Effect of lattice distortion on U magnetic moments in $\mathrm{U_{4}Ru_{7}Ge_{6}}$ studied by polarized neutron diffraction

TL;DR

This study investigates how lattice distortion influences U magnetic moments in U4Ru7Ge6, revealing site splitting, hybridization effects, and the coupling between magnetic ordering and structural changes through polarized neutron diffraction and DFT calculations.

Contribution

It provides experimental and theoretical evidence of lattice-induced U site splitting and magnetic moment variations in U4Ru7Ge6, emphasizing the role of spin-orbit coupling and hybridization.

Findings

Lattice distortion causes U site splitting into U1 and U2.

Different magnetic moments observed on U1 and U2 sites.

Strong hybridization between U 5f and Ru 4d orbitals.

Abstract

Small spontaneous lattice distortion of a cubic ferromagnet at temperatures below $T_{\mathrm{C}}$ is expected, but usually a neglected phenomenon. This study is focused on such effect on the example of $\mathrm{U_{4}Ru_{7}Ge_{6}}$. We propose the lattice distortion from the cubic space group to a lower symmetry as a result of our DFT calculations. The strong spin-orbit coupling on the U site plays the most essential role, that should lower the symmetry of the system and give rise to the two different U sites (U1 and U2). We bring convincing experimental evidence for this splitting resulting from our polarized neutron diffraction experiment. Proper treatment of the flipping ratios collected in the ordered state and in the paramagnetic state reveals dramatically different magnetic moments on the U1 and U2 sites. We have reached good agreement of the results from the MAXENT method and from direct fitting of the data. The ratio of the orbital and spin component on the U2 site is lower than for the free $\mathrm{U^{3+}}$ or $\mathrm{U^{4+}}$ ion and points to a strong hybridization between the U $5f$ and Ru $4d$ wave functions. The same ratio on the U1 site is unexpectedly low. This would mean that its orbital component is almost quenched. We have observed the absence of a magnetic moment on the Ru1 site, but a rather large induced moment on the Ru2 site. Very similar results were obtained also in the paramagnetic state in the regime induced by the magnetic field. It points to the intimate coupling of the magnetic ordering and structural distortion as it is possible to split the U site in to two different polarizing paramagnetic U moments above $T_{\mathrm{C}}$ by a strong magnetic field. We propose that the difference between the magnetic moment on the U sites is caused by the change of local point symmetry of the sites that is tightly bound to the role of the strong spin-orbit interaction.