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Magnetization and magnetostriction of TmS and TmSe in high magnetic fields
Physica B 281&282 (2000) 595}596
Magnetization and magnetostriction of TmS and TmSe in high magnetic "elds Y. Nakanishi!,*, T. Sakon!, F. Takahashi!, T. Matsumura", T. Suzuki#, M. Motokawa! !Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan "Graduated School of Science, Tohoku University, Sendai 980-8578, Japan #ISSP, University of Tokyo, Roppongi, Tokyo 106-8666, Japan
Abstract Magnetization and magnetostriction measurements of single crystals TmS and TmSe have been performed for the "rst time in high magnetic "elds up to 20 T to study the anisotropies of these compounds. The magnetization curve of both materials for any axis is almost the same as those for other axes above 50 K, which means that anisotropies of these compounds are very small at high temperatures. At low temperature, however, a steep increase of magnetization is observed when an external "eld is applied to the [1 0 0] axis. We have also performed magnetostriction measurement on TmS and it has been turned out that the crystal stretches along the [1 0 0] direction in the ordered state for HE[1 0 0] above transition "eld. It is considered that the crystalline "eld parameter O probably plays an important role for the 20 anisotropy observed in the magnetization process of the [1 0 0] axis. ( 2000 Published by Elsevier Science B.V. All rights reserved. Keywords: Tm-monochalcogenides; High-"eld magnetization; Magnetostriction
The thulium monochalcogenides TmX (X"S, Se or Te) with the NaCl structure have been attracting much interest since the discovery of anomalous physical behaviors associated with multipole e!ect [1] on TmTe. These compounds show various type of physical properties depending on the valence of Tm. TmTe, for example, is a divalent semiconductor with 13 4f-electrons at Tm site. TmSe, in which the ion radius of Se is smaller than that of Te, shows valence #uctuating state due to the strong mixing of 4f-electrons with 5d-conduction electrons. As a matter of fact, both the lattice constant and the Curie constant of TmSe are intermediate between those of divalent and trivalent values [2]. Its resistivity shows the Kondo-like logarithmic temperature dependence at high temperature and steep increase near ¹ "3.5 K which is N presumably due to a transition into an insulating state. In addition, the response on the external magnetic "eld is
* Corresponding author. Tel.: #81-22-215-2017; fax: #8122-215-2016. E-mail address: [email protected] (Y. Nakanishi)
very complicated near ¹ [1]. In the case of TmS, the N lattice constant and the Curie temperature are close to those of the trivalent case. Its resistivity also shows the Kondo-like behavior [1]. The magnetoresistance at 0.1 K abruptly decreases around 4 T above which it is nearly constant [2]. The magnetization processes of TmSe and TmS are also strange but interestings as will be mentioned below. Such complicated behaviors of TmSe and TmS are related to the valence #uctuation and the mechanisms have not been completely understood. In this paper, we report the experimental results of magnetization and magnetostriction measurements in magnetic "elds up to 14 and 20 T, respectively, for single crystals of TmSe and TmS. Figs. 1(a) and (b) are the magnetization curves as a function of applied "elds parallel to the three principal axes for TmS and TmSe, respectively. At high temperatures above 50 K, magnetization curves for three axes are almost the same, which means that the anisotropy is very small for both compounds. Below 50 K, the magnetization curves are di!erent depending on the "eld direction. Induced magnetizations along the [1 0 0] axis are larger below around 13 T than
0921-4526/00/$ - see front matter ( 2000 Published by Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 0 8 8 5 - 6
596
Y. Nakanishi et al. / Physica B 281&282 (2000) 595}596
Fig. 2. Magnetostriction of TmS with "elds parallel to the [1 0 0] axis below ¹ . N
Fig. 3. The calculated magnetization process taking account of O0 with the crystal "eld splitting D of 20 K. 2 Fig. 1. (a) Magnetization curves of TmS with "elds parallel to the three principal axes. The dotted lines are the expected magnetization processes from the Brillouin function of Tm`2 and Tm`3, respectively at 4.2 K. The broken lines are the expected magnetization processes from the susceptibility at 100 and 285 K, respectively. (b) Magnetization curves of TmSe with "elds parallel to the three principal axes. The broken lines are the expected magnetization processes from the Brillouin function of Tm`2 and Tm`3 at 4.2 and 285 K.
