMgxGeO3 with 0 < x < 0.09 by a solid-state-reaction method. The temperature (T) dependence of the magnetic susceptibility was measured by a SQUID magnetometer. The magnetic field ( H ) dependence of the magnetization was measured by a vibrating sample magnetometer in H up to 15 T induced by a water-cooled magnet at High Field Laboratory for Superconducting Materials, Institute for Materials Research, Tohoku University. In Fig. 1, we show the T dependence of the magnetic susceptibility per 1 mol Cu ions [ x ( T , x)] of CUl_xMgxGeO3 with 0 < x < 0.09 below 20 K measured in 0.01 T. The SP transition indicated by a drop of x ( T , x) was seen around 11-14 K for x < 0.06. The value
are not shown here. The magnitude of x ( T , x) increases upon doping below 50 K, while it is almost independent of x from 50 to 300 K. This weak x dependence of x ( T , x) above 50 K is due to a short AF correlation length and a thermal fluctuation at high T. In Fig. 2, the x dependence of Tsp [Tsr(X)] of Cul_xMgxGeO3 (open circle) is shown. In Cul_ x MgxGeO3, the value of Tsp(x)/Tsr(O) linearly decreases up to x = 0.06 and is expressed as 1 - a x ( a = 2.8). The SP transition was not observed for x > 0.07. This figure also includes the x dependences of Tsp (filled circle) and the SG-like-transition temperature (filled square) of CUl_xZnxGeO3 [2]. It is emphasized that effects of substi-
of Tsp defined as the onset temperature of the drop reduces with doping. On the contrary, x ( T , x)'s for x > 0.07 monotonically increase with decreasing temperature below 20 K and exhibit no phase transition. We also measured x ( T , x)'s from 20 to 300 K, although these data
4 t~
0304-8853/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0304-8853(94)00635-0
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• x=0
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.,0 6 * Correspondingauthor. Email: [email protected]; fax: +81-3-5689-8265.
I
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Cul_xMxGeO3
O
,
I
,
I
~
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5 10 15 Temperature (K)
,
20
Fig. 1. The magneticsusceptibilityof Cu1- xMgxGeO3.
M. Hase et aL /Journal of Magnetism and Magnetic Materials 140-144 (1995) 1691-1692
1692
tution of Mg are weaker than those of Zn. There are two significant differences between the phase diagram of CUl_xMg~GeO 3 and that of Cu l_xZnxGeO3 . First of all, the decrease of Tsp with doping in CUl_~MgxGeO 3 is weaker than that in CUl_xZnxGeO 3 (or = 13.7). In addition, the value of x up to which the SP transition was seen in Cul_xMgxGeO 3 ( x = 0 . 0 6 ) is larger than that in Cui_xZnxGeO 3 ( x = 0.02). Second, the SG-like transition does not occur down to 2 K in Cul_~MgxGeO3, while this transition was observed around 2 - 5 K in Cu I _xZnxGeO 3 with 0.02 < x < 0.08. We consider the absence of the SG-like transition in CUl_xMgxGeO 3. As was discussed in the study of CUl_xZn~GeO 3 [2], the destruction of the SP order leads to the development of AF correlation in chains, and the SG-like transition appears. The SP order in CUl_ x MgxGeO 3 remains more stable than that in CUl_xZn x GeO 3 for the same x, because the decrease of Tsp(X) by Mg doping is smaller than that by Zn doping. Thus, in CUl_xMgxGeO3, the A F correlation in chains does not develop enough to cause the SG-like transition. In addition to the T dependence of x(T, x), we measured the H dependence of the magnetization [ M ( H ) ] of Cul_xMg~GeO 3 with 0 < x < 0.08 at 4.2 K up to 15 T. Below x = 0.06, a characteristic change of M ( H ) was observed around 12 T and indicates a phase transition
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0.00 0.02 0.04 0.06 X Fig. (M
3. The = Mg
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in
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and Zn).
between dimerized and magnetic phases [3]. The critical field ( H c) related to this transition was defined as a field of a peak position in a d M ( H ) / d H curve. The x dependence of Hc of Cu l_xMxGeO3 (M = Mg and Zn [4]) at 4.2 K is shown in Fig. 3. In the data of CUl_xMgxGeO3, H~ decreases slightly upon doping. The value of H~ at increasing H is larger than that at decreasing H, which indicates a first-order transition at 4.2 K. The hysteresis was also observed in the data of CUl_xZn~GeO 3 at low T. It is noted that the reductions of H~ as well as Tsp upon doping in Cul_xMgxGeO 3 are weaker than those in Cu 1- • Zn xGeO3. Acknowledgements: We thank H. Obara and A. Sawa of Electrotechnical Laboratory, and T. Hirayama and K. Miyano of The University of Tokyo for collaborations.
