Ultrafast Laser-Induced Demagnetization and Valley Polarization in a Room-Temperature Altermagnetic Cr ₂ MoS ₄ Monolayer

Achieving ultrafast room-temperature magnetism control in two-dimensional (2D) altermagnetic (AM) materials is a major challenge in spintronics. In this study, we identify Cr₂MoS₄ as a promising material to overcome these limitations, combining exceptional stability, robust altermagnetism, and laser-tunable electronic and magnetic properties. Cr₂MoS₄ features an in-plane magnetic easy axis with a magnetic anisotropy energy of 0.24 meV/Cr and a high critical temperature of 400 K, well-above room temperature. Upon laser pulse irradiation, Cr₂MoS₄ undergoes rapid electron and phonon temperature rises, resulting in ultrafast demagnetization within ∼200 fs─comparable to antiferromagnetic Dirac semimetals. Transient thermal effects further induce structural distortions, breaking mirror symmetry and generating valley polarization in Cr₂MoS₄. Beyond single-pulse excitation, we apply a double-pulse with adjustable time delays (200-1000 fs), enabling precise modulation of spin dynamics. Tuning the delay allows for control over demagnetization times, amplification of demagnetization effects, and even the reversal of the recovery process into repeated demagnetization. These findings establish Cr₂MoS₄ as an excellent platform for spintronic applications and offer new insights into ultrafast optical control of AM spin dynamics.