Decoupled spin dynamics in the rare-earth orthoferrite YbFeO3 : Evolution of magnetic excitations through the spin-reorientation transition

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 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...

Low-temperature spin dynamics in the TmFeO3 orthoferrite with a non-Kramers ion

“…The phase purity of the resulting compound was checked with a conventional x-ray diffractometer. The crystal growth was carried out using an optical floating-zone method (see [20] for details).…”

“…Experimental hallmarks of exotic quantum states have been recently reported in a number of Yb-based (4 f 13 ) compounds by the observation of Tomonaga-Luttinger liquid behavior and spinon confinement-deconfinement transitions in YbAlO 3 [13]; spinon continuum and spinon confinement in Yb 2 Pt 2 Pb [14,15] and in the triangular-lattice systems YbMgGaO 4 [16,17]; the quantum dimer magnet state in Yb 2 Si 2 O 7 [18] and a Higgs transition from a magnetic Coulomb liquid to a ferromagnet in Yb 2 Ti 2 O 7 [19]. In the rare-earth orthoferrite YbFeO 3 the Yb magnetic sublattice also exhibits rich quantum spin dynamics [20]. At temperatures below the iron spin-reorientation (SR) transition T SR = 7.6 K 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 a gapped broad spinonlike continuum dominates the spectrum.…”

“…The quantum states of the ground Kramers doublet, separated by a large energy gap from the first excited state, can be viewed as an effective spin-1/2 [14]. It was predicted and demonstrated experimentally that the magnetic properties at low temperatures and in low magnetic fields can be described by a pseudospin S = 1/2 model [13,14,17,20].…”

“…In particular, the absence of the long range magnetic order in the rare-earth sublattice down to milli-Kelvin temperatures plays an important role. It was shown recently that the coupling within the rare-earth subsystem in Yb-based orthorhombic perovskites is essentially quasi-onedimensional [13,20,21], which results in unusual spin dynamics on the low-energy scale. Experiments to reveal the nature of the rare-earth magnetic state as well as intra-and inter-sublattice correlations have so far been limited and were mainly concerned for single-ion CF considerations.…”

“…The aim of this study is to present a quantitative analysis of the magnetic excitations in the orthoferrite TmFeO 3 , with a non-Kramers rare-earth ion, and to compare the results to the Kramers ion case YbFeO 3 [20] and YbAlO 3 [13,21]. Our target compound is TmFeO 3 , in which the Tm sublattice does not exhibit long range magnetic order down to T = 1.6 K [22].…”