Low temperature magnetic properties of NpNi$_5$

TL;DR

This study provides a detailed experimental analysis of NpNi$_5$, revealing its magnetic properties, electron localization, and ground state characteristics, and compares findings with theoretical models.

Contribution

It offers new insights into the magnetic behavior and electronic structure of NpNi$_5$, combining multiple experimental techniques with theoretical discussion.

Findings

Np ions have a 5f-shell occupation number close to 4.

The compound exhibits ferromagnetism below 16 K with specific orbital and spin moments.

The 5f electrons show localized behavior, and the ground state involves slow relaxation phenomena.

Abstract

We present the result of an extended experimental characterization of the hexagonal intermetallic Haucke compound NpNi$_{5}$. By combining macroscopic and shell-specific techniques, we determine the 5$f$-shell occupation number $n_f$ close to 4 for the Np ions, together with orbital and spin components of the ordered moment in the ferromagnetic phase below T$_C$ = 16 K ($\mu_{S}$ = -1.88~$\mu_{B}$ and $\mu_{L}$ = 3.91~$\mu_{B}$). The apparent coexistence of ordered and disordered phases observed in the M\"{o}ssbauer spectra is explained in terms of slow relaxation between the components of a quasi-triplet ground state. The ratio between the expectation value of the magnetic dipole operator and the spin magnetic moment ($3\langle T_{z}\rangle/ \langle S_{z}\rangle$ = +1.43) is positive and large, suggesting a localized character of the 5$f$ electrons. The angular part of the spin-orbit coupling ($\langle\vec{\ell}\cdot\vec{s}\rangle$ = -5.55) is close to the value of -6.25 calculated for trivalent Np ions in intermediate coupling approximation. The results are discussed against the prediction of first-principle electronic structure calculations based on the spin-polarized local spin density approximation plus Hubbard interaction, and of a mean field model taking into account crystal field and exchange interactions.