Magnetic order and crystalline electric field excitations of the quantum critical heavy fermion ferromagnet CeRh$_6$Ge$_4$

TL;DR

This study investigates the magnetic order and crystalline electric field excitations in CeRh$_6$Ge$_4$, a heavy fermion ferromagnet near a quantum critical point, revealing in-plane ferromagnetic order and anisotropic hybridization effects.

Contribution

It provides the first detailed characterization of the magnetic structure and CEF scheme in CeRh$_6$Ge$_4$, highlighting the role of orbital anisotropy in its quantum critical behavior.

Findings

In-plane ferromagnetic order with reduced Ce moments

CEF scheme explaining easy-plane anisotropy

Strong hybridization in the first excited CEF level

Abstract

CeRh$_6$Ge$_4$ is an unusual example of a stoichiometric heavy fermion ferromagnet, which can be cleanly tuned by hydrostatic pressure to a quantum critical point. In order to understand the origin of this anomalous behavior, we have characterized the magnetic ordering and crystalline electric field (CEF) scheme of this system. While magnetic Bragg peaks are not resolved in neutron powder diffraction, coherent oscillations are observed in zero-field $\mu$SR below $T_{\rm C}$, which are consistent with in-plane ferromagnetic ordering consisting of reduced Ce moments. From analyzing the magnetic susceptibility and inelastic neutron scattering, we propose a CEF-level scheme which accounts for the easy-plane magnetocrystalline anisotropy, where the low lying first excited CEF exhibits significantly stronger hybridization than the ground state. These results suggest that the orbital anisotropy of the ground state and low lying excited state doublets are important for realizing anisotropic electronic coupling between the $f$- and conduction electrons, which gives rise to the highly anisotropic hybridization observed in photoemission experiments.