Spin wave resonances and optical activity in simple chromites RCrO$_{\mathrm{3}}$ (R$=$Pr, Sm, Er) at ultralow temperatures in the THz region

We show that the spin reorientation temperature T$_{\mathrm{SR\thinspace }}$in polycrystalline RCrO$_{\mathrm{3}}$ (R$=$Pr, Sm, Er) is determinant on spin wave resonances,$^{\mathrm{1}}$ ferromagnetic-like (FM) and antiferromagnetic-like (AFM), being optically active. We also report on crystal field temperature and field dependences. Pr$^{\mathrm{3+}}$ non-Kramers lowest energy main transition emerges at 100 K. The no detection of spin wave resonances is attributed to Pr$^{\mathrm{3+}}$ remaining paramagnetic disrupting Cr$^{\mathrm{3+}}$-Pr$^{\mathrm{3+\thinspace }}$exchanges. In SmCrO$_{\mathrm{3\thinspace }}$we propose magnetic compensation for not detecting Sm$^{\mathrm{3+\thinspace }}$ground state transitions. The FM and AFM harden upon lowering temperature and split linearly on applied fields at 5 K. In ErCrO$_{\mathrm{3}}$ the Er$^{\mathrm{3+}}$ Kramers doublet becomes active at \textasciitilde T$_{\mathrm{SR}}$ onset. Each Zeeman line splits further under magnetic fields. The field-induced spin reversal at \textasciitilde 1.5 T yields a secondary split at the highest Zeeman level. The $\Gamma_{\mathrm{2}}$ 5 K resonances show concerted frequency-intensity temperature dependence. A shoulder in the AFM profile points to subtle distortions by Er$^{\mathrm{3+}}$ smaller ion size. Both modes merge into an induced continuum prompted by the external field. At 2 K, in the $\Gamma _{\mathrm{1}}$ representation, the resonances reduce to one excitation. [1] F. Keffer and C. Kittel. Phys. Rev. \textbf{85}, 329 (1952); G. F. Herrmann. Phys. Rev. \textbf{133}, A1334 (1964).