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Study on CuGe0.9885Si0.0115O3 in high magnetic fields by specific heat measurements
Physica B 281&282 (2000) 669}670
Study on CuGe Si O in high magnetic "elds 0.9885 0.0115 3 by speci"c heat measurements Masahiko Hiroi!,*, Masafumi Sera!,1, Norio Kobayashi!, Jun Akimitsu" !Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan "Department of Physics, Aoyama-Gakuin University, Tokyo 157-8572, Japan
Abstract Speci"c heat is measured for a single crystal of CuGe Si O (x"0.0115) in magnetic "elds up to 14.5 T. In zero "eld 1~x x 3 two peaks at &3 and &10 K were observed, suggesting that the coexistence of the antiferromagnetic and spin-Peierls orders are realized. In our previous measurements of speci"c heat in high-"eld region around 15 T a small but clear anomaly was observed at &4.2 K for x"0.01, while a sharp peak was observed at &5 K for x"0.015, which corresponds to the transition in the magnetic-"eld-induced phases between M phase and M@ phase. They appear very di!erent from each other. In the present study for x"0.0115 we observe a clear peak at 14.5 T which is just intermediate between those for x"0.01 and 0.015 in temperature and in size. This systematic change in the transition with varying x shows that these transitions in the high-"eld phases are of the same kind in this range of x, though it also shows that small change in Si concentration a!ects the magnetic-"eld-induced states to a large extent. ( 2000 Elsevier Science B.V. All rights reserved. Keywords: Spin-Peierls transition; CuGeO ; Impurity e!ect; Magnetic "eld 3
The discovery of spin-Peierls (SP) transition in an inorganic material, CuGeO [1] has promoted the stud3 ies on impurity e!ects on this ordered state. These led to the surprising discovery of the coexistence of the SP order and an antiferromagnetic (AF) order, which had been considered to be mutually exclusive [2]. Impurity e!ects on the magnetic phase diagram in CuGeO have 3 been studied [3]. We have revealed that in addition to D (dimerized) and M (high "eld magnetic) phases which exist in the undoped material, the D#AF (coexisting phase of the dimerized and AF ordered states) and M@ phase (high-"eld phase where the M and AF states likely
* Corresponding author. Present address: Department of Physics, Faculty of Science, Kagoshima University, Kagoshima 890-0065, Japan. Fax: #81-99-285-8081. E-mail address: [email protected] (M. Hiroi) 1 Present address. Department of Quantum Matter, Graduate School of Advanced Sciences of Matter, Hiroshima University, Higashi-Hiroshima 739-8526, Japan.
coexist) exist from the measurements of speci"c heat, C, thermal expansion and magnetostriction [4}6]. Our results are basically in agreement with other measurements. However, the nature of M@ is not clear at present. Looking at the C in high-"eld region around 15 T, for x"0.01 a small but sharp anomaly appears at &4.2 K, which indicates a transition from M@ to M phase [4]. On the other hand, a sharp peak is observed for x"0.015 at &5 K, which resembles that observed for x"0.02 where only an AF transition occurs. Because of the large di!erence in the appearance of the anomalies at these transitions, it was not obvious whether the anomaly changes continuously with varying x between 0.01 and 0.015 and whether the natures of the transition for x"0.01 and 0.015 are of the same kind or not. In this paper we report the results for speci"c heat measurements for x"0.0115 in between 0.01 and 0.015, particularly focusing on the transition between M and M@. A single crystal of CuGe Si O was pre0.9885 0.0115 3 pared by the #oating zone method. Speci"c heat was measured with a usual adiabatic heat pulse method in magnetic "eld up to 14.5 T.
0921-4526/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 0 8 9 5 - 9
670
M. Hiroi et al. / Physica B 281&282 (2000) 669}670
Fig. 2. The speci"c heat of CuGe Si O (x"0.01, 0.0115, 1~x x 3 0.015, and 0.02) in high magnetic "eld around 15 T parallel to the a-axis in the form of C/¹-¹. The origins are shifted for clarity. Fig. 1. Temperature dependence of the speci"c heat of CuGe Si O in magnetic "eld parallel to the a-axis in 0.9885 0.0115 3 the form of C/¹-¹. The origin of the vertical axis is shifted by 40 mJ/mol K2.
