Hyperfine Interaction and Susceptibility in Some Actinide Metals and Intermetallic Compounds

Abstract

Magnetic effects in a number of actinide metals and intermetallic compounds are being investigated utilizing the 59.6 keV Mössbauer resonance of 237Np in conjunction with bulk susceptibility measurements. The cubic Laves phase compound NpAl2 is found to be ferromagnetic. The paramagnetic moment is 2.30 μB, somewhat lower than in other cubic Np magnetic materials. The saturation magnetic hyperfine field is found to be (3155±50) kOe. Taking H(T)/H(0) = D[1−T/Tc]β, we find D = 1.48±0.10, Tc = 55.8±0.1°K and β = 0.355±0.017 for 0.994>T/Tc>0.83. At lower temperatures, departure from molecular field behavior is seen. In cubic actinide magnetic materials, an electric-quadrupole hyperfine interaction will be observed due to alignment of the unquenched orbital momentum. In NpAl2 we find e2qQ = − (27±1) MHz. For NpC, which has the NaCl structure, we obtain the hyperfine parameters Hi = (4789±50) kOe and e2qQ = (577±58) MHz at 4.2°K. Metallic neptunium (orthorhombic) and americium (hexagonal) show no magnetic hyperfine coupling at temperatures as low as 1.7°K.