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Magnetic phase diagram of CexLa1−xB6 studied by magnetization measurement
Physica B 259—261 (1999) 32—33
Magnetic phase diagram of Ce La B studied V \V by magnetization measurement T. Tayama *, S. Honma , K. Tenya , H. Amitsuka , T. Sakakibara , S. Kunii Division of Physics, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan Department of Physics, Tohoku University, Sendai 980-8577, Japan
Abstract From DC magnetic susceptibility s(¹) measurement on Ce La B (x"0.75, 0.7, 0.65 and 0.4), Ce concentration V \V dependence of the zero-field ordering temperature is established. The new magnetic state (phase-IV) characterized by an isotropic cusp in s(¹) is clearly seen for x"0.75, 0.7 and 0.65. Distinct irreversibility in s(¹) observed for x"0.4 below the cusp temperature indicates that phase-IV changes into a certain glass-like state for x)0.5. 1999 Elsevier Science B.V. All rights reserved. Keywords: CeB ; Ce La B ; Antiferromagnetic order; Quadrupolar order; Magnetic susceptibility V \V
The cubic compound CeB successively undergoes an antiferro-quadrupolar (AFQ, “phase-II”) and antiferromagnetic (AF, “phase-III”) orderings at ¹ "3.3 K / and ¹ "2.4 K, respectively. These phase transitions are , due to the orbital and spin degrees of freedom of the ! quartet ground state. Recently, the La dilute system Ce La B have been studied extensively [1—6]. When V \V H&0, the AFQ ordering is strongly suppressed by the La doping, and ¹ decreases to 0 at x&0.7. In contrast, / ¹ shows a much weaker dependence on the La concen, tration. Accordingly, a new type of magnetic phase (“phase-IV”) is observed for x)0.75 at low field. Quite interestingly, the sample of x"0.75 exhibits a successive I—IV—III phase transitions [2,4,5] As the complex magnetic structure of phase-III is considered to be due to the AFQ ordering, the IV—III transition should be a sort of AFQ transition. Normally, when magnetic order first sets in, orbital degrees of freedom (if any) also orders at the same time because of a strong spin—orbit coupling; no further quadrupolar transition can occur. The successive phase transitions in x"0.75 is therefore quite unusual.
* Corresponding author Fax: #81-11-746-5444; e-mail: [email protected].
The nature of the phase-IV is in fact not yet clearly understood. Moreover, to what extent the phase-IV exists in the ¹—x phase diagram is somewhat controversial. While the resistivity measurement suggests antiferromagnetic ordering [5], no sharp anomaly is observed in the specific heat measurement [3] for x"0.5. The latter experiment claims that the ground state of the sample of x"0.5 is a Kondo singlet, because the specific heat exhibits a broad peak at &0.9 K followed by a large ¹-linear contribution of C/¹&1.8 J/mol K at lower temperatures [3]. In order to obtain further information on the phase-IV, we have measured the static magnetization M(¹,H) of Ce La B with 0.4)x) V \V 0.75 at very low temperatures down to 40 mK and in magnetic fields H up to 8.5 T [7]. Fig. 1 shows the thermal variation of the magnetic susceptibility M/H(,s) of Ce La B , x"0.75, 0.65, V \V 0.5 and 0.4, obtained in steady magnetic field of 0.1 T (0.03 T for x"0.4) applied parallel to the [1 0 0] direction. Most of the data were taken by slowly warming up the specimen from the base temperature, after field-cooling (FC) from well above the transition temperatures. For x"0.75, the s(¹) curve shows a clear peak due to the phase I—IV transition at ¹ &1.7 K. Surprisingly, no + appreciable anisotropy is found to develop in s(¹) in the
0921-4526/99/$ — see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 8 ) 0 0 8 1 8 - 7
T. Tayama et al. / Physica B 259—261 (1999) 32—33
Fig. 1. Thermal variation of the magnetic susceptibility M/H of Ce La B x"0.75 (lower panel), 0.7, 0.65, 0.5 and 0.4 (upper V \V panel) with H#[1 0 0] at weak fields (H)0.1 T). FC (ZFC) denotes the field-cooling (zero-field cooling) curves.
