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Ce3Pt4 — a local moment antiferromagnet with two metamagnetic transitions
Solid State Communications, Vol. 108, No. 8, pp. 535–537, 1998 䉷 1998 Elsevier Science Ltd. All rights reserved 0038–1098/98 $ - see front matter
Pergamon
PII: S0038–1098(98)00406-2
Ce 3Pt 4 ¹ A LOCAL MOMENT ANTIFERROMAGNET WITH TWO METAMAGNETIC TRANSITIONS G.-F. von Blanckenhagen and G.R. Stewart* Institut fu¨r Physik, Universita¨t Augsburg, Universita¨tsstr. 1, 86159 Augsburg, Germany (Received 25 March 1998; accepted in revised form 12 August 1998 by F.J. Di Salvo) In this paper we present the data of the magnetic susceptibility and the low temperature specific heat of Ce 3Pt 4. The large anomaly in the specific heat and the behaviour of the magnetic susceptibility are explained by an antiferromagnetic phase transition at TN ¼ 2:8 K. The entropy due to the phase transition is Rln2 per mole Cerium indicating a local moment antiferromagnet. 䉷 1998 Elsevier Science Ltd. All rights reserved Keywords: A. magnetically ordered materials, D. heat capacity.
1. INTRODUCTION Cerium intermetallic compounds show a large variety in their physical behaviour at low temperatures because of the crucial role of the 4f-electrons in these compounds. Different ground states like superconductivity, antiferroand ferromagnetic order and heavy fermion behaviour have been reported (for an overview see example [1]). In order to look for new cerium compounds showing interesting physical behaviour we measured the magnetic susceptibility and the low temperature specific heat of Ce 3Pt 4. This compound crystallizes in the rhombohedral Pu 3Pd 4-structure [2]. The lattice parameters, given in the ˚ hexagonal setting, have been reported to a ¼ 13:657 A ˚ [2]. The smallest Ce–Ce distance is 3.5 A ˚. and c ¼ 5:781 A To our knowledge no previous measurements of the low temperature physical properties of this compound have been published. 2. EXPERIMENTAL Samples of Ce 3Pt 4, La 3Pt 4 and (Ce 0.5La 0.5) 3Pt 4 were prepared by arc melting together stoichiometric amounts of the constituting elements. Since Ce 3Pt 4 and La 3Pt 4 form peritectically (see for example [3] for phase diagrams) the samples have been annealed at 900⬚C for 3 weeks.
* Corresponding author. Also at the Department of Physics, University of Florida, Gainesville, FL 32611-8440, U.S.A.
For X-ray diffraction we used a Siemens D5000 diffractometer. Magnetic measurements were performed with a Quantum Design SQUID magnetometer. We measured the specific heat with a relaxation method in the temperature range 0.35 K–20 K. The error of the specific heat is ⫾3% between 1.3 K and 20 K, ⫾5% below 1.3 K. 3. RESULTS AND DISCUSSION The X-ray diffractometer patterns show that the samples are single phase. All lines can be indexed assuming the rhombohedral Pu 3Pd 4-structure [2]. The ˚ and c ¼ 5:780 A ˚ for lattice parameters are a ¼ 13:653 A Ce 3Pt 4. These values are in good agreement with those obtained in [2]. For (Ce 0.5La 0.5)Pt 4 and La 3Pt 4 we find ˚ and c ¼ 5:801 A ˚ and a ¼ 13:812 A ˚ and a ¼ 13:733 A ˚ c ¼ 5:812 A respectively (see also Table 1). Figure 1 shows the specific heat divided by temperature of Ce 3Pt 4, La 3Pt 4 and (Ce 0.5La 0.5)Pt 4. At temperatures above about 15 K the three curves show similar behaviour. Since La has no f-electrons this supports the view that the f-electrons are responsible for the observed anomaly, but do not contribute to the behaviour at higher temperatures. The results of the magnetic measurements show that the anomaly in C/T is due to an antiferromagnetic phase transition. Figure 2 shows the magnetic susceptibility (M/H) of Ce 3Pt 4 measured in a magnetic field of 50 G, 5 kG and 20 kG. The typical kink of an antiferromagnetic transition is observed and the maximum of M/H (T) shifts to lower temperatures with increasing magnetic field. In
