- Email: [email protected]
Ce4Sb3: A ferromagnetic system with Kondo behavior
Physica B 163 (1990) North-Holland
Ce,Sb,:
131-133
A FERROMAGNETIC
T. SUZUKI, 0. NAKAMURA”, and T. KASUYA
SYSTEM
WITH KONDO BEHAVIOR
N. TOMONAGA,
S. OZEKI,
Y.S. KWON,
A. OCHIAI,
T. TAKEDA”
Department of Physics, Faculty of Science, Tohoku University. Sendai 980, Japan “Hachioji Research Center, Casio Computer Co., Ltd., Chioji 192, Japan
Specific heat measurements on Ce,SB, were carried out in various magnetic fields and the phase diagram was determined. A large linear specific heat coefficient y = 160 mJ/K2 mol Ce was obtained at H = 0. The entropy at the phase transition temperature was only 40% of In2 and largely suppressed by applied magnetic field. Similarity between Ce,Bi, and Ce,Sb, was found. Various anomalous properties suggest competition between Kondo and ferromagnetic interactions in both compounds.
1. Introduction
2. Experimental
There are only a few cases of a Kondo system coexisting with ferromagnetic ordering, e.g. CeSi, [l], UTe [Z], CeRh,B, [3,4], Sm,X, (X = As, Sb, Bi) [5] and Ce,Bi, [6, 71. In our systematic study of abnormal properties in Ce, Sm and Yb-based rare earth pnictides with the anti-Th,P, crystal structure, we have found that Ce,Bi, shows a first order ferromagnetic transition at T, = 3.5 K, with positive slope of the phase boundary. A large linear specific heat coefficient y = 200 mJ/K* mol Ce in the paramagnetic region was obtained in zero magnetic field. The entropy at the phase transition temperature amounts to only 40% of Rln2. We observed a saturation magnetic moment much lower than that expected for Ce3+. The temperature dependence of the magnetic susceptibility suggested a r,-like ground state with a small crystal field splitting. The ordered anisotropy of the magnetization, however, cannot be explained by a simple crystal field model. Various anomalous properties suggest that there is strong competition between the Kondo, p-f mixing, quadrupole-quadrupole and ferromagnetic interactions in Ce,Be,. The p-f mixing in Ce,Sb, is expected to be smaller than that in Ce,Bi,, as for other Ce monopnictides, and we can clearly see these effects in rather larger crystal field splittings obtained from susceptibility measurements, as previously reported. More detailed studies in Ce,Sb, are reported here, in particular specific heat measurements under various magnetic fields. The data are discussed in connection with the Kondo effect.
Single crystals were obtained as described by Ochiai et al. [5]. Specific heat measurments were fully automatic and carried out by the usual adiabatic method using a carbon glass resistance thermometer (CGR1000) calibrated both for magnetic fields and temperatures in a superconducting magnet up to 100 kOe. The temperature dependence of the specific heat under various magnetic fields is shown in fig. 1. There is a very sharp peak at 3.9 K in zero magnetic field (H = 0). corresponding to the phase transition from paramagnetic to ferromagnetic order. This peak shifts strongly to higher temperatures and generally broadens out with increasing applied magnetic field. The specific heat does not depend on magnetic field strength above 35 K. The y value extrapolated from
0921-4526/90/$03.50 (North-Holland)
@ Elsevier
Science
Publishers
B.V.
C(mJ/mi
results
K)
lcoood
.,,/ I
T(K)
Fig.
1. Logarithmic
plot
of specific
heat
and
temperature.
T. Suzuki et al. I Ferromagnetic
132
system with Kondo behavior
Xld 6
I 0
x0
603
4LQ
ml
0
T*(K?
Fig. 2. C‘lT vs. T’ plot in zero magnetic
field.
100
Fig. 4. Temperature susceptibility.
