Terahertz emission spectroscopy of laser-induced spin dynamics inTmFeO3andErFeO3orthoferrites

Abstract

Using the examples of laser-induced spin-reorientation phase transitions in ${\mathrm{TmFeO}}_{3}$ and ${\mathrm{ErFeO}}_{3}$ orthoferrites, we demonstrate that terahertz emission spectroscopy can obtain novel information about ultrafast laser-induced spin dynamics, which is not accessible by more common all-optical methods. The power of the method is evidenced by the fact that, in addition to the expected quasi-ferromagnetic and quasi-antiferromagnetic modes of the iron sublattices, terahertz emission spectroscopy enables detection of a resonance optically excited at an unexpected frequency of \ensuremath{\sim}0.3--0.35 THz. By recording how the amplitude and phase of the excited oscillations depend on temperature and applied magnetic field, we show that the unexpected mode has all the features of a spin resonance of the ${\mathrm{Fe}}^{3+}$ ions. We suggest that it can be assigned to transitions between the multiplet sublevels of the ${}^{6}{A}_{1}$ ground state of the ${\mathrm{Fe}}^{+3}$ ions occupying rare-earth positions.