Domain wall dynamics driven by a transient laser-induced magnetization

One of the fundamental effects of laser-matter interaction is the appearance of induced transient magnetization. While the underlying phenomena differ in their microscopic origin and cover a diverse array of materials, here we address a fundamental question about the possibility of driving domain-wall dynamics on the femtosecond timescale of the exchange interactions solely by longitudinal changes of the magnetic moments. We verify the viability of this hypothesis in the case of a generic ferromagnetic system described in the framework of a high-temperature micromagnetic model based on the Landau-Lifshitz-Bloch equation. The effect is investigated in a 1D model at constant temperature as well as in a full micromagnetic framework considering realistic laser-induced heating. Our results demonstrate that domain-wall deformation in a femtosecond time frame leads to the displacement of the wall on a larger timescale up to nanoseconds accompanied by a release of excess energy in the form of spin waves. The domain-wall deformation leads to the appearance of a magnetization gradient across the wall, which promotes the motion towards the region consisting of spins with decreased magnetization length. The total displacement is enhanced at larger temperatures and smaller damping caused by an increase of the longitudinal relaxation time, which ensures the longer presence of the induced magnetization gradient. Our results are important for understanding ultrafast magnetism phenomena on the sub-picosecond timescale.