phase—IV state. The steep rise of s(¹) below &1.3 K corresponds to the AFQ ordering from phase-IV to phaseIII AF state [2]. In contrast, the s(¹) curves for 0.5)x)0.7 show only a single peak which is very similar to that of I—IV transition in x"0.75. Thus, it is likely that these samples undergo phase-I—IV transition. On cooling below ¹ , no + apparent history dependence was observed in s(¹) for x"0.65. Recent resistivity o(¹) measurement [6] also concludes the magnetic ordering in x"0.65, though the present transition temperature ¹ ("1.1 K) is slightly + lower than that determined from o(¹). In the s(¹) data of x"0.4, both ZFC (zero-field cooling) and FC curves are shown. A distinct irreversibility is found at ¹:0.5 K; while the ZFC curve decreases with decreasing ¹, the FC curve is almost flat at low T. We examined the field dependence of s(¹) and confirmed that the temperature where a difference between FC and ZFC emerges decreases rapidly as the field increases. The s(¹) curve at 1 T was almost independent of ¹ below &0.5 K, without any appreciable irreversibility. Such a behavior can be often seen in the spin glass system. Therefore, we may say that a certain glass-like ordering develops at low fields for x"0.4. This result is in contrast to the conclusion of Ref. [3] that the Kondo singlet is formed for x)0.5. Fig. 2 shows the zero-field ordering temperatures in Ce La B obtained from our magnetization measureV \V ments. We defined ¹ at the peak position in s(¹) + (x*0.5), or at the temperature where irreversibility of s(¹) begins to appear (x"0.4). Judging from this figure, it is clear that a certain magnetic transition still occurs at finite temperature for x"0.4. The present result for x"0.4 suggests that the glass-like ordering starts to
33
Fig. 2. The Ce concentration dependence of the zero-field ordering temperatures in Ce La B . The closed square denotes the V \V AF transition temperature, the open circles the AFQ transition temperatures and the closed circles certain magnetic transition temperatures. The behavior around the broken line is not still clear. A certain glass-like magnetic behavior can be seen at ¹ for x)0.5. +
develop already at x"0.5; the broad peak of C(¹) at &0.9 K and a very large value of C/¹ at lower ¹ [3] can be explained by the glass-like ordering. We can also understand why the temperature of a peak in s(¹) (&0.7 K) is lower than that in C(¹). Up to present, however, no appreciable history dependence is observed in s(¹) for x"0.5. This point would require careful reexamination of the data. The nature of phase-IV is quite unusual. Recent sound velocity measurement [4] revealed that the c mode exhibits a giant softening in phase-IV of x"0.75. This fact as well as our results strongly suggest that certain quadrupolar degeneracy still remains in phase-IV. This point has recently been discussed by Kuramoto introducing different Kondo temperatures for the spin and the orbital parts of the ! orbit [8]. We suggest that the real order parameter of phase-IV might be a magnetic octupolar moment of the type J J J , which is active in the V W X ! orbit [9]. Further experimental study would be needed to clarify these points. The present work was supported by a Grant-in-Aid for the Scientific Research on Priority Areas, from the Ministry of Education, Science, Sports and Culture. References [1] [2] [3] [4] [5] [6] [7] [8] [9]
T. Sakakibara et al., Physica B 230—232 (1997) 307. T. Tayama et al., J. Phys. Soc. Japan 66 (1997) 2268. S. Nakamura et al., J. Phys. Soc. Japan 66 (1997) 552. T. Goto, private communication. M. Hiroi et al., J. Phys. Soc. Japan 66 (1997) 1762. M. Hiroi et al., J. Phys. Soc. Japan 67 (1998) 53. T. Sakakibara et al., Japan J. Appl. Phys. 33 (1994) 5067. Y. Kuramoto, private communication. R. Shiina, H. Shiba, P. Thalmeier, J. Phys. Soc. Japan 66 (1997) 1741.
