Low-energy spin dynamics in rare-earth perovskite oxides

Abstract

We review recent studies of spin dynamics in rare-earth orthorhombic perovskite oxides of the type RMO 3 , where R is a rare-earth ion and M is a transition-metal ion, using single-crystal inelastic neutron scattering (INS). After a short introduction to the magnetic INS technique in general, the results of INS experiments on both transition-metal and rare-earth subsystems for four selected compounds (YbFeO 3 , TmFeO 3 , YFeO 3 , YbAlO 3 ) are presented. We show that 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 <70 meV, associated with the antiferromagnetically (AFM) ordered transition-metal subsystem, and AFM fluctuations of <5 meV within the rare-earth subsystem, with no hybridization of those modes. We discuss the high-energy conventional magnon excitations of the 3d subsystem only briefly, and focus in more detail on the spectacular dynamics of the rare-earth sublattice in these materials. We observe that the nature of the ground state and the low-energy excitation strongly depends on the identity of the rare-earth ion. In the case of non-Kramers ions, the low-symmetry crystal field completely eliminates the degeneracy of the multiplet state, creating a rich magnetic field-temperature phase diagram. In the case of Kramers ions, the resulting ground state is at least a doublet, which can be viewed as an effective quantum spin-1/2. Equally important is the fact that in Yb-based materials the nearest-neighbor exchange interaction dominates in one direction, despite the three-dimensional nature of the orthoperovskite crystal structure. The observation of a fractional spinon continuum and quantum criticality in YbAlO 3 demonstrates that Kramers rare-earth based magnets can provide realizations of various aspects of quantum low-dimensional physics.

Spin dynamics and exchange interaction in orthoferrite TbFeO$_3$ with non-Kramers rare-earth ion

“…in which two terms were taken into account: the first term is the Heisenberg exchange interaction between neighboring ions and the second term is the effective anisotropy. The DMI causes splitting of the magnon modes in the vicinity of the Gamma point (Γ) [21]; however, the presence of the CEF excitation of the terbium subsystem around 17 meV makes the signal noisy in this region, so the effect of the DMI on the spin dynamics cannot be reliably established. The first summation is made over different sets of neighbors of Fe atoms.…”

“…These parameters of the CEF can be determined using the point charge model (PCM). The experience in using this model was demonstrated in [21,32]. In the PCM, atoms localized in the selected unit volume are considered to be point charges in the crystallographic sites; this environment affects electrically the selected ion (in our case, Tb 3+ ) and, in this approach, any special interactions, spin parameters, etc.…”

“…Previously, several orthoferrites were investigated [21] and isostructural perovskite compounds with different rare-earth ions using inelastic neutron scattering as a basic research tool for studying the energy spectra of excitations in the magnetic subsystems. In the YbFeO 3 compound [22], the authors found quasi-one-dimensional Yb 3+ chains and "shadow modes" in the low-temperature spin dynamics of the rare-earth subsystem.…”

Quasi One-Dimensional Ising-like Antiferromagnetism in the Rare-earth Perovskite Oxide TbScO$_3$

“…The family of orthorhombic rare-earth perovskite oxides RM O 3 (where R is a 4f rare-earth ion, M is a 3d transitionmetal ion) attract continued attention due to their numerous intriguing physical phenomena and potential applications. Remarkable examples include ferroelectric and multiferroic properties [1,2], magneto-optical effects [3,4], magnetocaloric effects [5,6], exotic quantum spin states [7][8][9], and complex spin reorientation transitions [10,11] induced by temperature and magnetic field. Many of these interesting properties arise from the peculiarity of 3d and 4f magnetic sublattices and the diversity of exchange interactions between them.…”

“…As the temperature decreases, the R − M coupling becomes more significant and induces spin-reorientation transitions around several dozen Kelvin [15,16]. The strong coupling between 3d and 4f magnetic sublattices gives rise to a series of nontrivial magnetism, including exotic solitonic lattice [17], gigantic magnetoelectric effect [18], unconventional low-energy spin excitations [7,9]. In contrast, due to the weak exchange coupling between the 4f moments, the rare-earth sublattice orders at much lower temperatures [8,19,20], usually below 10 K. From naive expectation, these rare-earth ions would behave rather classically, considering the large saturation moments and the strong anisotropy.…”