References
n
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0.00
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oTsp Mg • Tsp Zn • TsG Zn
(13 I
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,
0.05
,
,
,
0.10
X
Fig. 2. The x dependences of Tsp and the SG-like-transition temperature (TsG) in CU~_xMxGeO3 (M = Mg and Zn).
[1] M. Hase, I. Terasaki and K. Uchinokura, Phys. Rev. Lett. 70 (1993) 3651. [2] M. Hase, I. Terasaki, Y. Sasago, K. Uchinokura and H. Obara, Phys. Rev. Lett. 71 (1993) 4059. [3] M. Hase, I. Terasaki, K. Uchinokura, M. Tokunaga, N. Miura and H. Obara, Phys. Rev. B 48 (1993) 9616. [4] M. Hase, I. Terasaki, Y. Sasago, K. Uchinokura, M. Toknnaga, N. Miura, G. Kido, T. Hamamoto and H. Obara, Physiea B. to be published.
~
Journalof magnetism and magnetic materials
ELSEVIER
Effect of substitution on magnetic properties of CuGeO 3 M. Hase a,*, y . Sasago a, K. Uchinokura a, G. Kido b,c, T. Hamamoto b a Department of Applied Physics, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan b Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai-shi, Miyagi 980, Japan c National Research Institute for Metals, Tsukuba-shi, Ibaraki 305, Japan
Abstract We measured the magnetization of CUl_xMgxGeO3. A spin-Peierls-transitiontemperature and a critical field associated with a transition to a magnetic phase reduce up to x = 0.06. A spin-glass-like transition was not seen, although it was observed in Cu I_~ZnxGeO3. The effects of substitution of Mg are weaker than those of Zn.
A spin-Peierls (SP) transition was discovered in an inorganic compound CuGeO 3 [1]. Besides, effects of impurities in antiferromagnetic (AF) chains were studied in the SP system for the first time [2]. In CUl_xZnxGeO3, the value of the SP-transition temperature (Tsp) decreases rapidly with doping up to x = 0.02, and a spin-glass-like (SG-like) phase transition appears in the samples with 0.02 < x < 0.08 below 5 K. It is thought that the reduction of Tsp is due to disorder in AF chains, and that the destruction of the SP order leads to the occurrence of the SG-like transition. In this paper, we will report further studies of influence of substitution in the SP system. We made polycrystalline CUl_xMgxGeO3 with 0 < x < 0.09 by a solid-state-reaction method. The temperature (T) dependence of the magnetic susceptibility was measured by a SQUID magnetometer. The magnetic field ( H ) dependence of the magnetization was measured by a vibrating sample magnetometer in H up to 15 T induced by a water-cooled magnet at High Field Laboratory for Superconducting Materials, Institute for Materials Research, Tohoku University. In Fig. 1, we show the T dependence of the magnetic susceptibility per 1 mol Cu ions [ x ( T , x)] of CUl_xMgxGeO3 with 0 < x < 0.09 below 20 K measured in 0.01 T. The SP transition indicated by a drop of x ( T , x) was seen around 11-14 K for x < 0.06. The value
are not shown here. The magnitude of x ( T , x) increases upon doping below 50 K, while it is almost independent of x from 50 to 300 K. This weak x dependence of x ( T , x) above 50 K is due to a short AF correlation length and a thermal fluctuation at high T. In Fig. 2, the x dependence of Tsp [Tsr(X)] of Cul_xMgxGeO3 (open circle) is shown. In Cul_ x MgxGeO3, the value of Tsp(x)/Tsr(O) linearly decreases up to x = 0.06 and is expressed as 1 - a x ( a = 2.8). The SP transition was not observed for x > 0.07. This figure also includes the x dependences of Tsp (filled circle) and the SG-like-transition temperature (filled square) of CUl_xZnxGeO3 [2]. It is emphasized that effects of substi-
of Tsp defined as the onset temperature of the drop reduces with doping. On the contrary, x ( T , x)'s for x > 0.07 monotonically increase with decreasing temperature below 20 K and exhibit no phase transition. We also measured x ( T , x)'s from 20 to 300 K, although these data
4 t~
0304-8853/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0304-8853(94)00635-0
_
I
M=Mg
• x=0
\ ~\
"
^\\
t~
o 0.05
0.01 ° 0.06
" 0.02
* 0.07
• 0.03
- o.o8
3
;~. i l
0.01 T
.,0 6 * Correspondingauthor. Email: [email protected]; fax: +81-3-5689-8265.