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Ce 3Pt 4 ¹ A LOCAL MOMENT ANTIFERROMAGNET
Vol. 108, No. 8
Table 1. Lattice parameters, Ne´el temperature and entropy of the f-electron system for the compounds Ce 3Pt 4, (Ce 0.5La 0.5) 3Pt 4 and La 3Pt 4
Ce 3Pt 4 (Ce 0.5La 0.5) 3Pt 4 La 3Pt 4 a b
˚] a [A ˚) (⫾0.005 A
˚] c [A ˚) (⫾0.005 A
T N [K] a (⫾0.15 K)
S 4f [Rln2] b (⫾0.1 Rln2)
13.653 13.733 13.812
5.780 5.801 5.812
2.8 1.6 –
3 1.5 –
Maximum in the specific heat C. At 18 K. To calculate the entropy of the f-electron system the specific heat of La 3Pt 4 has been subtracted from the data. We set C 4f/T (0 K) to zero, whichR is supported by the extrapolation of the measured data points to 0 K. The entropy at 18 K is calculated via S4f ¼ C4f =T dT between 0 K and 18 K. At 18 K the contribution of the f-electrons to the specific heat vanishes (see Fig. 1).
accordance with Fig. 2 the magnetisation M(H) measured at 10 K is linear up to 10 kG. From the data of the specific heat measurements shown in Fig. 1 we calculated the entropy of the magnetic transition. For this reason the specific heat of La 3Pt 4 has been subtracted from the data and C 4f/T (0 K) has been R set to zero. The entropy at 18 K is calculated via S4f ¼ C4f =T dT between 0 K and 18 K. At 18 K the contribution of the f-electrons to the specific heat becomes negligible. The results are given in Table 1. For both compounds, Ce 3Pt 4 and (Ce 0.5La 0.5) 3Pt 4, one obtains Rln2 per mole Cerium at 18 K, which is shown in Fig. 3. This indicates that all degrees of freedom of the presumed doublet ground state take part in the magnetic order. Thus Ce 3Pt 4 is a local moment magnet. As can be seen in Fig. 1 by the high temperature tail of C/T there exist antiferromagnetic correlations well above T N. The plot of the entropy of the f-electron system (Fig. 3) further illuminates this point. At the Ne´el-temperature for both compounds only about have the saturation value
Fig. 1. Low temperature specific heat divided by temperature C/T of Ce 3Pt 4, (Ce 0.5La 0.5) 3Pt 4 and La 3Pt 4. The inset shows the date for Ce 3Pt 4 at temperature close to the phase transition. Lines are guide to the eye.
of S 4f is found (0.4 Rln2/Ce-mol and 0.45 Rln2/Ce-mol for x ¼ 1 and x ¼ 0:5 respectively). In Fig. 4 we show the magnetisation as a function of the applied field M(B 0) at temperatures below (2 K) and above (3.2 K) the Ne´el-temperature (TN ¼ 2:8 K). For both temperatures the saturation value of the magnetisation is nearly the same. This indicates that the fluctuating moment in the paramagnetic phase and the ordered moment in the antiferromagnetic phase are of the same magnitude as expected for a local moment magnet. From the saturation value of the magnetization curves a moment of 1.07 m B/Ce-ion is obtained. The inverse susceptibility plotted vs the temperature T follows a straight line between about 15 K and 150 K (not shown). Fitting a Curie–Weiss law to the data yields an effective moment of 2.2 m B/Ce-ion. The inset of Fig. 4 shows the low field region of the M(B 0)-curve in the ordered regime. One observes two metamagnetic transitions by maxima in dM/dB 0 located at 1.9 kG and 6.5 kG respectively. No hysteresis in the M(B 0) data is found. The fact that we observe two metamagnetic transitions while there is only one crystallographic
Fig. 2. Magnetic susceptibility of Ce 3Pt 4 measured in applied fields of 50 G, 5 kG and 20 kG.