qualitatively explains the observed magnetization process mentioned above. It suggests that the crystal "eld parameter O0 plays an important role for the anisotropy of the magnet2 ization along the [1 0 0] axis. However, cross of the magnitude of the magnetization around 13 T is not explained by this model. The reduction in the absolute value may be attributed to the Kondo e!ect as well as that of CeB 6 system [3]. Their absolute values are also open problems. Acknowledgements
those along the other principal axes ([1 1 0], [1 1 1]), but become smaller above it on TmS and TmSe at 4.2 K. On the other hand, magnetostriction of TmS shows expansion of the crystal toward the [1 0 0] direction for HE[1 0 0] above the magnetic transition "eld at around 4 T [2] in the ordered state as shown in Fig. 2. This phenomenon is consistent with the strong anomalies observed at around ¹ for the low-temperature thermal expansion N coe$cients along the [1 0 0] axis [4]. We consider that the crystal "eld symmetry changes from cubic to tetragonal in low temperature and it is enhanced by magnetic "elds. In a tetragonal crystal structure the crystal "eld parameter O0 is 2 considered to be more dominant than the others, i.e. O0 , 4 O4 , O0 , O4 . We calculated the magnetization using only 4 6 6 O0 . When the total crystal "eld splitting D is assumed to be 2 about 20 K, the result is shown in Fig. 3. This model
This work was performed at the High Field Laboratory for Superconducting Materials, and Center for Low Temperature Science, Institute for Materials Research, Tohoku University. References [1] T. Matsumura, Ph.D. thesis, Tohoku University, 1996. [2] Y. Nakanishi, F. Takahashi, T. Sakon, H. Nojiri, M. Motokawa, T. Matsumura, T. Suzuki, Physica B 259}261 (1999) 329. [3] N. Sato, S. Kunii, I. Oguro, T. Komatsubara, T. Kasuya, J. Phys. Soc. Japan 53 (1984) 3967. [4] H.R. Ott, B. Luthi, P.S. Wang, in: R.D. Parks (Ed.), Valence Instabilities and Related Narrow Band Phenomena, Plenum, New York, 1977, p. 293.
Magnetization and magnetostriction of TmS and TmSe in high magnetic "elds Y. Nakanishi!,*, T. Sakon!, F. Takahashi!, T. Matsumura", T. Suzuki#, M. Motokawa! !Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan "Graduated School of Science, Tohoku University, Sendai 980-8578, Japan #ISSP, University of Tokyo, Roppongi, Tokyo 106-8666, Japan
Abstract Magnetization and magnetostriction measurements of single crystals TmS and TmSe have been performed for the "rst time in high magnetic "elds up to 20 T to study the anisotropies of these compounds. The magnetization curve of both materials for any axis is almost the same as those for other axes above 50 K, which means that anisotropies of these compounds are very small at high temperatures. At low temperature, however, a steep increase of magnetization is observed when an external "eld is applied to the [1 0 0] axis. We have also performed magnetostriction measurement on TmS and it has been turned out that the crystal stretches along the [1 0 0] direction in the ordered state for HE[1 0 0] above transition "eld. It is considered that the crystalline "eld parameter O probably plays an important role for the 20 anisotropy observed in the magnetization process of the [1 0 0] axis. ( 2000 Published by Elsevier Science B.V. All rights reserved. Keywords: Tm-monochalcogenides; High-"eld magnetization; Magnetostriction
The thulium monochalcogenides TmX (X"S, Se or Te) with the NaCl structure have been attracting much interest since the discovery of anomalous physical behaviors associated with multipole e!ect [1] on TmTe. These compounds show various type of physical properties depending on the valence of Tm. TmTe, for example, is a divalent semiconductor with 13 4f-electrons at Tm site. TmSe, in which the ion radius of Se is smaller than that of Te, shows valence #uctuating state due to the strong mixing of 4f-electrons with 5d-conduction electrons. As a matter of fact, both the lattice constant and the Curie constant of TmSe are intermediate between those of divalent and trivalent values [2]. Its resistivity shows the Kondo-like logarithmic temperature dependence at high temperature and steep increase near ¹ "3.5 K which is N presumably due to a transition into an insulating state. In addition, the response on the external magnetic "eld is
