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 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 ∼60 meV, associated with the antiferromagnetically (AFM) ordered Fe subsystem, and quasi-1D AFM fluctuations of ∼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 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 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 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 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. entropy evolution [9], laser-pulse induced ultrafast spinreorientation [10-12] etc. Magnetic property investigations of the rare-earth orthoferrites RFeO 3 have shown that the Fe 3+ moments (S = 5 2 ) are ordered in a canted AFM structure Γ 4 at high temperature with T N ≈ 600 K (details of the notations are given in [13]), and the spin canting gives a weak net ferromagnetic moment along the c axis [ Fig. 1(c)] [13][14][15]. Furthermore, symmetry analysis and careful neutron diffraction measurements have found a second "hidden" canting along the b-axis, which is symmetric relative to the ac-plane and does not create a net moment [16,17]. With decreasing temperature, a spontaneous spin-reorientation (SR) transition from Γ 4 to the Γ 2 magnetic configuration occurs in many orthoferrites with magnetic R-ions [13,14] in a wide temperature range from T SR ≈ 450 K for SmFeO 3 down to T SR ≈ 7.6 K for YbFeO 3 , and the net magnetic moment rotates from the a to the c axis [see Fig. 1(c-e)]. Most of previous work that was devoted to the investigation of the SR transition in RFeO 3 , associated this phenomenon with the R-Fe exchange interaction, because orthoferrites with nonmagnetic R =La, Y or Lu preserve the Γ 4 magnetic structure down to the lowest temperatures.Taking into account three characteristic t...

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By the single crystal inelastic neutron scattering the orthoferrite HoFeO3 was studied. We show that the spin dynamics of the Fe subsystem does not change through the spin-reorientation transitions. The observed spectrum of magnetic excitations was analyzed in the frames of linear spin-wave theory. Within this approach the antiferromagnetic exchange interactions of nearest neighbors and next nearest neighbors were obtained for Fe subsystem. Parameters of Dzyaloshinskii-Moriya interactions at Fe subsystem were refined. The temperature dependence of the gap in Fe spin-wave spectrum indicates the temperature evolution of the anisotropy parameters. The estimations for the values of Fe-Ho and Ho-Ho exchange interaction were made as well.

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