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Magnetism and crystalline electric field in CePt4B
Physica B 223&224 (1996) 347-350
ELSEVIER
Magnetism and crystalline electric field in CePt4B S. Siillow*, B. Ludoph, G.J. Nieuwenhuys, A.A. Menovsky, J.A. Mydosh Kamerlingh Onnes Laboratory, Leiden University, The Netherlands
Abstract We present data on the new intermetallic compound CePt4B crystallizing in the hexagonal CeCo4B-structure. From measurements of the specific heat, the resistivity and the susceptibility we deduce the basic physical properties of this system. It orders antiferromagnetically below TN = 2.3 K. In the specific heat a pronounced 2-anomaly at TN is found with a peak value of about 7 J/mol K. A second anomaly can be seen in the specific heat between Ty and 10 K, which we attribute to low-lying crystalline electric field levels. We discuss the connections between the crystalline electric field scheme and the magnetic behavior.
The physics of intermetallic Ce- and U-compounds crytallizing in the hexagonal CeCo4B-structure attracted considerable interest in recent years. Especially the magnetic properties were the subject of detailed investigations. F o r example, CeCo4B itself is a ferromagnet with Tc = 280 K [1], while CeNi4B was found to be a Pauli paramagnet [2]. UNi4B has been reported to be an antiferromagnet (TN = 20 K) with a very peculiar and, for U-compounds, unique magnetic structure [3]. Although CeT4B-systems are well studied for the lighter transition metals (T = Co, Ni), less work has been performed on compounds with heavier 4d or 5d elements. The reason is that with the heavy transition metals the CeCo4B-structure tends to become instable. Nevertheless, we have been able to synthesize, to our knowledge for the first time, a system of the 141-class containing the heavy transition metal Pt. Here we report our results of the physical properties of CePt4B. A polycrystalline sample was arc-melted in stoichiometric ratio (Ce: 3N, Pt: 4N, B: 3N) under argon atmosphere and was annealed subsequently in an evacuated quartz tube at different temperatures (800°C for 10 days and 1000°C for 5 days). The sample was checked by
* Corresponding author.
X-ray diffraction and electron probe micro analysis (EPMA) after each step. The second heat treatment appeared to be sufficient to obtain a homogeneous sample with the correct 1 : 4 : 1 stoichiometry. The X-ray diffraction pattern could be indexed within the CeCo4B-structure and we derive lattice constants of a = 5.446(4) A and c = 7.585(10) A. A comparison with the latticeparameters of CeCo4B (a = 5.024(4)A; c = 6.944(4)A [4]) shows the strong effect of the Pt substitution for Co on the lattice. That CePt4B is on the borderline of crystallographic stability is illustrated by the fact we were not able to synthesize LaPt4B in the same crystal structure. The X-ray pattern of nearly single phase LaPt4B (proven by EPMA) consisted of much more Bragg reflections than that of CePt4B, indicating a more complicated structure. Unfortunately, we were not able to resolve this structure. For measurements of the resistivity p a 4-point ACtechnique between 0.45 and 300 K was employed. The DC-susceptibility X in an external field of 0.005 T was obtained at temperatures between 1.6 and 300 K using a commercial SQUID. For the specific heat cp a standard adiabatic technique from 2.5 to 20 K in zero field and in fields up to 6 T between 5 and 20 K was utilized. In Fig. 1 we show the resistivity p of CePt4B. While at high temperatures ( >i 40 K; inset Fig. 1) it shows only
0921-4526/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved PII S 0 9 2 1 - 4 5 2 6 ( 9 6 ) 0 0 1 1 9 - 6
348
S. Siillow et al. / Physica B 223&224 (1996) 347-350
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T (K) Fig. l. The low-temperature resistivity p of CePt4B. The inset shows the overall behavior of the resistivity.
