Several groups of scientists found in 2017 that long-range magnetic order can stably existence at atomic layer thickness in two-dimensional van der Waals (vdW) materials. This significant discovery provides ideal materials platform for the realization of two-dimensional vdW spintronic devices. In order to construct new high-speed spintronics devices, it is necessary to study the ultrafast spin dynamics of vdW magnetic system.

Recently, Prof. SHENG Zhigao and LUO Xuan from Hefei Institutes of Physical Science (HFIPS), Chinese Academy of Sciences, collaborated with Dr. XIAO Ruichun from Anhui University, discovered the ultra-long spin relaxation behavior in two-dimensional van der Waals magnetic materials by an all-optical time-resolved magneto-optical Kerr effect technique under high magnetic fields. At the same time, based on the modified three-temperature model, they revealed the key role of dimensionality and thermal diffusion anisotropy in the spin dynamics of two-dimensional vdW magnets.
The results were published in recent 2D Materials.

Aiming at the frontier field of spin dynamics of two-dimensional magnetic materials, researchers utilized the ultrafast magneto-optical technology (TR-MOKE) under high magnetic fields to conduct a detailed comparative study on the laser-induced spin dynamics of two-dimensional vdW semiconductor Cr₂Ge₂Te₆, two-dimensional vdW metal Fe₃GeTe₂, and three-dimensional metal Cr₃Te₄. They found that the regenerative magnetization rates of two-dimensional vdW magnetic materials were significantly slower than that of three-dimensional materials after excited by femtosecond lasers.

"We found, for the first time, that Cr₂Ge₂Te₆ has ultra-long spin relaxation behavior, that is, there is no obvious recovery of magnetism in the time scale of 3500 ps (1 ps=10-12 s) after ultrafast demagnetization,” said SHENG from the High Magnetic Field Laboratory, “This is the most attracting part of this experiment.”

Based on the model analysis and experimental research results, researchers revealed that the lattice dimension and thermal diffusion anisotropy play a key role in the spin dynamics of two-dimensional magnetic materials.

The ultrafast dynamics study not only effectively expand the research field of two-dimensional magnetism, but also further reveal the dimensional specificity of two-dimensional vdW magnetics, which provides a research basis for their application in the high frequency devices.

This work was supported by the National Key R&D Program of China, the National Natural Science Foundation of China, the Plan for Major Provincial Science&Technology Project, the Key Research Program of Frontier Sciences, CAS, and Users with Excellence Program of Hefei Science Center CAS. A portion of this work was performed on the Steady High Magnetic Field Facilities (Ultrafast optical measurement system under superconducting magnet), High Magnetic Field Laboratory, CAS and supported by the High Magnetic Field Laboratory of Anhui Province.

Link to paper: Ultra-long spin relaxation in two-dimensional ferromagnet Cr2Ge2Te6 flake

Compared with the other two materials, Cr₂Ge₂Te₆ exhibits ultra-long spin relaxation behavior after complete demagnetization. (Image by SUN Tao)