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

The Kondo-lattice system CeRh₆Ge₄ is a quasi-one-dimensional easy-plane ferromagnet that provides an extremely rare example of a ferromagnetic quantum critical point (QCP) under a hydrostatic pressure of ~0.8 GPa [1-3]. We report ⁷³Ge nuclear quadrupole resonance (NQR) and magnetic resonance (NMR) spectroscopy in the paramagnetic and ferromagnetic phases of this compound [4]. NQR and NMR spectral measurements at ambient pressure at the two nonequivalent 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 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 μ_B/Ce confined within the ab plane. The ordered moment shows a notable in-plane magnetic stiffness against out-of-plane AC 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.

[1] Shen et al, Nature 579, 51 (2020)

[2] Kotegawa et al, J. Phys. Soc. Jpn. 88, 093702 (2019)

[3] Thomas et al, Phys. Rev. B 109, L121105 (2024)

[4] Yamamoto et al, Phys. Rev. B 112, 155152 (2025), Editors’ Suggestions