Identification of spin wave resonances and crystal field levels in simple chromites RCrO₃ (R = Pr, Sm, Er) at low temperatures in the THz spectral region

Abstract

We report on THz absorption spectroscopy combined with high magnetic fields of polycrystalline RCrO₃ (R = Pr, Sm, Er) aiming understanding spin wave resonances at their low temperature magnetic phases. Our measurements show that the temperature, and the implicit anisotropies at which the Cr³⁺ spin reorientation at TSR takes place, are determinant on the ferromagnetic-like (FM) and the antiferromagnetic-like (AFM) spin modes being optically active. It is found that they are dependent on Rare Earth 4f moment and ion size. We also studied temperature and field dependence of crystal field levels in the same spectroscopic region. Pr³⁺ non-Kramers emerges at 100 K and Zeeman splits. An observed absence of spin wave resonances in PrCrO₃ is attributed to Pr³⁺ remaining paramagnetic. In SmCrO₃ near cancelation of the spin and orbital moments is proposed as the possible reason for not detecting Sm³⁺ ground state transitions. Here, the FM and AFM resonant modes harden when the temperature decreases and split linearly under applied fields at 5 K and below. In ErCrO₃ the Er³⁺ Kramers doublet becomes active at about the TSR onset. Each line further experiences Zeeman splitting under magnetic fields while an spin reversal induced by a ∼2.5 T field, back to the Γ4 (Fz) from the Γ1 phase at 2 K, produces a secondary splitting. The 5 K AFM and FM excitations in ErCrO₃ have a concerted frequency-intensity temperature dependence and a shoulder pointing to the Er³⁺ smaller ion size also disrupting the two magnetic sublattice approximation. Both resonances reduce to one when the temperature is lowered to 2 K in the Γ1 representation. Our findings have important implications on the complex interplay in the magneto-electrodynamics associated with the Rare-Earth 4f – 3d transition metal spin coupling and the structural A site instabilities in perovskite multiferroics.