Distinguishing charge transfer and lattice heat impacts on the ultrafast demagnetization in antiferromagnetic CoO films

The time-resolved magneto-optical (MO) Voigt effect is utilized to study the ultrafast quenching and recovery process of antiferromagnetic (AFM) spin order of CoO films. The impacts of charge transfer and lattice heat are distinguished via comparison of the MO dynamics for the pure CoO films with different thickness and the CoO films capped with the Au layer. For excitation with the charge transfer absorption at 400 nm, all pure CoO films far below the Néel temperature (TN) display the fast quenching of AFM order before the lattice thermalization and have a similar recovery speed, pointing to the dominant role of direct charge-transfer induced AFM quenching. In contrast, the lattice heat plays a significant role in the AFM quenching close to TN; thus, the overall recovery process is significantly slower for the thicker film due to its longer heat diffusion time. In consistent with these findings, it is also revealed that for T ≪ TN, the pure CoO film excited at 400 nm has much faster and larger AFM quench-induced MO response compared to the CoO film capped with Au excited at 800 nm, which only produces lattice heat. Whereas near TN, their AFM quenching-induced MO responses have a similar amplitude as a result of the pronounced heat diffusion in Au/CoO. This work clarifies the mechanism of ultrafast AFM quenching in CoO and offers important implication of ultrafast spin modulation via charge excitation in other AFM materials.