S.E. Nave

Magnetism of the heavy 5f elements

Abstract Magnetic properties of the trivalent lanthanide ions have been classically used to demonstrate the applicability of L-S coupling and Hunds rule to the 4f element series. An examination of similar behavior for the more spatially extended orbitals of the 5f element series has, until the last few years, been limited to lighter members (actinium through americium). Characterization of magnetic properties of heavier ions (curium, berkelium, californium and einsteinium) has recently led to a comparison with the 4f ion homologues of the actinides and to an extension of the test of L-S coupling to a new region of the periodic table. Investigations of the magnetic properties of heavy element compounds and metals have additionally led to some unexpected results which suggest a rich structure of low temperature magnetic ordering. The increasing degree of spatial localization of the heavy actinide orbitals thereby provides another vehicle for studying the theory of magnetic ordering via the conduction electrons.

Magnetic susceptibility of cerium pnictides

Abstract The magnetic susceptibility of microgram quantities of 248 CmP and 248 CmSb has been determined with the use of a SQUID micromagnetic susceptometer over the temperature range 4.2–340 K and in the applied magnetic field range of 0.45–1600 G. The fcc (NaCl-type) samples yield magnetic transitions at 73 K and 162 K for the phosphide and antimonide, respectively. Together, with published magnetic data for CmN and CmAs, these results indicate spatially extended exchange interactions between the relatively localized 5f electrons of the metallic actinide atoms.

Magnetic susceptibility of californium ovides

Magnetic susceptibility measurements have been performed on the ovide system of californium. Poly crystalline samples of Cf2O3 (cubic and monoclinic), Cf7O12, CfO2 and BaCfO3 have been studied at magnetic fields up to 50 kG and temperatures between 4.2 and 340 K. All the ovides evhibit Curie-Weiss behavior above 100 K. In all cases, the paramagnetic effective moments are in reasonable agreement with the free-ion, intermediate-coupling value for the 3+ and 4+ charge states assigned on the basis of the stoichiometry. Antiferromagnetic ordering is observed in all these ovides at temperatures below 20 K. Crystal field effects are evident in the temperature range between about 20 and 100 K.

Magnetic susceptibility of CfN, CfAs and CfSb☆

Magnetic susceptibility measurements are reported for the compounds CfN, CfAs and CfSb. These semimetallic compounds evhibit the NaCl-type cubic crystal structure. For these magnetic measurements the temperature was varied from 4.2 K to 340 K and the magnetic field from 1 to 50 kG. The paramagnetic effective moments derived from Curie-Weiss fits to the data agree, within evperimental error, with a theoretical value of 10.2 μB calculated for the 5f9 electronic state of Cf3+ assuming an intermediate-coupling model. Deviations of the susceptibility from Curie-Weiss behavior at lower temperatures are indicative of magnetic ordering and crystal-field splitting of the ground state. The ordering temperatures of these californium compounds are considerably lower than those observed previously for pnictides of curium and berkelium. This decrease in ordering temperature with increasing atomic number is consistent with the trend observed for the lanthanide monopnictides.

The magnetic susceptibility of 248Cm metal

A SQUID-based micromagnetic susceptometer [1,2] has been used to determine the magnetic susceptibility of 248Cm metal in the temperature range 4.2–320 K and in the applied magnetic field range of 0.45–1400 G. X-ray diffraction indicated that two of the samples of mass 37.2 and 41.2 μg exhibit the dhcp phase and one of mass 152 μg the fcc structure. The nominally dhcp samples exhibit an antiferromagnetic transition at ≈ 65 K. A small second transition near 200 K is observed and influences the high temperature Curie-Weiss fit to the data. For 270 K < T < 307 K and an applied field of 1200 G, x = C/(T + θ), with C = Nμ2/3k and μ = 5.5 μB, θ = −176 K. The fcc sample exhibits a ferrimagnetic transition in the neighborhood of 200 K and at low temperature has a saturated magnetic moment per atom of 0.4 Bohr magnetons in applied fields above 1200 G. At high temperatures three interpretations of the susceptibility may be made. 1. (a) Two regions of Curie-Weiss behavior x = C/(T + θ) with C = Nμ2/3k. For 200 K < T < 300 K, μ = 6.2 μB, θ = −202 K. For 300 K < T < 340 K, μ = 7.7 μB, and θ = −138 K. 2. (b) A modified Curie-Weiss behavior x = x0 + C/(T + θ), with C = Nμ2/3k. Here x0 = 530 × 10−6, μ = 5.36 μB, and θ = −204 K. 3. (c) A ferrimagnetic behavior x = (C1 + C2)T − 2λC1C2T2 − TN2 with Ci = Nμ2i/6k and μi constrained = μ ± 0.4 μB. Here λ = 231 ± 49, μ = 6.13 ± 0.2 μB, and TN = (205 ± 0.6) K.

Magnetism of californium metal

Magnetic susceptibility measurements have been made on samples of californium-249 metal having the dhcp crystal structure. At temperatures between 100 and 340 K and at fields up to 50 kilogauss, the samples exhibit Curie-Weiss behavior. Previous measurements extending only to 1.6 kilogauss gave a magnetic moment per atom of μeff = (10.7±0.2)μB and paramagnetic Weiss temperatures. Θp, in the range of −2 to −16 K for two samples. These values of μeff are in good agreement with the value expected (10.62μB) for a free-ion 5f9 configuration based on an L-S coupling scheme and Hunds rule. In this work extended to higher fields, two additional samples give the values μeff = (9.7 ± 0.2)μB and Θp = −40 K. At low temperatures the samples exhibit an ordered magnetic transition to a state with a saturated moment of 6.1μB/atom when extrapolated to infinitely high field. The low temperature ordered phase exists at temperatures below Tc = (51 ± 2) K as determined from constant magnetization plots.

Magnetic susceptibility of CfCl3 and its dependence on crystal structure

Abstract Magnetic measurements are presented for polycrystalline samples of californium trichloride in both a hexagonal (UC1 3 type; californium(III) CN = 9) and an orthorhombic (PuBr 3 type; californium(III) CN = 8) crystal form. In the Curie-Weiss temperature regions (orthorhombic, 100–340 K, hexagonal, 60–340 K) the Weiss constant θ p is significantly larger for the orthorhombic form. As the temperature decreases, the susceptibility of the orthorhombic form begins to deviate from Curie-Weiss behavior at a higher temperature than the hexagonal form indicating a larger crystal-field splitting in the orthorhombic form. At temperatures below 15 K a field-dependent transition is observed. In a field of 5 kG an antiferromagnetic maximum in the susceptibility is observed at 7 K and 13 K for the hexagonal and orthorhombic forms respectively. For 30 kG both types exhibit monotonically increasing susceptibilities with decreasing temperature and the hexagonal form shows saturation behavior in the susceptibility vs . temperature plots with a saturation moment of about 6 μ B atom −1 .

The Magnetic Susceptibility of 249Bk Metal

Magnetic susceptibility measurements have been made on a well characterized sample of 249 Bk metal using a SQUID micromagnetic susceptometer1 which has been specifically designed for small volume samples. The sample had ∿12% Cf impurity at the time of measurement and was in the pure dhcp crystal structure. The only previous magnetic measurements on berkelium metal were on samples of mixed crystal structure or with higher concentrations of californium daughter and were made with a mechanical balance susceptometer.2