Magnetism in californium

A SQUID-based magnetic susceptometer has been constructed for studying small radioactive samples at temperatures below 350 K and in magnetic fields up to 50 kilogauss. The device has been used to study californium (element 98) in a number of solid-state forms: the dhcp metal, several oxides (Cf₂O₃ in both the bcc and monoclinic structures, Cf₇O₁₂, CfO₂ and BaCfO₃), several monopnictides (CfN, CfAs and CfSb) and the trichloride (in both the hexagonal and orthorhombic structures). All of these materials were studied in polycrystalline form, and hexagonal CfCl₃ was studied in single-crystal form as well. The susceptometer has the sensitivity to measure samples containing less than 10 micrograms of californium.

The magnetic susceptibilities of all of the californium materials at temperatures above about 100 K are described well by the Curie-Weiss relationship χ_M(T) = (N_Aμ²_eff) ÷ (3k_B(T-θ_P). This behavior is consistent with the assumption that the magnetic 5f electrons are localized and that the paramagnetic behavior can be interpreted in terms of the properties of the free ion. The measured values of the effective paramagnetic moment, μ_efffor all the californium materials that were studied are reasonably consistent with theoretical values based on intermediate coupling models. All of the californium materials showed some indications of cooperative magnetic effects. The dhcp metal was observed to order ferromagnetically at 52 K, and all of the californium compounds studied showed signs of antiferromagnetic ordering, mostly at temperatures below 25 K.

Although the 5f electrons of californium are not severely influenced by the solid state environment, crystal field effects are evident in the magnetic properties. For the metal, oxides and monopnictides, estimates of the crystal field strengths have been made based on extrapolations from the known crystal fields of analogous lanthanide and light actinide materials. For hexagonal CfCl₃, where single-crystal samples are available, anisotropy in the magnetic susceptibility allows a direct determination of the crystal field parameters.