Collapse of the hyperfine magnetic field at the Ru site in ferromagnetic rare earth intermetallics

TL;DR

This study reveals that in certain ferromagnetic rare-earth intermetallics, the hyperfine magnetic field at Ru sites collapses despite magnetic order, due to the delocalized nature of Ru electrons and localized rare-earth moments.

Contribution

The paper demonstrates through experiments and ab initio calculations that Ru sites do not exhibit hyperfine fields in ferromagnetic intermetallics, challenging assumptions about magnetic hyperfine interactions.

Findings

Ru sites show no hyperfine magnetic field despite ferromagnetic order.

Spin polarization is localized on rare-earth sites, not Ru sites.

4d Ru electrons form broad conduction bands mediating magnetic interactions.

Abstract

The M\"{o}ssbauer Effect(ME) is frequently used to investigate magnetically ordered systems. One usually assumes that the magnetic order induces a hyperfine magnetic field, $B_{hyperfine}$, at the ME active site. This is the case in the ruthenates, where the temperature dependence of $B_{hyperfine}$ at $^{99}$Ru sites tracks the temperature dependence of the ferromagnetic or antiferromagnetic order. However this does not happen in the rare-earth intermetallics, GdRu$_2$ and HoRu$_2$. Specific heat, magnetization, magnetic susceptibility, M\"{o}ssbauer effect, and neutron diffraction have been used to study the nature of the magnetic order in these materials. Both materials are found to order ferromagnetically at 82.3 and 15.3 K, respectively. Despite the ferromagnetic order of the rare earth moments in both systems, there is no evidence of a correspondingly large $B_{hyperfine}$ in the M\"{o}ssbauer spectrum at the Ru site. Instead the measured spectra consist of a narrow peak at all temperatures which points to the absence of magnetic order. To understand the surprising absence of a transferred hyperfine magnetic field, we carried out {\it ab initio} calculations which show that spin polarization is present only on the rare-earth site. The electron spin at the Ru sites is effectively unpolarized and, as a result, $B_{hyperfine}$ is very small at those sites. This occurs because the 4$d$ Ru electrons form broad conduction bands rather than localized moments. These 4$d$ conduction bands are polarized in the region of the Fermi energy and mediate the interaction between the localized rare earth moments.