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Magnetic properties of UxPty compounds
Journal of Magnetism and Magnetic Materials 31-34 (1983) 240-242
240
MAGNETIC
PROPERTIES
P.H. FRINGS,
OF UxPty COMPOUNDS
J.J.M. FRANSE,
F . R . d e B O E R a n d A. M E N O V S K Y
Natuurkundig Laboratorium, Valckenierstraat 65, 1018 XE Amsterdam, The Netherlands
Magnetization, magnetic susceptibility and specific heat studies have been performed on a series of uranium-platinum compounds. The magnetic order of UPt is confirmed, whereas in UPt 2 and UP% no magnetic order is observed. A n antiferromagnetic type of order, suggested for UPt 3 by a peak in the susceptibility near 19 K and by the magnetic transition in the magnetization curve at 4.2 K near 19 T is not supported by the specific heat data in which no anomaly near 19 K was found. Additional magnetization and susceptibility measurements on a single-crystalline sample exclude a simple type of antiferromagnetic order. The low-temperature specific heat of UPt 3 is rather similar to that of the "spin-fluctuation" compound UA12.
In the series of u r a n i u m - p l a t i n u m compounds the nearest U - U distance varies from 3.61 A in U P t to 5.25 ,~ in U P t s , causing a considerable decrease in the overlap of the 5f orbitals. Magnetism is supposed to be of an itinerant nature in UPt whereas for the other compounds the localized character is supposed to increase due to the increasing U - U distances. In this contribution we report on high-temperature susceptibility, specific heat, and high-magnetic-field measurements on UPt, UPt 2, UPt 3 and U P t 5. Parts of the results have already been published [1 ]. In analysing the high-temperature susceptibility measurements on polycrystalline samples (see fig. 1) we derive two clearly different values for the effective magnetic moments: (3.55 + 0.1)# B for U P t and UPt 2 and ( 3 . 0 _ 0.1)# s for U P t 3. As far as Hunds rules are obeyed for the U-ion we conclude that the value of 3.55#s is close to the value for the effective moment in a 5f 2 and a 5f 3 configuration, being 3.57 and 3.62#B, respectively. The X - l versus T plot for UPt is curved at lower temperatures leading to a value for the effective moment just above the ordering temperature of 27 K, approximately, of (2 + 0.2)~ s. The curvature in the X- i versus T plot of a polycrystalline UPt 3 sample (see fig. 1) could be due to the strong magnetic anisotropy that has been observed in magnetization curves of a single-crystalline sample at 4.2 K. Susceptibility measurements on different UPt s samples at room temperature differed by about 15%. For one of the samples the susceptibility measurements at high temperatures point to a value for the effective moment of - 4 # B and to an unreasonably large value for the paramagnetic Curie-temperature of about 1000 K. In analysing the low-temperature susceptibility data it is clear that no magnetic order is present in UPt 2 and U P t s. The ferromagnetism of UPt 2 that has been reported by Schneider and Laubschat [2] is probably due to the presence of a second phase (UPt). For UPt, where magnetic order starts at 27 K, additional ac-susceptibility measurements reveal a pronounced peak at 0304-8853/83/0000-0000/$03.00
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TEK] Fig. 1. Reciprocal susceptibility versus temperature for UPt (O), UPt 2 (zx), OPt 3 (D) and UPt 5 (v). Data for UPt 3 below 300 K on a single crystal along the a-axis. (The data below and above 300 K have been measured in different equipment on different samples.) 19 K and two weak anomalies around 6 K and 27 K, suggesting different types of magnetic order. Similar phenomena have also been observed for PuPt by Smith and Hill [3]. The peak in the susceptibility at 19 K corresponds to a peak in the C / T versus T curve which was already evident in the experiments by Luengo et al. [4] but was ignored at that time. Single-crystalline data on UPt 3 show an anisotropy in the susceptibility between the c-axis and the basal plane, see fig. 2. In the basal plane X - l versus T follows a Curie-Weiss law with #eft = (2.6 + 0.2)/a s and 0 = 50 K. Next we discuss the specific heat data, shown in fig. 3. A remarkable feature is the large value for lim r ~ o C / T in all compounds, especially in UPt 3. There
© 1983 N o r t h - H o l l a n d
241
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Fig. 2. Susceptibility versus temperature of single-crystalline UPt 3 along the a (ra), b ( × ) and c ( 0 ) directions.
is hardly any effect of an applied magnetic field of 5 T on the specific heat, with the exception of the peak in the C / T versus T z curve of UPt at 19 K that is very sensitive to the field. A description of the specific heat with the usual expression C = 7 T + f l T 3 is far from appropriate for UPt 3 and is not more than a first
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Fig. 3. Specific heat of U P t ( O ) , UPt2 (zx), U P t 3 (12) and U P t s (v), plotted as C/T versus T 2. Results in a field of 5 T for the various compounds: ( + ).
