Laser-driven formation of transient local ferromagnetism in FeRh thin films

The magnetic phase transition from antiferromagnetic to ferromagnetic order in FeRh can be induced globally by heating the material above its phase transition temperature, applying mechanical strain or magnetic fields. To induce this phase transition locally requires a confined source of energy such as a focused laser beam. Here we combine optical excitation with X-ray magnetic imaging to determine the effect of laser heating on the magnetization of FeRh using time-resolved photoelectron emission microscopy. Excitation by a 100 fs laser pulse generates a ferromagnetic state within 0.6 ns which recovers its initial antiferromagnetic state within a further 2 ns. The form of magnetic domains during the growth and disappearance of magnetization suggests an intrinsic speed limit for magnetic and structural changes.