Ultrafast laser-induced spin-lattice dynamics in the van der Waals antiferromagnet CoPS₃

CoPS₃ stands out in the family of the van der Waals antiferromagnets XPS₃ (X = Mn, Ni, Fe, and Co) due to the unquenched orbital momentum of the magnetic Co²⁺ ions, which is known to facilitate the coupling of spins to both electromagnetic waves and lattice vibrations. Here, using a time-resolved magneto-optical pump-probe technique, we experimentally study the ultrafast laser-induced dynamics of mutually correlated spins and lattice. It is shown that a femtosecond laser pulse acts as an ultrafast heater and, thus, results in the melting of the antiferromagnetic order. At the same time, the resonant pumping of the ⁴T_1g → ⁴T_2g electronic transition in Co²⁺ ions effectively changes their orbital momentum, giving rise to a mechanical force that moves the ions in the direction parallel to the orientation of their spins, thus generating a coherent B_g phonon mode at the frequency of about 4.7 THz.