Magneto-optical probe of ultrafast spin dynamics in antiferromagnetic CoO thin films

Abstract

The probe of antiferromagnetic (AFM) spin dynamics in thin films has traditionally relied on the short x-ray pulses based on the magnetic dichroism or diffraction effect. Here, we demonstrate the direct optical probe of ultrafast laser-induced AFM spin dynamics of thin AFM CoO films using time-resolved magnetic linear dichroism effect in reflection geometry. The ultrafast laser excitation of the CoO film leads to the quench of AFM order, manifested as large polarization rotations of the reflected probe light. Far below the N\'eel temperature $({T}_{\mathrm{N}})$, the quench of the AFM order occurs within 300 fs, which is faster than the lattice thermalization $(\ensuremath{\sim}1\phantom{\rule{0.16em}{0ex}}\mathrm{ps})$ via electron-lattice scattering. This AFM quench process, however, slows down near ${T}_{\mathrm{N}}$, where an additional slower quench process with the time constant longer than 20 ps is emergent. Such an AFM spin dynamics is dramatically different than the transient reflectivity dynamics which is nearly invariant in a wide temperature range across ${T}_{\mathrm{N}}$. We attribute the quench of AFM order in CoO to the charge-transfer excitation and thermal effect, but the latter mechanism only plays a significant role near ${T}_{\mathrm{N}}$.