Effect of lattice distortion on uranium magnetic moments in U4Ru7Ge6 studied by polarized neutron diffraction
Abstrakt
In a cubic ferromagnet, small spontaneous lattice distortions are expected below the Curie temperature, but the phenomenon is usually neglected. This study focuses on such an effect in the U4Ru7Ge6 compound.
Based on DFT calculations, we propose a lattice distortion from the cubic Im-3m space group to a lower, rhombohedral, symmetry described by the R-3m space group. The strong spin-orbit coupling of the uranium ions plays an essential role in lowering the symmetry, giving rise to two different U sites (U1 and U2).
Using polarized neutron diffraction in appliedmagnetic fields of 1 and 9 T in the ordered state (1.9K) and in the paramagnetic state (20K), we bring convincing experimental evidence of this splitting of the U sites, with different magnetic moments. The data have been analyzed both by maximum entropy calculations and by a direct fit in the dipolar approximation.
In the ordered phase, the mu(L)/mu(S) ratio of the orbital and spin moments on the U2 site is remarkably lower than for the free U3+ or U4+ ion, which points to a strong hybridization of the U 5f wave functions with the 4d wave functions of the surrounding Ru. On the U1 site, the mu(L)/mu(S) ratio exhibits an unexpectedly low value: the orbital moment is almost quenched, like in metallic alpha-uranium.
As a further evidence of the 5f-4d hybridization in the U4Ru7Ge6 system, we observe the absence of a magnetic moment on the Ru1 site, but a rather large induced moment on the Ru2 site, which is in closer coordination with both U positions. Very similar results are obtained at 20K in the ferromagnetic regime induced by themagnetic field of 9 T.
This shows that applying a strong magnetic field above the Curie temperature also leads to the splitting of the uranium sites, which further demonstrates the intimate coupling of the magnetic ordering and structural distortion. We propose that the difference between the magnetic moment on the U1 and U2 sites results from the strong spin-orbit interaction with different local point symmetries.