The Multi-Scale Dynamics of All-Optical Exchange Bias Reversal

Abstract

Pinning magnetization in a ferromagnetic thin film is commonly realized through exchange biasing with an adjacent antiferromagnet. Field-cooling from above the Néel temperature is a reliable yet slow re-pinning method in exchange-biased systems. For on-demand reprogrammable devices, localized and rapid exchange bias repinning methods are essential. Recent work has shown that femtosecond laser pulses enable field-free reversal of exchange bias in tailored multilayer stacks. Contrary to field-cooling, our experiments with ultrafast excitation reach hitherto unexplored regimes in the exchange bias setting process. Here, we unravel these observations by considering both ultrafast magnetization dynamics on the femto- to picosecond timescale and slow heat-driven dynamics on millisecond timescales and upwards. We develop a microscopic framework of exchange bias setting in a polycrystalline antiferromagnetic thin film like IrMn that provides a complete description of the observations in our present experiments and those found in literature. We expand the use of our model by identifying material platforms and stack designs that lead to optimized performance, aiding further development of optically reprogrammable devices.