a slight negative curvature, an anomaly is observable below 8 K. Here the resistivity drops down to 2 K and changes to a smaller slope below 2 K. This low-temperature anomaly of p is partially due to the onset of magnetic ordering. A sharp kink in the susceptibility (Fig. 2) reveals the antiferromagnetic nature of the transition. The transition temperature is determined as a maximum of d(zT ) / d T to TN = 2.3 K. At high temperatures the susceptibility cannot be described by a local moment picture, since Z-1 bends over the whole temperature range (inset Fig. 2). The curvature of Z-1 indicates that crystalline electric field levels are important over the whole temperature range. In Fig. 3 the specific heat cp versus T of CePt4B in zero field and in fields of 3 and 6 T is plotted. The zero field specific heat exhibits a sharp upturn below ~ 3 K with a highest value of 7 J/mol K indicating the onset of magnetic ordering. Unfortunately, with our specific heat apparatus we were restricted to temperatures ~> 2.5 K, and, therefore, we could not measure the whole magnetic transition. As a consequence, we cannot estimate the entropy connected with this transition. Moreover, since LaPt4B does not crystallize in the CeCo4Bstructure, we cannot easily correct cp for the lattice contribution. Besides the magnetic anomaly an additional broad feature is visible at higher temperatures in the specific heat ( ~< 10 K). In magnetic field this anomaly shifts to higher temperatures. The anomaly coincides with the
downturn of the resistivity at low temperatures and we identify it as a Schottky anomaly. The J = ~ state of Ce is split in a hexagonal symmetry into three doublets, [ _+ 2z), [ _+ ~2) and [ _+ ~2). In view of the susceptibility results (curvature) and the anomalies in cp and p below 10 K, we suggest the splitting between the lower doublets to be of the order of 10 K, while the last doublet is estimated to be at 300 K. Since we have no data on a single crystal, we are not able to deduce the ground state. But, at least, we can test our assumption by a model with the [ _+ 2~)-state as ground state and apply a simple molecular field model. We then calculate cp with the [ _ ~2)-level as first excited state at 10 K, while the ] _+ ~)-state situated at higher energies does not contribute to the low-temperature cp. Now, we can determine the effect of a magnetic field on the specific heat by calculating cp at 0 and 6 T within the molecular field model and subsequently subtracting the 0 T data from the result at 6 T. This calculated "field-induced" specific heat we add to the actually measured values at 0 T, thus obtaining data for cp at 6 T. The result of the calculation is included in Fig. 3 as a solid line. Obviously, the broad anomaly in cp is indeed shifted to higher temperatures with magnetic field and reproduces qualitatively the measurement of cp of 6 T. And, although the calculated values are somewhat larger than the measured ones, we regard this disparity of minor importance in view of the simplicity of our model. Based on the calculated CEF contribution we can derive the lattice and electronic specific heat of CePt4B.
349
S. Siillow et al. /Physica B 223&224 (1996) 347-350
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T (K) Fig. 2. The susceptibilityZ of CePtaB as a function of temperature. In the inset the inverse susceptibilityZ- 1 of CePtaB as a function of temperature is plotted.
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T (K) Fig. 3. The specific heat cp versus temperature at different magnetic fields: ( + ) 0 T; (~) 3 T; (V) 6 T. The solid line is the result of a molecular field calculation for a magnetic field of 6 T as described in the text.
We describe the lattice part with a Debye function for Ce and Pt and an Einstein mode for B with Oo = 182 K and OE = 500 K and extrapolate the electronic contribution to y = 19 mJ/mol K 2. Therefore, we suppose the overall situation in CePt+B to be as follows: The large A-anomaly is essentially formed in the presence of strong internal magnetic fields by the ordered low-lying states, while the anomaly above
TN arises from the I + z2)-level. In magnetic field the i + ~2) and I + ~) doublets split into singlets, leading to the observed shift of the anomaly in cp. The downturn of the resistivity then appears to be a cooperative effect of the depopulation of the CEF levels and the onset of long-range magnetic ordering. In fact, the maximum of dp/dT ( = 2.6 K) gives a reasonable value for TN even in the presence of the CEF. A last open question refers to the comparatively small electronic contribution ?. Such small ? and the absence of the Kondo effect indicates a strong magnetic moment at the Ce-site. However, the ordering temperature is small and, hence, the RKKY-exchange is also small. This, we suppose, results from the large increase of the lattice parameters by the substitution of Pt in CeCoaB. Compared to other 141-compounds it leads to a strong decrease of the magnetic exchange interaction J, implying a low magnetic transition temperature. In conclusion, we presented data on the new intermetallic compound CePt+B. At low temperatures we found the system to undergo an antiferromagnetic transition at TN = 2.3 K. The low-temperature specific heat is governed by the magnetic transition and low-lying CEF levels, while the overall CEF splitting is of the order of several hundred K. From the specific heat we found the hybridization strength of the electrons to be small. This and the small magnetic ordering temperature can be qualitatively understood as a consequence of Pt
350
S. Siillow et al. /Physica B 223&224 (1996) 347-350
substitution on the crystallographic structure, since in CePt4B the lattice parameters are increased by about 10% compared to the homologous compound CeCo4B. This work was partially supported by the Stichting FOM.
References [1] E. Burzo et al., J. Less-Common Met. 155 (1989) 281. [2] C. Mazumdar et al., Proc. Solid State Phys. Symp., Dept. Atomic Energy, India, 33C (1991) 265. [3] S.A.M. Mentink et al., Phys. Rev. Lett. 73 (1994) 1031. [4] A. Lindbaum et al., J. Appl. Phys. 73 (1993) 6153.
