Ultra-long spin relaxation in two-dimensional ferromagnet Cr2Ge2Te6 flake

By utilizing time-resolved magneto-optical Kerr effect technique, we investigated both demagnetization and remagnetization processes of a typical two-dimensional (2D) van der Waals (vdW) ferromagnetic (FM) semiconductor Cr2Ge2Te6 (CGT) and two comparative Te-based crystals, i.e. three-dimensional (3D) FM metal Cr3Te4 (CT) and 2D vdW FM metal Fe3GeTe2 (FGT). Different from 3D CT showing one-step laser-induced demagnetization, two-steps demagnetization behavior is dominated in 2D vdW FGT and CGT, which indicates that the 2D vdW magnets have relatively weaker electron-spin coupling. Of particular interest, we discovered an ultra-long remagnetization behavior in the 2D vdW CGT flake. The laser pumping induced spin demagnetization would not recover even in the time scale of 3500 ps (experimental setup limitation). To the best of our knowledge, this is the longest remagnetization process observed so far and it might be one of the particular characterizations of 2D vdW ferromagnet semiconductors. Based on the modified three temperature model, the key roles played by the crystal dimensionality and thermal diffusion anisotropy were revealed in the spin dynamics of 2D vdW magnets. Our findings would not only open the door to the thoroughly understanding of the origin of the magnetism in 2D FM materials but also undoubtedly find suitable applications in spintronics and magnonics devices.