Magnetic phase diagram of Ce La B studied V \V by magnetization measurement T. Tayama *, S. Honma , K. Tenya , H. Amitsuka , T. Sakakibara , S. Kunii Division of Physics, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan Department of Physics, Tohoku University, Sendai 980-8577, Japan
Abstract From DC magnetic susceptibility s(¹) measurement on Ce La B (x"0.75, 0.7, 0.65 and 0.4), Ce concentration V \V dependence of the zero-field ordering temperature is established. The new magnetic state (phase-IV) characterized by an isotropic cusp in s(¹) is clearly seen for x"0.75, 0.7 and 0.65. Distinct irreversibility in s(¹) observed for x"0.4 below the cusp temperature indicates that phase-IV changes into a certain glass-like state for x)0.5. 1999 Elsevier Science B.V. All rights reserved. Keywords: CeB ; Ce La B ; Antiferromagnetic order; Quadrupolar order; Magnetic susceptibility V \V
The cubic compound CeB successively undergoes an antiferro-quadrupolar (AFQ, “phase-II”) and antiferromagnetic (AF, “phase-III”) orderings at ¹ "3.3 K / and ¹ "2.4 K, respectively. These phase transitions are , due to the orbital and spin degrees of freedom of the ! quartet ground state. Recently, the La dilute system Ce La B have been studied extensively [1—6]. When V \V H&0, the AFQ ordering is strongly suppressed by the La doping, and ¹ decreases to 0 at x&0.7. In contrast, / ¹ shows a much weaker dependence on the La concen, tration. Accordingly, a new type of magnetic phase (“phase-IV”) is observed for x)0.75 at low field. Quite interestingly, the sample of x"0.75 exhibits a successive I—IV—III phase transitions [2,4,5] As the complex magnetic structure of phase-III is considered to be due to the AFQ ordering, the IV—III transition should be a sort of AFQ transition. Normally, when magnetic order first sets in, orbital degrees of freedom (if any) also orders at the same time because of a strong spin—orbit coupling; no further quadrupolar transition can occur. The successive phase transitions in x"0.75 is therefore quite unusual.
* Corresponding author Fax: #81-11-746-5444; e-mail: [email protected].
The nature of the phase-IV is in fact not yet clearly understood. Moreover, to what extent the phase-IV exists in the ¹—x phase diagram is somewhat controversial. While the resistivity measurement suggests antiferromagnetic ordering [5], no sharp anomaly is observed in the specific heat measurement [3] for x"0.5. The latter experiment claims that the ground state of the sample of x"0.5 is a Kondo singlet, because the specific heat exhibits a broad peak at &0.9 K followed by a large ¹-linear contribution of C/¹&1.8 J/mol K at lower temperatures [3]. In order to obtain further information on the phase-IV, we have measured the static magnetization M(¹,H) of Ce La B with 0.4)x) V \V 0.75 at very low temperatures down to 40 mK and in magnetic fields H up to 8.5 T [7]. Fig. 1 shows the thermal variation of the magnetic susceptibility M/H(,s) of Ce La B , x"0.75, 0.65, V \V 0.5 and 0.4, obtained in steady magnetic field of 0.1 T (0.03 T for x"0.4) applied parallel to the [1 0 0] direction. Most of the data were taken by slowly warming up the specimen from the base temperature, after field-cooling (FC) from well above the transition temperatures. For x"0.75, the s(¹) curve shows a clear peak due to the phase I—IV transition at ¹ &1.7 K. Surprisingly, no + appreciable anisotropy is found to develop in s(¹) in the
0921-4526/99/$ — see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 8 ) 0 0 8 1 8 - 7
T. Tayama et al. / Physica B 259—261 (1999) 32—33
Fig. 1. Thermal variation of the magnetic susceptibility M/H of Ce La B x"0.75 (lower panel), 0.7, 0.65, 0.5 and 0.4 (upper V \V panel) with H#[1 0 0] at weak fields (H)0.1 T). FC (ZFC) denotes the field-cooling (zero-field cooling) curves.