Vol. 108, No. 8
Ce 3Pt 4 ¹ A LOCAL MOMENT ANTIFERROMAGNET
Fig. 3. Entropy of the f-electron system S 4f for the magnetically ordered compounds Ce 3Pt 4 and (Ce 0.5La 0.5) 3Pt 4. The Ne´el-temperatures are marked by the vertical lines. Note that for both compound S 4f saturates at Rln2/Ce-mol well above T N. At the Ne´eltemperature for both compounds only about have the saturation value is found (0.4 Rln2/Ce-mol and 0.45 Rln2/Ce-mol for x ¼ 1 and x ¼ 0:5 respectively). site for the cerium atoms in the Pu 3Pd 4-structure is in agreement with the results obtained for strongly anisotropic antiferromagnets (see for example [4]). 4. CONCLUSION We measured the magnetic susceptibility and the low temperature specific heat of the compounds Ce 3Pt 4 (Ce 0.5La 0.5) 3Pt 4 and La 3Pt 4. Ce 3Pt 4 orders antiferromagnetically at TN ¼ 2:8 K. Two metamagnetic transitions are observed in the M(B 0)-curves of Ce 3Pt 4. From the analysis of the magnetic and caloric data we conclude that Ce 3Pt 4 is a local moment antiferromagnet.
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Fig. 4. Magnetisation M(B 0) of Ce 3Pt 4 measured at 2 K and 3.2 K. These temperatures are below and above the Ne´el temperature (TN ¼ 2:8 K) of this compound. The inset shows the low field data in the ordered state. Here, two metamagnetic transitions are observed. Acknowledgements—Work at the University of Florida was supported by the U.S. Department of Energy, Grant No. De-FG05-86ER45268. REFERENCES 1. Sereni, J.G., in Handbook on the Physics and Chemistry of Rare Earths, Vol. 15 (Edited by K.A. Gschneidner Jr. and L. Eyring). Elsevier, Amsterdam, 1991. 2. Palenzona, A., Journal of the Less-Common Metals, 53, 1977, 133. 3. Moffatt’s Handbook of Binary Phase Diagrams (Edited by J.H. Westbrook). Genium Publishing Corporation, Schenectady, U.S.A., 1992. 4. Gignoux, D., in Materials Science and Technology, Vol. 3A (Edited by R.W. Cahn, P. Haasen, E.J. Kramer and K.H.J. Buschow). VCH Verlagsgesellschaft, Weinheim, 1992.
Pergamon
PII: S0038–1098(98)00406-2
Ce 3Pt 4 ¹ A LOCAL MOMENT ANTIFERROMAGNET WITH TWO METAMAGNETIC TRANSITIONS G.-F. von Blanckenhagen and G.R. Stewart* Institut fu¨r Physik, Universita¨t Augsburg, Universita¨tsstr. 1, 86159 Augsburg, Germany (Received 25 March 1998; accepted in revised form 12 August 1998 by F.J. Di Salvo) In this paper we present the data of the magnetic susceptibility and the low temperature specific heat of Ce 3Pt 4. The large anomaly in the specific heat and the behaviour of the magnetic susceptibility are explained by an antiferromagnetic phase transition at TN ¼ 2:8 K. The entropy due to the phase transition is Rln2 per mole Cerium indicating a local moment antiferromagnet. 䉷 1998 Elsevier Science Ltd. All rights reserved Keywords: A. magnetically ordered materials, D. heat capacity.
1. INTRODUCTION Cerium intermetallic compounds show a large variety in their physical behaviour at low temperatures because of the crucial role of the 4f-electrons in these compounds. Different ground states like superconductivity, antiferroand ferromagnetic order and heavy fermion behaviour have been reported (for an overview see example [1]). In order to look for new cerium compounds showing interesting physical behaviour we measured the magnetic susceptibility and the low temperature specific heat of Ce 3Pt 4. This compound crystallizes in the rhombohedral Pu 3Pd 4-structure [2]. The lattice parameters, given in the ˚ hexagonal setting, have been reported to a ¼ 13:657 A ˚ [2]. The smallest Ce–Ce distance is 3.5 A ˚. and c ¼ 5:781 A To our knowledge no previous measurements of the low temperature physical properties of this compound have been published. 2. EXPERIMENTAL Samples of Ce 3Pt 4, La 3Pt 4 and (Ce 0.5La 0.5) 3Pt 4 were prepared by arc melting together stoichiometric amounts of the constituting elements. Since Ce 3Pt 4 and La 3Pt 4 form peritectically (see for example [3] for phase diagrams) the samples have been annealed at 900⬚C for 3 weeks.