I
I
Cul_xMxGeO3
O
,
I
,
I
~
I
5 10 15 Temperature (K)
,
20
Fig. 1. The magneticsusceptibilityof Cu1- xMgxGeO3.
M. Hase et aL /Journal of Magnetism and Magnetic Materials 140-144 (1995) 1691-1692
1692
tution of Mg are weaker than those of Zn. There are two significant differences between the phase diagram of CUl_xMg~GeO 3 and that of Cu l_xZnxGeO3 . First of all, the decrease of Tsp with doping in CUl_~MgxGeO 3 is weaker than that in CUl_xZnxGeO 3 (or = 13.7). In addition, the value of x up to which the SP transition was seen in Cul_xMgxGeO 3 ( x = 0 . 0 6 ) is larger than that in Cui_xZnxGeO 3 ( x = 0.02). Second, the SG-like transition does not occur down to 2 K in Cul_~MgxGeO3, while this transition was observed around 2 - 5 K in Cu I _xZnxGeO 3 with 0.02 < x < 0.08. We consider the absence of the SG-like transition in CUl_xMgxGeO 3. As was discussed in the study of CUl_xZn~GeO 3 [2], the destruction of the SP order leads to the development of AF correlation in chains, and the SG-like transition appears. The SP order in CUl_ x MgxGeO 3 remains more stable than that in CUl_xZn x GeO 3 for the same x, because the decrease of Tsp(X) by Mg doping is smaller than that by Zn doping. Thus, in CUl_xMgxGeO3, the A F correlation in chains does not develop enough to cause the SG-like transition. In addition to the T dependence of x(T, x), we measured the H dependence of the magnetization [ M ( H ) ] of Cul_xMg~GeO 3 with 0 < x < 0.08 at 4.2 K up to 15 T. Below x = 0.06, a characteristic change of M ( H ) was observed around 12 T and indicates a phase transition
511
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,
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,
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tCUl_xMxGeO3 4.2 K 1 4 I M g : :up downZn: up down "
2
o a []
11
o []
o []
'
'
*
In
'
'
'
'
'
0.00 0.02 0.04 0.06 X Fig. (M
3. The = Mg
x
dependence
of
H c
at 4.2
K
in
Cul_xMxGeO
3
and Zn).
between dimerized and magnetic phases [3]. The critical field ( H c) related to this transition was defined as a field of a peak position in a d M ( H ) / d H curve. The x dependence of Hc of Cu l_xMxGeO3 (M = Mg and Zn [4]) at 4.2 K is shown in Fig. 3. In the data of CUl_xMgxGeO3, H~ decreases slightly upon doping. The value of H~ at increasing H is larger than that at decreasing H, which indicates a first-order transition at 4.2 K. The hysteresis was also observed in the data of CUl_xZn~GeO 3 at low T. It is noted that the reductions of H~ as well as Tsp upon doping in Cul_xMgxGeO 3 are weaker than those in Cu 1- • Zn xGeO3. Acknowledgements: We thank H. Obara and A. Sawa of Electrotechnical Laboratory, and T. Hirayama and K. Miyano of The University of Tokyo for collaborations.
References
n
•
0.00
'
oTsp Mg • Tsp Zn • TsG Zn
(13 I
'
" " I
,
0.05
,
,
,
0.10
X
Fig. 2. The x dependences of Tsp and the SG-like-transition temperature (TsG) in CU~_xMxGeO3 (M = Mg and Zn).
[1] M. Hase, I. Terasaki and K. Uchinokura, Phys. Rev. Lett. 70 (1993) 3651. [2] M. Hase, I. Terasaki, Y. Sasago, K. Uchinokura and H. Obara, Phys. Rev. Lett. 71 (1993) 4059. [3] M. Hase, I. Terasaki, K. Uchinokura, M. Tokunaga, N. Miura and H. Obara, Phys. Rev. B 48 (1993) 9616. [4] M. Hase, I. Terasaki, Y. Sasago, K. Uchinokura, M. Toknnaga, N. Miura, G. Kido, T. Hamamoto and H. Obara, Physiea B. to be published.