ELSEVIER
Magnetism and crystalline electric field in CePt4B S. Siillow*, B. Ludoph, G.J. Nieuwenhuys, A.A. Menovsky, J.A. Mydosh Kamerlingh Onnes Laboratory, Leiden University, The Netherlands
Abstract We present data on the new intermetallic compound CePt4B crystallizing in the hexagonal CeCo4B-structure. From measurements of the specific heat, the resistivity and the susceptibility we deduce the basic physical properties of this system. It orders antiferromagnetically below TN = 2.3 K. In the specific heat a pronounced 2-anomaly at TN is found with a peak value of about 7 J/mol K. A second anomaly can be seen in the specific heat between Ty and 10 K, which we attribute to low-lying crystalline electric field levels. We discuss the connections between the crystalline electric field scheme and the magnetic behavior.
The physics of intermetallic Ce- and U-compounds crytallizing in the hexagonal CeCo4B-structure attracted considerable interest in recent years. Especially the magnetic properties were the subject of detailed investigations. F o r example, CeCo4B itself is a ferromagnet with Tc = 280 K [1], while CeNi4B was found to be a Pauli paramagnet [2]. UNi4B has been reported to be an antiferromagnet (TN = 20 K) with a very peculiar and, for U-compounds, unique magnetic structure [3]. Although CeT4B-systems are well studied for the lighter transition metals (T = Co, Ni), less work has been performed on compounds with heavier 4d or 5d elements. The reason is that with the heavy transition metals the CeCo4B-structure tends to become instable. Nevertheless, we have been able to synthesize, to our knowledge for the first time, a system of the 141-class containing the heavy transition metal Pt. Here we report our results of the physical properties of CePt4B. A polycrystalline sample was arc-melted in stoichiometric ratio (Ce: 3N, Pt: 4N, B: 3N) under argon atmosphere and was annealed subsequently in an evacuated quartz tube at different temperatures (800°C for 10 days and 1000°C for 5 days). The sample was checked by
* Corresponding author.
X-ray diffraction and electron probe micro analysis (EPMA) after each step. The second heat treatment appeared to be sufficient to obtain a homogeneous sample with the correct 1 : 4 : 1 stoichiometry. The X-ray diffraction pattern could be indexed within the CeCo4B-structure and we derive lattice constants of a = 5.446(4) A and c = 7.585(10) A. A comparison with the latticeparameters of CeCo4B (a = 5.024(4)A; c = 6.944(4)A [4]) shows the strong effect of the Pt substitution for Co on the lattice. That CePt4B is on the borderline of crystallographic stability is illustrated by the fact we were not able to synthesize LaPt4B in the same crystal structure. The X-ray pattern of nearly single phase LaPt4B (proven by EPMA) consisted of much more Bragg reflections than that of CePt4B, indicating a more complicated structure. Unfortunately, we were not able to resolve this structure. For measurements of the resistivity p a 4-point ACtechnique between 0.45 and 300 K was employed. The DC-susceptibility X in an external field of 0.005 T was obtained at temperatures between 1.6 and 300 K using a commercial SQUID. For the specific heat cp a standard adiabatic technique from 2.5 to 20 K in zero field and in fields up to 6 T between 5 and 20 K was utilized. In Fig. 1 we show the resistivity p of CePt4B. While at high temperatures ( >i 40 K; inset Fig. 1) it shows only
0921-4526/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved PII S 0 9 2 1 - 4 5 2 6 ( 9 6 ) 0 0 1 1 9 - 6
348
S. Siillow et al. / Physica B 223&224 (1996) 347-350
14.00
.
.
.
.
i
.
.
.
.
i
.
.
.
.
~
,
,
1
13.50
¢9
13.00
/
12.50
/
t
,/
12.00
. . . .
J
'° J . . . .
5
t
J
..... 2'co ..... ,
,
10
,
, T !K)a
15
. . . .
20
T (K) Fig. l. The low-temperature resistivity p of CePt4B. The inset shows the overall behavior of the resistivity.