approximation for the other compounds. Instead, the specific heat data for UPt 3 are rather similar to those of the "spin-fluctuation" compound UA12. We want to draw special attention to the results for UPt 3 in the region 15-20 K, where no sign of magnetic order is found, although Schneider and Laubschaft [2] concluded from low-field magnetization measurements to an antiferromagnetic transition at 16 K. This conclusion could also be drawn from high-field magnetization measurements on a polycrystalline sample of UPt 3 at 4.2 K and 1.4 K. Finally we present some high-magnetic-field data of the uranium-platinum compounds and the pressure dependence of the magnetization of UPt, see fig. 4. The magnetization curves at 4.2 K confirm the magnetic order in UPt, with a spontaneous magnetization of (0.46 + 0.04)/xB/f.u. and the paramagnetic behaviour of UPt 2 and UPt 5. Previous magnetization measurements under pressure revealed a magnetic transition in UPt between 2 and 4 T [1] and an enormous decrease of the magnetization at fields less than 2 T [5,1]. This transition is under study in neutron diffraction experiments under high pressure. For the hexagonal compound UPt 3, magnetization curves have been measured on a singlecrystalline sphere along the three different crystallographic directions (a-, b-, and c-axis). The hexagonal axis turns out to be the hard direction for the magnetization and no anisotropy is observed in the basal plane. The dependence of these high-field results on the direction of the field with respect to the crystal axes excludes a simple antiferromagnetic order in this compound.
242
P.H. Frings et al. / Magnetic properties of UxPty compounds
This dependence of the magnetization in various directions is observed, for instance, for a spiral arrangement of moments around the c-axis, as is the case in M n A u 2, in which c o m p o u n d a large anisotropy between the c-axis and the basal plane is present. However, the maximum in the susceptibility of UPt 3 at 19 K in zero field and at 18 T for T = 4.2 K could equally point to spin-fluctuation phenomena.
References [1] J.J.M. Franse, P.H. Frings, F.R. de Boer and A. Menovsky, in: Physics of Solids under High Pressure, eds. J.S. Schilling and R.N. Shelton (1981) p. 181. [2] W.D. Schneider and C. Laubschat, Phys. Rev. B23 (1981) 997. [3] J.L. Smith and H.H. Hill, Phys. Stat. Sol. (b)64 (1974) 343. [4] C.A. Luengo, M.B. Maple and J.O. Huber, J. Magn. Magn. Mat. 3 (1976) 305. [5] J.G. Huber, M.B. Maple and D. Wohlleben, J. Magn. Magn. Mat. 1 (1975) 58.
240
MAGNETIC
PROPERTIES
P.H. FRINGS,
OF UxPty COMPOUNDS
J.J.M. FRANSE,
F . R . d e B O E R a n d A. M E N O V S K Y
Natuurkundig Laboratorium, Valckenierstraat 65, 1018 XE Amsterdam, The Netherlands
Magnetization, magnetic susceptibility and specific heat studies have been performed on a series of uranium-platinum compounds. The magnetic order of UPt is confirmed, whereas in UPt 2 and UP% no magnetic order is observed. A n antiferromagnetic type of order, suggested for UPt 3 by a peak in the susceptibility near 19 K and by the magnetic transition in the magnetization curve at 4.2 K near 19 T is not supported by the specific heat data in which no anomaly near 19 K was found. Additional magnetization and susceptibility measurements on a single-crystalline sample exclude a simple type of antiferromagnetic order. The low-temperature specific heat of UPt 3 is rather similar to that of the "spin-fluctuation" compound UA12.