C/T vs. T’ plot in the temperature range of 12-23 K at H = 0 was 180 mJ/K’ mol Ce, as shown in fig. 2. The abnormal increase of C/T below 12 K may be due to short range order competing with the Kondo effect. The temperature dependence of the magnetic entropy at various constant magnetic fields, assuming the coefficient of the /3T3 term to be independent of magnetic field, is shown in fig. 3. The entropy at the transition for H = 0 is only 40% of R In2 and reduced to below 10% by an applied magnetic field H = 10 T. Even at 10 K, the entropy is reduced by 50% in 10 T compared to that of H = 0. All entropy curves coincide above 35 K. Above 13 K, the entropy reaches Rln2. However, the temperature dependence of the magnetic susceptibility as shown in fig. 4 is explained
dependence
T(K)
200
of the
300
inverse
magnetic
the
by a free CeZ ’ ion, or with a crystal field ground state like I-“. Thus the saturation entropy of Rln2 should be much smaller for a real ground state. It is very interesting to show the magnetic field dependence of the entropy at several constant temperatures, as shown in fig. 5. We can obtain the temperature dependence of the magnetization under constant magnetic fields from the thermodynamical relation of (&S/&H), = (SMi ST),. The magnetic phase diagram obtained from specific heat measurements versus temperature is shown in fig. 6. The temperature dependence of the critical field for para to ferromagnetic transition, dH,./ dT, is similar to that of Ce,Bi,, but more enhanced than the case of CeB, [9], which was caused by quadrupole-quadrupole interaction. At low temperatures below the peak, all curves obey a T3 depen-
0
Fig. 3. Temperature dependence several magnetic fields.
of magnetic
2K
10 H(T)
T(K)
entropy
under
Fig. 5. Magnetic several constant
field dependence temperatures.
of the magnetic
entropy
at
T. Suzuki et al. I Ferromagnetic system with Kondo behavior
I 0 Fig. 6. Magnetic measurements.
I
0
5
133
temperature dependent y values, etc. More detailed examination is necessary. The various anomalous properties in both compounds seem to be due to a competition between the Kondo and ferromagnetic exchange interaction. T, of the ground state is estimated to be about 20 K, where the special heat at T, deviates from yT + /3T3. The coherence of the Kondo state may be established below T,. There is no pnictogen dependence for the overall features of both compounds, which is rather mysterious.
15
10 T(K)
phase
diagram
obtained
from
specific
heat
References [II N. Sato, M. Kohgi, T. Satoh,
dence. Thus more antiferromagnetic components may exist in the ferromagnetic state, similar to the case for U,P,(8). The field dependence of the coefficient of this T3 term is strongly decreased with increased magnetic field. The data suggest that the stiffness of the antiferromagnetic spin waves are enhanced by an applied magnetic field. This unusual phenomenon may be explained by the existence of quadrupole-quadrupole interactions, as in the case of phase II in CeB, [9]. The similarity of Ce,Sb, and Ce,Bi, is clarified. The temperature dependence of the critical field, the profile of the specific heat, and T, are similar in both compounds, except for existence of an additional small peak due to some magnetic phase transition in Ce,Bi,. It is rather difficult to interpret the PT3 term in the specific heat as a pure phonon term, because the /? value obtained for Ce,Sb, is larger than that of Ce,Bi,. Larger /I values mean smaller Debye temperatures. Thus there are other contributions to /3 due to some magnetic origin such as spin fluctuations or
PI [31 [41
PI [61
[71
PI [91
[lOI
Y. Ishikawa, H. Hiroyoshi and H. Takei, J. Magn. Magn. Mat. 52 (1985) 360. B. Frick, J. Schoenes and 0. Vogt, J. Magn. Magn. Mat. 47 & 48 (1985) 549. R. Vijayaraghavan, J. Magn. Magn. Mat. 47 & 48 (1985) 561. T. Kasuya, M. Kasaya, K. Takegahara, F. Iga, B. Liu and N. Kobayashi, 17th Rare Earth Research Conference, Hamilton (1986). A. Ochiai, T. Suzuki and T. Kasuya, J. Magn. Magn. Mat. 52 (1985) 13. A. Ochiai, Y. Nakabayashi, Y.S. Kwon, K. Takeuchi, K. Takegahara, T. Suzuki and T. Kasuya, J. Magn. Magn. Mat. 52 (1985) 304. T. Suzuki, Y. Nakabayashi, A. Ochiai, T. Kasuya, K. Sugiyama and M. Date, J. Magn. Magn. Mat. 63 & 64 (1987) 58. A. Ochiai, S. Nakai, A. Oyamada, T. Suzuki and T. Kasuya, J. Magn. Magn. Mat. 47 & 48 (1985) 570. M. Takigawa, H. Yasuoka, T. Tanaka and Y. Ishizawa, J. Phys. Sot. Jpn. 52 (1983) 728. N. Sato, S. Kunii, I. Oguro, T. Komatsubara and T. Kasuya, J. Phys. Sot. Jpn. 53 (1984) 3967. P. Burlet, J. Rossat-Mignod, R. Trot and Z. Henkie, Solid State Commun. 38 (1981) 745.
