Optical detection of ultra-fast magnetization dynamics in 2D ferromagnet Cr₂Ge₂Ge₆

Two-dimensional magnetic materials provide a unique platform for the study of magnetism in reduced dimensions. Moreover, they provide the opportunity of coupling with other 2D materials in heterostructures exploiting magnetic exchange through proximity. Understanding the magnetization dynamics of these thin materials on ultra-short time scales is vital, both for designing applications and for understanding the fundamentals of magnetism in the two-dimensional limit. Here, we study the magnetization dynamics of the layered magnetic material Cr₂Ge₂Te₆ using an all-optical technique based on time-resolved Faraday rotation, and change the magnetic properties using electrostatic gating and heterostructure engineering. By heating the two-dimensional magnet with an ultrashort intense laser pulse and probing the magnetization using a second, weaker laser pulse, we measure the demagnetization and subsequent remagnetisation at sub-picosecond timescales. Moreover, we measure a damped oscillation on top of this signal, which is due to precession of the magnetization around the externally applied magnetic field. Our first results show a fast, two-step demagnetization, consistent with a type-II demagnetization, followed by a recovery time in the order of nanoseconds. From these measurements, the interaction between the electron, phonon and spin degrees of freedom can be extracted using the microscopic three-temperature model. The magnetic field dependence of the frequency and decay time of the precession is well described by the Landau-Lifshitz-Gilbert equation, and from this we extract the magnetization dynamics parameters such as the gilbert damping. The possibility of tuning these parameters with a gate voltage or heterostructure engineering makes two-dimensional magnets an appealing system for the development of novel magnetic devices for data processing and storage at ultrafast timescales.