Bridging the Gap: A New Theory for Laser-Induced Demagnetisation

It is a well-established fact that a laser pulse can strip a ferromagnet of its magnetism, a phenomenon known as demagnetisation. However, a mathematical void has long existed between the fields of nonlinear optics and magnetism, leaving this process without a comprehensive analytic theory. Researchers have now attempted to fill this gap by developing a framework centred on the 'spin moment'—a measure of magnetic strength arising from the intrinsic angular momentum of electrons—rather than traditional magnetic susceptibility.

By employing group theory, the team determined that second-order interactions are the lowest order necessary to affect the spin in centrosymmetric systems. Specifically, they identified that among possible nonlinear processes, 'Difference Frequency Generation' (DFG)—where light waves mix to produce a new frequency—produces the largest change in the spin moment. When applied to common recording materials like FePt and FePd, the theory successfully predicted that FePt exhibits a stronger light-induced response. This lays a solid foundation for femtomagnetism, allowing scientists to compute demagnetisation effects without relying on time-consuming real-time simulations.