Ce,Sb,:
131-133
A FERROMAGNETIC
T. SUZUKI, 0. NAKAMURA”, and T. KASUYA
SYSTEM
WITH KONDO BEHAVIOR
N. TOMONAGA,
S. OZEKI,
Y.S. KWON,
A. OCHIAI,
T. TAKEDA”
Department of Physics, Faculty of Science, Tohoku University. Sendai 980, Japan “Hachioji Research Center, Casio Computer Co., Ltd., Chioji 192, Japan
Specific heat measurements on Ce,SB, were carried out in various magnetic fields and the phase diagram was determined. A large linear specific heat coefficient y = 160 mJ/K2 mol Ce was obtained at H = 0. The entropy at the phase transition temperature was only 40% of In2 and largely suppressed by applied magnetic field. Similarity between Ce,Bi, and Ce,Sb, was found. Various anomalous properties suggest competition between Kondo and ferromagnetic interactions in both compounds.
1. Introduction
2. Experimental
There are only a few cases of a Kondo system coexisting with ferromagnetic ordering, e.g. CeSi, [l], UTe [Z], CeRh,B, [3,4], Sm,X, (X = As, Sb, Bi) [5] and Ce,Bi, [6, 71. In our systematic study of abnormal properties in Ce, Sm and Yb-based rare earth pnictides with the anti-Th,P, crystal structure, we have found that Ce,Bi, shows a first order ferromagnetic transition at T, = 3.5 K, with positive slope of the phase boundary. A large linear specific heat coefficient y = 200 mJ/K* mol Ce in the paramagnetic region was obtained in zero magnetic field. The entropy at the phase transition temperature amounts to only 40% of Rln2. We observed a saturation magnetic moment much lower than that expected for Ce3+. The temperature dependence of the magnetic susceptibility suggested a r,-like ground state with a small crystal field splitting. The ordered anisotropy of the magnetization, however, cannot be explained by a simple crystal field model. Various anomalous properties suggest that there is strong competition between the Kondo, p-f mixing, quadrupole-quadrupole and ferromagnetic interactions in Ce,Be,. The p-f mixing in Ce,Sb, is expected to be smaller than that in Ce,Bi,, as for other Ce monopnictides, and we can clearly see these effects in rather larger crystal field splittings obtained from susceptibility measurements, as previously reported. More detailed studies in Ce,Sb, are reported here, in particular specific heat measurements under various magnetic fields. The data are discussed in connection with the Kondo effect.
Single crystals were obtained as described by Ochiai et al. [5]. Specific heat measurments were fully automatic and carried out by the usual adiabatic method using a carbon glass resistance thermometer (CGR1000) calibrated both for magnetic fields and temperatures in a superconducting magnet up to 100 kOe. The temperature dependence of the specific heat under various magnetic fields is shown in fig. 1. There is a very sharp peak at 3.9 K in zero magnetic field (H = 0). corresponding to the phase transition from paramagnetic to ferromagnetic order. This peak shifts strongly to higher temperatures and generally broadens out with increasing applied magnetic field. The specific heat does not depend on magnetic field strength above 35 K. The y value extrapolated from
0921-4526/90/$03.50 (North-Holland)
@ Elsevier
Science
Publishers
B.V.
C(mJ/mi
results
K)
lcoood
.,,/ I
T(K)
Fig.
1. Logarithmic
plot
of specific
heat
and
temperature.
T. Suzuki et al. I Ferromagnetic
132
system with Kondo behavior
Xld 6
I 0
x0
603
4LQ
ml
0
T*(K?
Fig. 2. C‘lT vs. T’ plot in zero magnetic
field.
100
Fig. 4. Temperature susceptibility.
