Mössbauer Spectra and Magnetic Susceptibility of Neptunium Tetrachloride below 85°K

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Abstract
Magnetic properties of NpCl4 were studied from 2.6 to 8.5°K, with the Mössbauer resonance of the 59.54‐keV γ ray in 237Np, and with magnetic susceptibility measurements from 4.3 to 43°K using a vibrating‐sample magnetometer at applied fields of 50 to 10 000 Oe. A ferromagnetic transition was found at 6.7 ± 0.1°K. Above the Curie temperature, NpCl4 obeys the Curie–Weiss law χ = C / (T − θ) with θ equal to 6.9°K, giving an effective moment of 3.08 μ B . The Mössbauer data define the temperature dependence of the hyperfine fields in NpCl4. The behavior of the magnetic hyperfine field at the Np nucleus parallels that of the magnetization in the ferromagnetic region, giving an extrapolated H(0) of 3000 kOe. In the paramagnetic region only a small temperature‐independent quadrupole splitting was observed.

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