In Fig. 1 temperature (¹) dependence of the C of CuGe Si O in magnetic "eld up to 14.5 T 0.9885 0.0115 3 parallel to a-axis is shown in the form of C/¹-¹. At 0 T rather broad but clear peaks are observed at ¹ &3 K N and ¹ &10 K, corresponding to the AF and the SP SP transitions, respectively. In this compound also the coexistence of the SP and the AF orders is realized. The appearance of the ¹ dependence of the C for x"0.0115 is just intermediate between those for x"0.01 and 0.015. For x"0.0115 the peaks are clearer than those for x"0.015, where the peaks are very broad. With increasing magnetic "eld the peak at SP transition becomes broader and ¹ becomes lower, while the peak at SP ¹ becomes smaller and ¹ shifts to higher temperN N atures with increasing "eld. The peak at ¹ becomes N small and broad at 9 T. On further increasing magnetic "eld from 9 to 12 T a fairly sudden increase of ¹ is N observed. Above &12 T the peak at ¹ becomes sharp N and large. At the same time the anomaly at ¹ becomes SP so broad that it is rather di$cult to know precisely where ¹ is. This kind of behavior of the C with varying SP magnetic "eld was also observed for x"0.01 [4]. In Fig. 2 C for x"0.01, 0.0115, 0.015, and 0.02 in the high"eld region around 15 T are shown in the form of C/¹-¹. For x"0.02 only one peak indicating an AF transition is observed, and the SP order does not exist in this sample. This AF transition is hardly a!ected by magnetic "eld up to 15 T [4]. With x increasing from 0.01 to 0.015, where the coexistence of the SP and the AF states is observed,
the peak at ¹ changes systematically in temperature N and in sharpness and size. Judging from this systematic change, the transitions at ¹ in the magnetic-"eld-inN duced states in this range of x are of the same kind. However, the large change in this transition with slight variation of Si concentration shows sensitiveness of M and M@ to impurities. At a "eld around &15 T the peak at ¹ for x"0.015 resembles that for x"0.02. N From this fact it can be presumed that the nature of ordered state M@ in high-"eld region becomes similar to that of the AF ordered state for x"0.02. However, ¹ for x"0.015 is a little higher than that for x"0.02 N and ¹ for x"0.015 appears to increase slightly with N increasing magnetic "eld still around 15 T [6]. Considering these results it is questionable at present that nature of M@ phase is directly related to that of the AF phase.
Acknowledgements The present work is partially supported by REIMEI Research Resources of Japan Energy Research Institute. References [1] M. Hase et al., Phys. Rev. Lett. 70 (1993) 3651. [2] L.P. Regnault et al., Europhys. Lett. 32 (1995) 579. [3] B.BuK chner et al., Phys. Rev. B 59 (1999) 6886 and references therein. [4] M. Hiroi et al., Phys. Rev. B 55 (1997) R6125. [5] M. Sera et al., Phys. Rev. B 56 (1997) 14 771. [6] M. Hiroi et al., Physica B 246}247 (1998) 242.
Study on CuGe Si O in high magnetic "elds 0.9885 0.0115 3 by speci"c heat measurements Masahiko Hiroi!,*, Masafumi Sera!,1, Norio Kobayashi!, Jun Akimitsu" !Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan "Department of Physics, Aoyama-Gakuin University, Tokyo 157-8572, Japan
Abstract Speci"c heat is measured for a single crystal of CuGe Si O (x"0.0115) in magnetic "elds up to 14.5 T. In zero "eld 1~x x 3 two peaks at &3 and &10 K were observed, suggesting that the coexistence of the antiferromagnetic and spin-Peierls orders are realized. In our previous measurements of speci"c heat in high-"eld region around 15 T a small but clear anomaly was observed at &4.2 K for x"0.01, while a sharp peak was observed at &5 K for x"0.015, which corresponds to the transition in the magnetic-"eld-induced phases between M phase and M@ phase. They appear very di!erent from each other. In the present study for x"0.0115 we observe a clear peak at 14.5 T which is just intermediate between those for x"0.01 and 0.015 in temperature and in size. This systematic change in the transition with varying x shows that these transitions in the high-"eld phases are of the same kind in this range of x, though it also shows that small change in Si concentration a!ects the magnetic-"eld-induced states to a large extent. ( 2000 Elsevier Science B.V. All rights reserved. Keywords: Spin-Peierls transition; CuGeO ; Impurity e!ect; Magnetic "eld 3
The discovery of spin-Peierls (SP) transition in an inorganic material, CuGeO [1] has promoted the stud3 ies on impurity e!ects on this ordered state. These led to the surprising discovery of the coexistence of the SP order and an antiferromagnetic (AF) order, which had been considered to be mutually exclusive [2]. Impurity e!ects on the magnetic phase diagram in CuGeO have 3 been studied [3]. We have revealed that in addition to D (dimerized) and M (high "eld magnetic) phases which exist in the undoped material, the D#AF (coexisting phase of the dimerized and AF ordered states) and M@ phase (high-"eld phase where the M and AF states likely
* Corresponding author. Present address: Department of Physics, Faculty of Science, Kagoshima University, Kagoshima 890-0065, Japan. Fax: #81-99-285-8081. E-mail address: [email protected] (M. Hiroi) 1 Present address. Department of Quantum Matter, Graduate School of Advanced Sciences of Matter, Hiroshima University, Higashi-Hiroshima 739-8526, Japan.