* Corresponding author. Also at the Department of Physics, University of Florida, Gainesville, FL 32611-8440, U.S.A.
For X-ray diffraction we used a Siemens D5000 diffractometer. Magnetic measurements were performed with a Quantum Design SQUID magnetometer. We measured the specific heat with a relaxation method in the temperature range 0.35 K–20 K. The error of the specific heat is ⫾3% between 1.3 K and 20 K, ⫾5% below 1.3 K. 3. RESULTS AND DISCUSSION The X-ray diffractometer patterns show that the samples are single phase. All lines can be indexed assuming the rhombohedral Pu 3Pd 4-structure [2]. The ˚ and c ¼ 5:780 A ˚ for lattice parameters are a ¼ 13:653 A Ce 3Pt 4. These values are in good agreement with those obtained in [2]. For (Ce 0.5La 0.5)Pt 4 and La 3Pt 4 we find ˚ and c ¼ 5:801 A ˚ and a ¼ 13:812 A ˚ and a ¼ 13:733 A ˚ c ¼ 5:812 A respectively (see also Table 1). Figure 1 shows the specific heat divided by temperature of Ce 3Pt 4, La 3Pt 4 and (Ce 0.5La 0.5)Pt 4. At temperatures above about 15 K the three curves show similar behaviour. Since La has no f-electrons this supports the view that the f-electrons are responsible for the observed anomaly, but do not contribute to the behaviour at higher temperatures. The results of the magnetic measurements show that the anomaly in C/T is due to an antiferromagnetic phase transition. Figure 2 shows the magnetic susceptibility (M/H) of Ce 3Pt 4 measured in a magnetic field of 50 G, 5 kG and 20 kG. The typical kink of an antiferromagnetic transition is observed and the maximum of M/H (T) shifts to lower temperatures with increasing magnetic field. In
535
536
Ce 3Pt 4 ¹ A LOCAL MOMENT ANTIFERROMAGNET
Vol. 108, No. 8
Table 1. Lattice parameters, Ne´el temperature and entropy of the f-electron system for the compounds Ce 3Pt 4, (Ce 0.5La 0.5) 3Pt 4 and La 3Pt 4
Ce 3Pt 4 (Ce 0.5La 0.5) 3Pt 4 La 3Pt 4 a b
˚] a [A ˚) (⫾0.005 A
˚] c [A ˚) (⫾0.005 A
T N [K] a (⫾0.15 K)
S 4f [Rln2] b (⫾0.1 Rln2)
13.653 13.733 13.812
5.780 5.801 5.812
2.8 1.6 –
3 1.5 –
Maximum in the specific heat C. At 18 K. To calculate the entropy of the f-electron system the specific heat of La 3Pt 4 has been subtracted from the data. We set C 4f/T (0 K) to zero, whichR is supported by the extrapolation of the measured data points to 0 K. The entropy at 18 K is calculated via S4f ¼ C4f =T dT between 0 K and 18 K. At 18 K the contribution of the f-electrons to the specific heat vanishes (see Fig. 1).
accordance with Fig. 2 the magnetisation M(H) measured at 10 K is linear up to 10 kG. From the data of the specific heat measurements shown in Fig. 1 we calculated the entropy of the magnetic transition. For this reason the specific heat of La 3Pt 4 has been subtracted from the data and C 4f/T (0 K) has been R set to zero. The entropy at 18 K is calculated via S4f ¼ C4f =T dT between 0 K and 18 K. At 18 K the contribution of the f-electrons to the specific heat becomes negligible. The results are given in Table 1. For both compounds, Ce 3Pt 4 and (Ce 0.5La 0.5) 3Pt 4, one obtains Rln2 per mole Cerium at 18 K, which is shown in Fig. 3. This indicates that all degrees of freedom of the presumed doublet ground state take part in the magnetic order. Thus Ce 3Pt 4 is a local moment magnet. As can be seen in Fig. 1 by the high temperature tail of C/T there exist antiferromagnetic correlations well above T N. The plot of the entropy of the f-electron system (Fig. 3) further illuminates this point. At the Ne´el-temperature for both compounds only about have the saturation value
Fig. 1. Low temperature specific heat divided by temperature C/T of Ce 3Pt 4, (Ce 0.5La 0.5) 3Pt 4 and La 3Pt 4. The inset shows the date for Ce 3Pt 4 at temperature close to the phase transition. Lines are guide to the eye.