a slight negative curvature, an anomaly is observable below 8 K. Here the resistivity drops down to 2 K and changes to a smaller slope below 2 K. This low-temperature anomaly of p is partially due to the onset of magnetic ordering. A sharp kink in the susceptibility (Fig. 2) reveals the antiferromagnetic nature of the transition. The transition temperature is determined as a maximum of d(zT ) / d T to TN = 2.3 K. At high temperatures the susceptibility cannot be described by a local moment picture, since Z-1 bends over the whole temperature range (inset Fig. 2). The curvature of Z-1 indicates that crystalline electric field levels are important over the whole temperature range. In Fig. 3 the specific heat cp versus T of CePt4B in zero field and in fields of 3 and 6 T is plotted. The zero field specific heat exhibits a sharp upturn below ~ 3 K with a highest value of 7 J/mol K indicating the onset of magnetic ordering. Unfortunately, with our specific heat apparatus we were restricted to temperatures ~> 2.5 K, and, therefore, we could not measure the whole magnetic transition. As a consequence, we cannot estimate the entropy connected with this transition. Moreover, since LaPt4B does not crystallize in the CeCo4Bstructure, we cannot easily correct cp for the lattice contribution. Besides the magnetic anomaly an additional broad feature is visible at higher temperatures in the specific heat ( ~< 10 K). In magnetic field this anomaly shifts to higher temperatures. The anomaly coincides with the
downturn of the resistivity at low temperatures and we identify it as a Schottky anomaly. The J = ~ state of Ce is split in a hexagonal symmetry into three doublets, [ _+ 2z), [ _+ ~2) and [ _+ ~2). In view of the susceptibility results (curvature) and the anomalies in cp and p below 10 K, we suggest the splitting between the lower doublets to be of the order of 10 K, while the last doublet is estimated to be at 300 K. Since we have no data on a single crystal, we are not able to deduce the ground state. But, at least, we can test our assumption by a model with the [ _+ 2~)-state as ground state and apply a simple molecular field model. We then calculate cp with the [ _ ~2)-level as first excited state at 10 K, while the ] _+ ~)-state situated at higher energies does not contribute to the low-temperature cp. Now, we can determine the effect of a magnetic field on the specific heat by calculating cp at 0 and 6 T within the molecular field model and subsequently subtracting the 0 T data from the result at 6 T. This calculated "field-induced" specific heat we add to the actually measured values at 0 T, thus obtaining data for cp at 6 T. The result of the calculation is included in Fig. 3 as a solid line. Obviously, the broad anomaly in cp is indeed shifted to higher temperatures with magnetic field and reproduces qualitatively the measurement of cp of 6 T. And, although the calculated values are somewhat larger than the measured ones, we regard this disparity of minor importance in view of the simplicity of our model. Based on the calculated CEF contribution we can derive the lattice and electronic specific heat of CePt4B.
349
S. Siillow et al. /Physica B 223&224 (1996) 347-350
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T (K) Fig. 2. The susceptibilityZ of CePtaB as a function of temperature. In the inset the inverse susceptibilityZ- 1 of CePtaB as a function of temperature is plotted.
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T (K) Fig. 3. The specific heat cp versus temperature at different magnetic fields: ( + ) 0 T; (~) 3 T; (V) 6 T. The solid line is the result of a molecular field calculation for a magnetic field of 6 T as described in the text.
We describe the lattice part with a Debye function for Ce and Pt and an Einstein mode for B with Oo = 182 K and OE = 500 K and extrapolate the electronic contribution to y = 19 mJ/mol K 2. Therefore, we suppose the overall situation in CePt+B to be as follows: The large A-anomaly is essentially formed in the presence of strong internal magnetic fields by the ordered low-lying states, while the anomaly above
TN arises from the I + z2)-level. In magnetic field the i + ~2) and I + ~) doublets split into singlets, leading to the observed shift of the anomaly in cp. The downturn of the resistivity then appears to be a cooperative effect of the depopulation of the CEF levels and the onset of long-range magnetic ordering. In fact, the maximum of dp/dT ( = 2.6 K) gives a reasonable value for TN even in the presence of the CEF. A last open question refers to the comparatively small electronic contribution ?. Such small ? and the absence of the Kondo effect indicates a strong magnetic moment at the Ce-site. However, the ordering temperature is small and, hence, the RKKY-exchange is also small. This, we suppose, results from the large increase of the lattice parameters by the substitution of Pt in CeCoaB. Compared to other 141-compounds it leads to a strong decrease of the magnetic exchange interaction J, implying a low magnetic transition temperature. In conclusion, we presented data on the new intermetallic compound CePt+B. At low temperatures we found the system to undergo an antiferromagnetic transition at TN = 2.3 K. The low-temperature specific heat is governed by the magnetic transition and low-lying CEF levels, while the overall CEF splitting is of the order of several hundred K. From the specific heat we found the hybridization strength of the electrons to be small. This and the small magnetic ordering temperature can be qualitatively understood as a consequence of Pt
350
S. Siillow et al. /Physica B 223&224 (1996) 347-350
substitution on the crystallographic structure, since in CePt4B the lattice parameters are increased by about 10% compared to the homologous compound CeCo4B. This work was partially supported by the Stichting FOM.
References [1] E. Burzo et al., J. Less-Common Met. 155 (1989) 281. [2] C. Mazumdar et al., Proc. Solid State Phys. Symp., Dept. Atomic Energy, India, 33C (1991) 265. [3] S.A.M. Mentink et al., Phys. Rev. Lett. 73 (1994) 1031. [4] A. Lindbaum et al., J. Appl. Phys. 73 (1993) 6153.