Uranium ferromagnet with negligible magnetocrystalline anisotropy - $\mathrm{U_{4}Ru_{7}Ge_{6}}$

TL;DR

This study reveals that $ ext{U}_4 ext{Ru}_7 ext{Ge}_6$ is an almost isotropic uranium ferromagnet with negligible magnetocrystalline anisotropy, contrasting typical uranium compounds, and combines experimental and theoretical analyses to understand its unique magnetic properties.

Contribution

The paper reports the discovery of near-isotropic ferromagnetism in $ ext{U}_4 ext{Ru}_7 ext{Ge}_6$, supported by comprehensive experimental measurements and first-principles calculations, highlighting its unusual magnetic anisotropy behavior.

Findings

$ ext{U}_4 ext{Ru}_7 ext{Ge}_6$ exhibits a low anisotropy field of ~0.3 T.

The easy magnetization axis switches from [111] to [001] at 5.9 K.

First-principles calculations support the experimental magnetic moment orientation.

Abstract

Strong magnetocrystalline anisotropy is a well-known property of uranium compounds. The almost isotropic ferromagnetism in $\mathrm{U_{4}Ru_{7}Ge_{6}}$ reported in this paper represents a striking exception. We present results of magnetization, AC susceptibility, thermal expansion, specific heat and electrical resistivity measurements performed on a $\mathrm{U_{4}Ru_{7}Ge_{6}}$ single crystal at various temperatures and magnetic fields and discuss them in conjunction with results of first-principles electronic-structure calculations. $\mathrm{U_{4}Ru_{7}Ge_{6}}$ behaves as an itinerant 5$f$-electron ferromagnet ($T_{\mathrm{C}}= 10.7 K$, $\mu_{\mathrm{S}}= 0.85 \mu_{\mathrm{B}}/f.u.$ at 1.9 K. The ground-state easy-magnetization direction is along the [111] axis of the cubic lattice. The anisotropy field $\mu_{0}H_{\mathrm{a}}$ along the [001] direction is only of $\sim0.3 T$, which is at least 3 orders of magnitude smaller value than in other U ferromagnets. At $T_{r}=5.9 K$ the easy magnetization direction changes for [001] which holds at temperatures up to $T_{\mathrm{C}}$. The magnetoelastic interaction induces a rhombohedral (tetragonal) distortion of the paramagnetic cubic crystal lattice in case of the [111]([001]) easy-magnetization direction. The rhombohedral distortion is connected with two crystallographically inequivalent U sites. The ab initio calculated ground-state magnetic moment of $1.01 \mu_{\mathrm{B}}/f.u.$ is oriented along [111]. The two crystallographically inequivalent U sites are a consequence of spin-orbit coupling of the U 5$f$-electrons. In the excited state which is only 0.9 meV above the ground state the moment points to the [001] direction in agreement with experiment.