coexist) exist from the measurements of speci"c heat, C, thermal expansion and magnetostriction [4}6]. Our results are basically in agreement with other measurements. However, the nature of M@ is not clear at present. Looking at the C in high-"eld region around 15 T, for x"0.01 a small but sharp anomaly appears at &4.2 K, which indicates a transition from M@ to M phase [4]. On the other hand, a sharp peak is observed for x"0.015 at &5 K, which resembles that observed for x"0.02 where only an AF transition occurs. Because of the large di!erence in the appearance of the anomalies at these transitions, it was not obvious whether the anomaly changes continuously with varying x between 0.01 and 0.015 and whether the natures of the transition for x"0.01 and 0.015 are of the same kind or not. In this paper we report the results for speci"c heat measurements for x"0.0115 in between 0.01 and 0.015, particularly focusing on the transition between M and M@. A single crystal of CuGe Si O was pre0.9885 0.0115 3 pared by the #oating zone method. Speci"c heat was measured with a usual adiabatic heat pulse method in magnetic "eld up to 14.5 T.
0921-4526/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 0 8 9 5 - 9
670
M. Hiroi et al. / Physica B 281&282 (2000) 669}670
Fig. 2. The speci"c heat of CuGe Si O (x"0.01, 0.0115, 1~x x 3 0.015, and 0.02) in high magnetic "eld around 15 T parallel to the a-axis in the form of C/¹-¹. The origins are shifted for clarity. Fig. 1. Temperature dependence of the speci"c heat of CuGe Si O in magnetic "eld parallel to the a-axis in 0.9885 0.0115 3 the form of C/¹-¹. The origin of the vertical axis is shifted by 40 mJ/mol K2.
In Fig. 1 temperature (¹) dependence of the C of CuGe Si O in magnetic "eld up to 14.5 T 0.9885 0.0115 3 parallel to a-axis is shown in the form of C/¹-¹. At 0 T rather broad but clear peaks are observed at ¹ &3 K N and ¹ &10 K, corresponding to the AF and the SP SP transitions, respectively. In this compound also the coexistence of the SP and the AF orders is realized. The appearance of the ¹ dependence of the C for x"0.0115 is just intermediate between those for x"0.01 and 0.015. For x"0.0115 the peaks are clearer than those for x"0.015, where the peaks are very broad. With increasing magnetic "eld the peak at SP transition becomes broader and ¹ becomes lower, while the peak at SP ¹ becomes smaller and ¹ shifts to higher temperN N atures with increasing "eld. The peak at ¹ becomes N small and broad at 9 T. On further increasing magnetic "eld from 9 to 12 T a fairly sudden increase of ¹ is N observed. Above &12 T the peak at ¹ becomes sharp N and large. At the same time the anomaly at ¹ becomes SP so broad that it is rather di$cult to know precisely where ¹ is. This kind of behavior of the C with varying SP magnetic "eld was also observed for x"0.01 [4]. In Fig. 2 C for x"0.01, 0.0115, 0.015, and 0.02 in the high"eld region around 15 T are shown in the form of C/¹-¹. For x"0.02 only one peak indicating an AF transition is observed, and the SP order does not exist in this sample. This AF transition is hardly a!ected by magnetic "eld up to 15 T [4]. With x increasing from 0.01 to 0.015, where the coexistence of the SP and the AF states is observed,
the peak at ¹ changes systematically in temperature N and in sharpness and size. Judging from this systematic change, the transitions at ¹ in the magnetic-"eld-inN duced states in this range of x are of the same kind. However, the large change in this transition with slight variation of Si concentration shows sensitiveness of M and M@ to impurities. At a "eld around &15 T the peak at ¹ for x"0.015 resembles that for x"0.02. N From this fact it can be presumed that the nature of ordered state M@ in high-"eld region becomes similar to that of the AF ordered state for x"0.02. However, ¹ for x"0.015 is a little higher than that for x"0.02 N and ¹ for x"0.015 appears to increase slightly with N increasing magnetic "eld still around 15 T [6]. Considering these results it is questionable at present that nature of M@ phase is directly related to that of the AF phase.
Acknowledgements The present work is partially supported by REIMEI Research Resources of Japan Energy Research Institute. References [1] M. Hase et al., Phys. Rev. Lett. 70 (1993) 3651. [2] L.P. Regnault et al., Europhys. Lett. 32 (1995) 579. [3] B.BuK chner et al., Phys. Rev. B 59 (1999) 6886 and references therein. [4] M. Hiroi et al., Phys. Rev. B 55 (1997) R6125. [5] M. Sera et al., Phys. Rev. B 56 (1997) 14 771. [6] M. Hiroi et al., Physica B 246}247 (1998) 242.