1. Absorption of Laser Pulse: When an ultrashort laser pulse, typically in the femtosecond to picosecond range, strikes a magnetic material, it is absorbed by the material's electrons through various mechanisms such as photoexcitation or multiphoton absorption. This absorption leads to a rapid increase in the electron temperature.
2. Hot Electron Generation: The absorbed laser energy excites a large number of electrons in the material, creating a non-equilibrium state with a high concentration of hot electrons. These hot electrons have sufficiently high energy to overcome the potential barriers at the material's interfaces.
3. Spin-Dependent Scattering: Hot electrons generated by the laser pulse can undergo spin-dependent scattering with the magnetic moments of the material's atoms. Specifically, the spin of the hot electrons interacts with the magnetic moments of the localized d-electrons of the magnetic atoms.
4. Transfer of Spin Angular Momentum: During these spin-dependent scattering events, the spin angular momentum of the hot electrons is transferred to the localized d-electrons of the magnetic atoms. This transfer of spin angular momentum exerts a torque on the magnetic moments of the atoms, causing them to precess around their easy axes.
5. Magnetization Dynamics: The transfer of spin angular momentum from the hot electrons to the localized d-electrons leads to the precession of the magnetic moments, giving rise to ultrafast magnetization dynamics. The direction and amplitude of this precession depend on the laser pulse's polarization, intensity, and duration.
6. Magnetic Switching: If the laser pulse has sufficient energy and duration, the precession of the magnetic moments can reach a critical angle, leading to the reversal of the magnetization direction. This is commonly known as all-optical switching or laser-induced magnetization reversal.
7. Femtosecond Timescales: The characteristic timescales for STT-induced magnetization dynamics are on the order of femtoseconds to picoseconds, making it an ultrafast process. This allows for the manipulation of magnetization on exceptionally short timescales.
Overall, laser pulses can transfer spin angular momentum to the localized d-electrons of magnetic materials via spin-transfer torque, enabling ultrafast manipulation and switching of magnetization. This opens up possibilities for exploring fundamental aspects of magnetism, developing high-speed spintronic devices, and advancing technologies such as magnetic random-access memory (MRAM) and ultrafast spintronic logic circuits.
