Researchers from CNRS, Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy, and the Leibniz Institute for Crystal Growth have demonstrated that all-optical helicity-independent magnetization switching (AO-HIS) in spintronic materials is a spatially inhomogeneous process along the depth of nanometer-thin magnetic films, challenging the traditional view of uniform, local switching.
Using femtosecond soft X-ray spectroscopy on a 9.4 nm-thick Gd25Co75 alloy film within a layered heterostructure, they observed an ultrafast formation and downward propagation of a magnetization boundary at about 2,000 m/s, sweeping through the magnetic layer in roughly 4.5 ps.
Initially, infrared laser pulses cause a near-uniform demagnetization, but within 2 picoseconds, two magnetic domains with opposite magnetization directions emerge: the surface-adjacent region reverses first due to additional heating from the platinum layer above, while the bottom remains unchanged. The boundary between these domains propagates through the film, enabling switching beyond the laser-excited surface slice.
This boundary-driven mechanism redefines AO-HIS as involving both local thermal effects and non-local spatial dynamics, likely driven by angular momentum exchange and ultrafast thermal gradients. Understanding these nanoscale transient inhomogeneities is crucial for optimizing ultrafast spin switching and phase transitions in magnetic heterostructures.
For spintronic applications, this insight enables new strategies to engineer light-actuated magnetic devices by tailoring film thickness, composition, and surrounding layers to control boundary nucleation and velocity. Such design flexibility paves the way for fast, energy-efficient memory and logic devices leveraging ultrafast, light-driven magnetization reversal.