* Corresponding author. Tel.: #81-22-215-2017; fax: #8122-215-2016. E-mail address: [email protected] (Y. Nakanishi)
very complicated near ¹ [1]. In the case of TmS, the N lattice constant and the Curie temperature are close to those of the trivalent case. Its resistivity also shows the Kondo-like behavior [1]. The magnetoresistance at 0.1 K abruptly decreases around 4 T above which it is nearly constant [2]. The magnetization processes of TmSe and TmS are also strange but interestings as will be mentioned below. Such complicated behaviors of TmSe and TmS are related to the valence #uctuation and the mechanisms have not been completely understood. In this paper, we report the experimental results of magnetization and magnetostriction measurements in magnetic "elds up to 14 and 20 T, respectively, for single crystals of TmSe and TmS. Figs. 1(a) and (b) are the magnetization curves as a function of applied "elds parallel to the three principal axes for TmS and TmSe, respectively. At high temperatures above 50 K, magnetization curves for three axes are almost the same, which means that the anisotropy is very small for both compounds. Below 50 K, the magnetization curves are di!erent depending on the "eld direction. Induced magnetizations along the [1 0 0] axis are larger below around 13 T than
0921-4526/00/$ - see front matter ( 2000 Published by Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 0 8 8 5 - 6
596
Y. Nakanishi et al. / Physica B 281&282 (2000) 595}596
Fig. 2. Magnetostriction of TmS with "elds parallel to the [1 0 0] axis below ¹ . N
Fig. 3. The calculated magnetization process taking account of O0 with the crystal "eld splitting D of 20 K. 2 Fig. 1. (a) Magnetization curves of TmS with "elds parallel to the three principal axes. The dotted lines are the expected magnetization processes from the Brillouin function of Tm`2 and Tm`3, respectively at 4.2 K. The broken lines are the expected magnetization processes from the susceptibility at 100 and 285 K, respectively. (b) Magnetization curves of TmSe with "elds parallel to the three principal axes. The broken lines are the expected magnetization processes from the Brillouin function of Tm`2 and Tm`3 at 4.2 and 285 K.
qualitatively explains the observed magnetization process mentioned above. It suggests that the crystal "eld parameter O0 plays an important role for the anisotropy of the magnet2 ization along the [1 0 0] axis. However, cross of the magnitude of the magnetization around 13 T is not explained by this model. The reduction in the absolute value may be attributed to the Kondo e!ect as well as that of CeB 6 system [3]. Their absolute values are also open problems. Acknowledgements
those along the other principal axes ([1 1 0], [1 1 1]), but become smaller above it on TmS and TmSe at 4.2 K. On the other hand, magnetostriction of TmS shows expansion of the crystal toward the [1 0 0] direction for HE[1 0 0] above the magnetic transition "eld at around 4 T [2] in the ordered state as shown in Fig. 2. This phenomenon is consistent with the strong anomalies observed at around ¹ for the low-temperature thermal expansion N coe$cients along the [1 0 0] axis [4]. We consider that the crystal "eld symmetry changes from cubic to tetragonal in low temperature and it is enhanced by magnetic "elds. In a tetragonal crystal structure the crystal "eld parameter O0 is 2 considered to be more dominant than the others, i.e. O0 , 4 O4 , O0 , O4 . We calculated the magnetization using only 4 6 6 O0 . When the total crystal "eld splitting D is assumed to be 2 about 20 K, the result is shown in Fig. 3. This model
This work was performed at the High Field Laboratory for Superconducting Materials, and Center for Low Temperature Science, Institute for Materials Research, Tohoku University. References [1] T. Matsumura, Ph.D. thesis, Tohoku University, 1996. [2] Y. Nakanishi, F. Takahashi, T. Sakon, H. Nojiri, M. Motokawa, T. Matsumura, T. Suzuki, Physica B 259}261 (1999) 329. [3] N. Sato, S. Kunii, I. Oguro, T. Komatsubara, T. Kasuya, J. Phys. Soc. Japan 53 (1984) 3967. [4] H.R. Ott, B. Luthi, P.S. Wang, in: R.D. Parks (Ed.), Valence Instabilities and Related Narrow Band Phenomena, Plenum, New York, 1977, p. 293.