Optical seminar | 15 May 2020

Understanding the speed and the way short-range interactions build up to long-range order is one of the most intriguing topics in condensed matter physics. This is a particularly interesting problem in magnetism, where the microscopic spin-spin exchange interaction is ultimately responsible for the ordering of spins and the emergence of a macroscopic net magnetization. FeRh is an outstanding material, which undergoes first order phase transition from antiferromagnetic to ferromagnetic state upon a temperature increase and thus offers a unique playground for investigations of ultrafast emergence of ferromagnetism. Using femtosecond laser pulses as an ultrafast heater and THz emission spectroscopy as an ultrafast magnetometer, we have revealed ultrafast dynamics of the emerging ferromagnetism in FeRh.  In particular, our data show that THz radiation emitted by ferromagnetic nuclei only emerges in 20 ps after FeRh was heated with a femtosecond laser pulse.  In an external magnetic field the nuclei’s will reorient their magnetizations and expand leading to an increase of the THz emission intensity on a time scale of 100 ps. Although the speed of the reorientation and expansion increases upon an increase of the external magnetic field, we will show that there is a fundamental limit on the speed of the phase transition to ferromagnetic state.