Decoupled Spin Dynamics in the Rare-Earth Orthoferrite YbFeO₃

We present an inelastic neutron scattering study of magnetic dynamics in YbFeO₃ at temperatures below and above the spin-reorientation (SR) transition T_SR = 7.6K and in magnetic field. The spectrum of magnetic excitations consists of two types of collective modes that are well separated in energy: gapped magnons with a typical bandwidth of ~60 meV, associated with the antiferromagnetically (AFM) ordered 3d subsystem, and quasi-1D AFM fluctuations of ~1 meV within the rare-earth subsystem. The spin dynamics of the Fe subsystem could be well described in the frame of semiclassical linear spin-wave theory. The rotation of the net moment of the Fe subsystem at T_SR drastically changes the excitation spectrum of the Yb subsystem, inducing the transition between two regimes with magnon and spinon fluctuations. At T < T_SR, the Yb spin chains have a well-defined field-induced ferromagnetic 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_SR only a gapped broad spinon continuum dominates the spectrum. The observation of a fractional spinon continuum in YbFeO₃ demonstrates that Kramers rare-earth based magnets can provide realizations of various aspects of quantum low-dimensional physics.