Magnetic Properties of Protactinium Tetrachloride

Magnetic susceptibility of PaCl₄ was measured from 3.2-296°K with a vibrating-sample magnetometer at fields up to 12 000 Oe. PaCl₄ has a ferromagnetic transition at T_c=182±2° K. Below T_c the magnetization deviates markedly from Brillouin behavior and is far from saturation at 12 kOe; hysteresis is absent above 3.3°K. A high degree of covalency is suggested to explain the high Curie point; a large anisotropy energy is indicated by the lack of saturation. From 180-210°K, PaCl₄ obeys the Curie—Weiss law, χ=C/(T-θ), with θ=157°K and μ _eff=1.04 μ B. The measured paramagnetic properties were compared to those calculated from crystal-field theory, with and without J mixing included. The comparison indicates that J mixing is significant in PaCl₄.