C/T vs. T’ plot in the temperature range of 12-23 K at H = 0 was 180 mJ/K’ mol Ce, as shown in fig. 2. The abnormal increase of C/T below 12 K may be due to short range order competing with the Kondo effect. The temperature dependence of the magnetic entropy at various constant magnetic fields, assuming the coefficient of the /3T3 term to be independent of magnetic field, is shown in fig. 3. The entropy at the transition for H = 0 is only 40% of R In2 and reduced to below 10% by an applied magnetic field H = 10 T. Even at 10 K, the entropy is reduced by 50% in 10 T compared to that of H = 0. All entropy curves coincide above 35 K. Above 13 K, the entropy reaches Rln2. However, the temperature dependence of the magnetic susceptibility as shown in fig. 4 is explained
dependence
T(K)
200
of the
300
inverse
magnetic
the
by a free CeZ ’ ion, or with a crystal field ground state like I-“. Thus the saturation entropy of Rln2 should be much smaller for a real ground state. It is very interesting to show the magnetic field dependence of the entropy at several constant temperatures, as shown in fig. 5. We can obtain the temperature dependence of the magnetization under constant magnetic fields from the thermodynamical relation of (&S/&H), = (SMi ST),. The magnetic phase diagram obtained from specific heat measurements versus temperature is shown in fig. 6. The temperature dependence of the critical field for para to ferromagnetic transition, dH,./ dT, is similar to that of Ce,Bi,, but more enhanced than the case of CeB, [9], which was caused by quadrupole-quadrupole interaction. At low temperatures below the peak, all curves obey a T3 depen-
0
Fig. 3. Temperature dependence several magnetic fields.
of magnetic
2K
10 H(T)
T(K)
entropy
under
Fig. 5. Magnetic several constant
field dependence temperatures.
of the magnetic
entropy
at
T. Suzuki et al. I Ferromagnetic system with Kondo behavior
I 0 Fig. 6. Magnetic measurements.
I
0
5
133
temperature dependent y values, etc. More detailed examination is necessary. The various anomalous properties in both compounds seem to be due to a competition between the Kondo and ferromagnetic exchange interaction. T, of the ground state is estimated to be about 20 K, where the special heat at T, deviates from yT + /3T3. The coherence of the Kondo state may be established below T,. There is no pnictogen dependence for the overall features of both compounds, which is rather mysterious.
15
10 T(K)
phase
diagram
obtained
from
specific
heat
References [II N. Sato, M. Kohgi, T. Satoh,
dence. Thus more antiferromagnetic components may exist in the ferromagnetic state, similar to the case for U,P,(8). The field dependence of the coefficient of this T3 term is strongly decreased with increased magnetic field. The data suggest that the stiffness of the antiferromagnetic spin waves are enhanced by an applied magnetic field. This unusual phenomenon may be explained by the existence of quadrupole-quadrupole interactions, as in the case of phase II in CeB, [9]. The similarity of Ce,Sb, and Ce,Bi, is clarified. The temperature dependence of the critical field, the profile of the specific heat, and T, are similar in both compounds, except for existence of an additional small peak due to some magnetic phase transition in Ce,Bi,. It is rather difficult to interpret the PT3 term in the specific heat as a pure phonon term, because the /? value obtained for Ce,Sb, is larger than that of Ce,Bi,. Larger /I values mean smaller Debye temperatures. Thus there are other contributions to /3 due to some magnetic origin such as spin fluctuations or
PI [31 [41
PI [61
[71
PI [91
[lOI
Y. Ishikawa, H. Hiroyoshi and H. Takei, J. Magn. Magn. Mat. 52 (1985) 360. B. Frick, J. Schoenes and 0. Vogt, J. Magn. Magn. Mat. 47 & 48 (1985) 549. R. Vijayaraghavan, J. Magn. Magn. Mat. 47 & 48 (1985) 561. T. Kasuya, M. Kasaya, K. Takegahara, F. Iga, B. Liu and N. Kobayashi, 17th Rare Earth Research Conference, Hamilton (1986). A. Ochiai, T. Suzuki and T. Kasuya, J. Magn. Magn. Mat. 52 (1985) 13. A. Ochiai, Y. Nakabayashi, Y.S. Kwon, K. Takeuchi, K. Takegahara, T. Suzuki and T. Kasuya, J. Magn. Magn. Mat. 52 (1985) 304. T. Suzuki, Y. Nakabayashi, A. Ochiai, T. Kasuya, K. Sugiyama and M. Date, J. Magn. Magn. Mat. 63 & 64 (1987) 58. A. Ochiai, S. Nakai, A. Oyamada, T. Suzuki and T. Kasuya, J. Magn. Magn. Mat. 47 & 48 (1985) 570. M. Takigawa, H. Yasuoka, T. Tanaka and Y. Ishizawa, J. Phys. Sot. Jpn. 52 (1983) 728. N. Sato, S. Kunii, I. Oguro, T. Komatsubara and T. Kasuya, J. Phys. Sot. Jpn. 53 (1984) 3967. P. Burlet, J. Rossat-Mignod, R. Trot and Z. Henkie, Solid State Commun. 38 (1981) 745.