Decoupled spin dynamics in the rare-earth orthoferrite YbFeO$_3$: Evolution of magnetic excitations through the spin-reorientation transition

TL;DR

This study investigates the magnetic excitations in YbFeO₃ across the spin-reorientation transition, revealing distinct behaviors of Fe and Yb subsystems and highlighting the role of weak quasi-1D Yb-Yb interactions in quantum spin dynamics.

Contribution

It provides the first detailed inelastic neutron scattering analysis of YbFeO₃'s magnetic excitations across the SR transition, emphasizing the impact of weak Yb-Yb coupling on low-energy spin dynamics.

Findings

Fe subsystem's spin dynamics are stable across the SR transition.

Yb subsystem exhibits a transition from magnon to spinon-like fluctuations.

Weak Yb-Yb interactions induce unusual quantum spin behavior.

Abstract

In this paper we present a comprehensive study of magnetic dynamics in the rare-earth orthoferrite YbFeO$_3$ at temperatures below and above the spin-reorientation (SR) transition $T_{\mathrm{SR}}=7.6$ K, in magnetic fields applied along the $a, b$ and $c$ axes. Using single-crystal inelastic neutron scattering, we observed that the spectrum of magnetic excitations consists of two collective modes well separated in energy: 3D gapped magnons with a bandwidth of $\sim$60 meV, associated with the antiferromagnetically (AFM) ordered Fe subsystem, and quasi-1D AFM fluctuations of $\sim$1 meV within the Yb subsystem, with no hybridization of those modes. The spin dynamics of the Fe subsystem changes very little through the SR transition and could be well described in the frame of semiclassical linear spin-wave theory. On the other hand, the rotation of the net moment of the Fe subsystem at $T_{\mathrm{SR}}$ drastically changes the excitation spectrum of the Yb subsystem, inducing the transition between two regimes with magnon and spinon-like fluctuations. At $T < T_{\mathrm{SR}}$, the Yb spin chains have a well defined field-induced ferromagnetic (FM) ground state, and the spectrum consists of a sharp single-magnon mode, a two-magnon bound state, and a two-magnon continuum, whereas at $T > T_{\mathrm{SR}}$ only a gapped broad spinon-like continuum dominates the spectrum. In this work we show that a weak quasi-1D coupling within the Yb subsystem $J_\text{Yb-Yb}$, mainly neglected in previous studies, creates unusual quantum spin dynamics on the low energy scales. The results of our work may stimulate further experimental search for similar compounds with several magnetic subsystems and energy scales, where low-energy fluctuations and underlying physics could be "hidden" by a dominating interaction.