of S 4f is found (0.4 Rln2/Ce-mol and 0.45 Rln2/Ce-mol for x ¼ 1 and x ¼ 0:5 respectively). In Fig. 4 we show the magnetisation as a function of the applied field M(B 0) at temperatures below (2 K) and above (3.2 K) the Ne´el-temperature (TN ¼ 2:8 K). For both temperatures the saturation value of the magnetisation is nearly the same. This indicates that the fluctuating moment in the paramagnetic phase and the ordered moment in the antiferromagnetic phase are of the same magnitude as expected for a local moment magnet. From the saturation value of the magnetization curves a moment of 1.07 m B/Ce-ion is obtained. The inverse susceptibility plotted vs the temperature T follows a straight line between about 15 K and 150 K (not shown). Fitting a Curie–Weiss law to the data yields an effective moment of 2.2 m B/Ce-ion. The inset of Fig. 4 shows the low field region of the M(B 0)-curve in the ordered regime. One observes two metamagnetic transitions by maxima in dM/dB 0 located at 1.9 kG and 6.5 kG respectively. No hysteresis in the M(B 0) data is found. The fact that we observe two metamagnetic transitions while there is only one crystallographic
Fig. 2. Magnetic susceptibility of Ce 3Pt 4 measured in applied fields of 50 G, 5 kG and 20 kG.
Vol. 108, No. 8
Ce 3Pt 4 ¹ A LOCAL MOMENT ANTIFERROMAGNET
Fig. 3. Entropy of the f-electron system S 4f for the magnetically ordered compounds Ce 3Pt 4 and (Ce 0.5La 0.5) 3Pt 4. The Ne´el-temperatures are marked by the vertical lines. Note that for both compound S 4f saturates at Rln2/Ce-mol well above T N. At the Ne´eltemperature for both compounds only about have the saturation value is found (0.4 Rln2/Ce-mol and 0.45 Rln2/Ce-mol for x ¼ 1 and x ¼ 0:5 respectively). site for the cerium atoms in the Pu 3Pd 4-structure is in agreement with the results obtained for strongly anisotropic antiferromagnets (see for example [4]). 4. CONCLUSION We measured the magnetic susceptibility and the low temperature specific heat of the compounds Ce 3Pt 4 (Ce 0.5La 0.5) 3Pt 4 and La 3Pt 4. Ce 3Pt 4 orders antiferromagnetically at TN ¼ 2:8 K. Two metamagnetic transitions are observed in the M(B 0)-curves of Ce 3Pt 4. From the analysis of the magnetic and caloric data we conclude that Ce 3Pt 4 is a local moment antiferromagnet.
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Fig. 4. Magnetisation M(B 0) of Ce 3Pt 4 measured at 2 K and 3.2 K. These temperatures are below and above the Ne´el temperature (TN ¼ 2:8 K) of this compound. The inset shows the low field data in the ordered state. Here, two metamagnetic transitions are observed. Acknowledgements—Work at the University of Florida was supported by the U.S. Department of Energy, Grant No. De-FG05-86ER45268. REFERENCES 1. Sereni, J.G., in Handbook on the Physics and Chemistry of Rare Earths, Vol. 15 (Edited by K.A. Gschneidner Jr. and L. Eyring). Elsevier, Amsterdam, 1991. 2. Palenzona, A., Journal of the Less-Common Metals, 53, 1977, 133. 3. Moffatt’s Handbook of Binary Phase Diagrams (Edited by J.H. Westbrook). Genium Publishing Corporation, Schenectady, U.S.A., 1992. 4. Gignoux, D., in Materials Science and Technology, Vol. 3A (Edited by R.W. Cahn, P. Haasen, E.J. Kramer and K.H.J. Buschow). VCH Verlagsgesellschaft, Weinheim, 1992.