In the series of u r a n i u m - p l a t i n u m compounds the nearest U - U distance varies from 3.61 A in U P t to 5.25 ,~ in U P t s , causing a considerable decrease in the overlap of the 5f orbitals. Magnetism is supposed to be of an itinerant nature in UPt whereas for the other compounds the localized character is supposed to increase due to the increasing U - U distances. In this contribution we report on high-temperature susceptibility, specific heat, and high-magnetic-field measurements on UPt, UPt 2, UPt 3 and U P t 5. Parts of the results have already been published [1 ]. In analysing the high-temperature susceptibility measurements on polycrystalline samples (see fig. 1) we derive two clearly different values for the effective magnetic moments: (3.55 + 0.1)# B for U P t and UPt 2 and ( 3 . 0 _ 0.1)# s for U P t 3. As far as Hunds rules are obeyed for the U-ion we conclude that the value of 3.55#s is close to the value for the effective moment in a 5f 2 and a 5f 3 configuration, being 3.57 and 3.62#B, respectively. The X - l versus T plot for UPt is curved at lower temperatures leading to a value for the effective moment just above the ordering temperature of 27 K, approximately, of (2 + 0.2)~ s. The curvature in the X- i versus T plot of a polycrystalline UPt 3 sample (see fig. 1) could be due to the strong magnetic anisotropy that has been observed in magnetization curves of a single-crystalline sample at 4.2 K. Susceptibility measurements on different UPt s samples at room temperature differed by about 15%. For one of the samples the susceptibility measurements at high temperatures point to a value for the effective moment of - 4 # B and to an unreasonably large value for the paramagnetic Curie-temperature of about 1000 K. In analysing the low-temperature susceptibility data it is clear that no magnetic order is present in UPt 2 and U P t s. The ferromagnetism of UPt 2 that has been reported by Schneider and Laubschat [2] is probably due to the presence of a second phase (UPt). For UPt, where magnetic order starts at 27 K, additional ac-susceptibility measurements reveal a pronounced peak at 0304-8853/83/0000-0000/$03.00
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TEK] Fig. 1. Reciprocal susceptibility versus temperature for UPt (O), UPt 2 (zx), OPt 3 (D) and UPt 5 (v). Data for UPt 3 below 300 K on a single crystal along the a-axis. (The data below and above 300 K have been measured in different equipment on different samples.) 19 K and two weak anomalies around 6 K and 27 K, suggesting different types of magnetic order. Similar phenomena have also been observed for PuPt by Smith and Hill [3]. The peak in the susceptibility at 19 K corresponds to a peak in the C / T versus T curve which was already evident in the experiments by Luengo et al. [4] but was ignored at that time. Single-crystalline data on UPt 3 show an anisotropy in the susceptibility between the c-axis and the basal plane, see fig. 2. In the basal plane X - l versus T follows a Curie-Weiss law with #eft = (2.6 + 0.2)/a s and 0 = 50 K. Next we discuss the specific heat data, shown in fig. 3. A remarkable feature is the large value for lim r ~ o C / T in all compounds, especially in UPt 3. There
© 1983 N o r t h - H o l l a n d
241
P.H. Frings et al. / Magnetic properties of UxPty compounds i v
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Fig. 2. Susceptibility versus temperature of single-crystalline UPt 3 along the a (ra), b ( × ) and c ( 0 ) directions.
is hardly any effect of an applied magnetic field of 5 T on the specific heat, with the exception of the peak in the C / T versus T z curve of UPt at 19 K that is very sensitive to the field. A description of the specific heat with the usual expression C = 7 T + f l T 3 is far from appropriate for UPt 3 and is not more than a first
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Fig. 3. Specific heat of U P t ( O ) , UPt2 (zx), U P t 3 (12) and U P t s (v), plotted as C/T versus T 2. Results in a field of 5 T for the various compounds: ( + ).
approximation for the other compounds. Instead, the specific heat data for UPt 3 are rather similar to those of the "spin-fluctuation" compound UA12. We want to draw special attention to the results for UPt 3 in the region 15-20 K, where no sign of magnetic order is found, although Schneider and Laubschaft [2] concluded from low-field magnetization measurements to an antiferromagnetic transition at 16 K. This conclusion could also be drawn from high-field magnetization measurements on a polycrystalline sample of UPt 3 at 4.2 K and 1.4 K. Finally we present some high-magnetic-field data of the uranium-platinum compounds and the pressure dependence of the magnetization of UPt, see fig. 4. The magnetization curves at 4.2 K confirm the magnetic order in UPt, with a spontaneous magnetization of (0.46 + 0.04)/xB/f.u. and the paramagnetic behaviour of UPt 2 and UPt 5. Previous magnetization measurements under pressure revealed a magnetic transition in UPt between 2 and 4 T [1] and an enormous decrease of the magnetization at fields less than 2 T [5,1]. This transition is under study in neutron diffraction experiments under high pressure. For the hexagonal compound UPt 3, magnetization curves have been measured on a singlecrystalline sphere along the three different crystallographic directions (a-, b-, and c-axis). The hexagonal axis turns out to be the hard direction for the magnetization and no anisotropy is observed in the basal plane. The dependence of these high-field results on the direction of the field with respect to the crystal axes excludes a simple antiferromagnetic order in this compound.
242
P.H. Frings et al. / Magnetic properties of UxPty compounds
This dependence of the magnetization in various directions is observed, for instance, for a spiral arrangement of moments around the c-axis, as is the case in M n A u 2, in which c o m p o u n d a large anisotropy between the c-axis and the basal plane is present. However, the maximum in the susceptibility of UPt 3 at 19 K in zero field and at 18 T for T = 4.2 K could equally point to spin-fluctuation phenomena.
References [1] J.J.M. Franse, P.H. Frings, F.R. de Boer and A. Menovsky, in: Physics of Solids under High Pressure, eds. J.S. Schilling and R.N. Shelton (1981) p. 181. [2] W.D. Schneider and C. Laubschat, Phys. Rev. B23 (1981) 997. [3] J.L. Smith and H.H. Hill, Phys. Stat. Sol. (b)64 (1974) 343. [4] C.A. Luengo, M.B. Maple and J.O. Huber, J. Magn. Magn. Mat. 3 (1976) 305. [5] J.G. Huber, M.B. Maple and D. Wohlleben, J. Magn. Magn. Mat. 1 (1975) 58.