Evidence for easy-plane XY ferromagnetism in heavy-fermion quantum-critical CeRh6Ge4

We report ⁷³Ge nuclear quadrupole resonance (NQR) and magnetic resonance (NMR) spectroscopy in the heavy-fermion quantum-critical ferromagnet CeRh₆Ge₄. NQR and NMR spectral measurements at the two non-equivalent Ge sites reveal electric field gradient tensors and the directions of their principal axes relative to the hexagonal basal plane. The spin-lattice relaxation rate 1/T₁ experiments reveal a clear critical slowing down approaching the ferromagnetic transition. 1/T₁ in the paramagnetic state is found to be predominantly caused by fluctuating 4f-local moments. The Knight shift shows Curie-Weiss behavior at high temperature and a deviation from this below T^ ≈ 25 K possibly due to a mixture of crystalline electric field effects and Kondo screening. Order-parameter-like behavior of hyperfine fields at the Ge sites and ferromagnetic signal enhancement are observed in Zeeman-perturbed NQR, with uniform ferromagnetic order and a small ordered moment of ≈ 0.26 μB/Ce confined within the $ab$-plane. The ordered moment shows a notable in-plane magnetic stiffness against out-of-plane radiofrequency fields and has an (XY-type) in-plane isotropic nature. Our results reveal a strong easy-plane anisotropy of 4f-electron moment and suggest an involved interplay of hybridization and local moment physics in this quantum critical heavy-fermion ferromagnet.