Destabilisation of local magnetic anisotropy in heavy-fermion compounds

TL;DR

This paper demonstrates that in heavy-fermion compounds, the local magnetic anisotropy is destabilized by coherent electron scattering, leading to unconventional magnetic orderings and anisotropy behaviors linked to the coherence energy scale.

Contribution

It introduces a new understanding of how electron coherence in heavy-fermion systems destabilizes local magnetic anisotropy, affecting magnetic order and response.

Findings

Destabilization of local anisotropy by electron coherence.

Competing splittings in Curie-Weiss constants and moments.

Correlation between anisotropy change temperature and coherence energy.

Abstract

The local magnetic anisotropy of a typical crystalline compound is usually attributed to the combined effect of crystal electric fields and spin-orbit coupling. We show that this simple local picture is transformed in heavy-fermion compounds by the development of coherent electron scattering from local spin degrees of freedom. Provided the dominance of the coherence energy scale over the magnetic energy scale is strong enough, the fractionalisation and delocalisation of the spins destabilises their single-ion anisotropy by generating an opposing anisotropy in the exchange. Experimentally, this can manifest as competing splittings in the Curie-Weiss constants and effective moments. We show that in the presence of orthorhombic or tetragonal symmetry the destabilisation of the anisotropy can result in either ferromagnetic or antiferromagnetic order that is perpendicular to the high-temperature easy axis. In the absence of destabilisation, we show that the order is more likely to be antiferromagnetic. In agreement with our theory, we also observe that the temperature at which the anisotropy of the uniform magnetic response changes tracks the coherence energy scale in a wide range of actinide and lanthanide compounds.