Laser-induced generation and quenching of magnetization on FeRh studied with time-resolved x-ray magnetic circular dichroism

Ultrafast magnetization dynamics triggered by femtosecond laser excitation of the FeRh alloy is investigated by time-resolved x-ray magnetic circular dichroism. We measure a gradual growth of magnetization within ≈100 ps for both Fe and Rh elements as the system is laser-driven through its first-order magnetic phase transition from the antiferromagnetic (AFM) to the ferromagnetic (FM) state. A dynamic AFM-FM phase coexistence is observed, identifying the magnetization growth mechanism as the rapid nucleation and subsequent slow expansion of FM regions within an AFM matrix. In contrast to the magnetization generation process, the photoinduced demagnetization of FeRh proceeds on a sub-picosecond time scale. We propose a demagnetization mechanism that follows the electronic temperature of the system developed after laser excitation.