Magnetic properties of neptunium Laves phases: NpMn2, NpFe2, NpCo2, and NpNi2

Abstract

The magnetic properties of the cubic Laves phases ($C-15\mathrm{}$ structure) Np${\mathrm{Mn}}_2$, Np${\mathrm{Fe}}_2$, Np${\mathrm{Co}}_2$, and Np${\mathrm{Ni}}_2$ have been studied from 4 to 300 K by means of magnetization, neutron-diffraction, and nuclear-$\gamma$-ray resonance (Mössbauer) experiments. The respective ordering temperatures (and types of ordering) are 18 K (ferro), ∼500 K (ferro), 15 K (antiferro), and 32 K (ferro). Magnetic moments are present on the neptunium atoms in all compounds with values varying from $1.2{\mu }_B$ in Np${\mathrm{Ni}}_2$ to $0.3{\mu }_B$ in Np${\mathrm{Mn}}_2$. On the transition-metal site, only the iron atom carries a magnetic moment in zero field; in high magnetic fields, a small moment of $\sim 0.2{\mu }_B$ is observed on the manganese atom. No simple correlation appears to exist among the values of the magnetic moments, the Np-Np distance, and the ordering temperature. A feature of the data is the large high-field susceptibility in the ferromagnetic compounds at low temperatures. Some magnetic properties, such as the large anisotropy ($〈111〉$ is the easy axis) and the linear relationship between the hyperfine field and the neptunium magnetic moment, are best interpreted in terms of localized $5f$ electrons on the actinide ion, whereas other properties, such as the low values of the neptunium moments and the high-field susceptibility, suggest strong hybridization between the $5f$ and $3d$ electrons.