phase—IV state. The steep rise of s(¹) below &1.3 K corresponds to the AFQ ordering from phase-IV to phaseIII AF state [2]. In contrast, the s(¹) curves for 0.5)x)0.7 show only a single peak which is very similar to that of I—IV transition in x"0.75. Thus, it is likely that these samples undergo phase-I—IV transition. On cooling below ¹ , no + apparent history dependence was observed in s(¹) for x"0.65. Recent resistivity o(¹) measurement [6] also concludes the magnetic ordering in x"0.65, though the present transition temperature ¹ ("1.1 K) is slightly + lower than that determined from o(¹). In the s(¹) data of x"0.4, both ZFC (zero-field cooling) and FC curves are shown. A distinct irreversibility is found at ¹:0.5 K; while the ZFC curve decreases with decreasing ¹, the FC curve is almost flat at low T. We examined the field dependence of s(¹) and confirmed that the temperature where a difference between FC and ZFC emerges decreases rapidly as the field increases. The s(¹) curve at 1 T was almost independent of ¹ below &0.5 K, without any appreciable irreversibility. Such a behavior can be often seen in the spin glass system. Therefore, we may say that a certain glass-like ordering develops at low fields for x"0.4. This result is in contrast to the conclusion of Ref. [3] that the Kondo singlet is formed for x)0.5. Fig. 2 shows the zero-field ordering temperatures in Ce La B obtained from our magnetization measureV \V ments. We defined ¹ at the peak position in s(¹) + (x*0.5), or at the temperature where irreversibility of s(¹) begins to appear (x"0.4). Judging from this figure, it is clear that a certain magnetic transition still occurs at finite temperature for x"0.4. The present result for x"0.4 suggests that the glass-like ordering starts to
33
Fig. 2. The Ce concentration dependence of the zero-field ordering temperatures in Ce La B . The closed square denotes the V \V AF transition temperature, the open circles the AFQ transition temperatures and the closed circles certain magnetic transition temperatures. The behavior around the broken line is not still clear. A certain glass-like magnetic behavior can be seen at ¹ for x)0.5. +
develop already at x"0.5; the broad peak of C(¹) at &0.9 K and a very large value of C/¹ at lower ¹ [3] can be explained by the glass-like ordering. We can also understand why the temperature of a peak in s(¹) (&0.7 K) is lower than that in C(¹). Up to present, however, no appreciable history dependence is observed in s(¹) for x"0.5. This point would require careful reexamination of the data. The nature of phase-IV is quite unusual. Recent sound velocity measurement [4] revealed that the c mode exhibits a giant softening in phase-IV of x"0.75. This fact as well as our results strongly suggest that certain quadrupolar degeneracy still remains in phase-IV. This point has recently been discussed by Kuramoto introducing different Kondo temperatures for the spin and the orbital parts of the ! orbit [8]. We suggest that the real order parameter of phase-IV might be a magnetic octupolar moment of the type J J J , which is active in the V W X ! orbit [9]. Further experimental study would be needed to clarify these points. The present work was supported by a Grant-in-Aid for the Scientific Research on Priority Areas, from the Ministry of Education, Science, Sports and Culture. References [1] [2] [3] [4] [5] [6] [7] [8] [9]
T. Sakakibara et al., Physica B 230—232 (1997) 307. T. Tayama et al., J. Phys. Soc. Japan 66 (1997) 2268. S. Nakamura et al., J. Phys. Soc. Japan 66 (1997) 552. T. Goto, private communication. M. Hiroi et al., J. Phys. Soc. Japan 66 (1997) 1762. M. Hiroi et al., J. Phys. Soc. Japan 67 (1998) 53. T. Sakakibara et al., Japan J. Appl. Phys. 33 (1994) 5067. Y. Kuramoto, private communication. R. Shiina, H. Shiba, P. Thalmeier, J. Phys. Soc. Japan 66 (1997) 1741.