Off-resonant magnetization dynamics in ferromagnetic thin films initiated by ultrastrong THz field

Abstract

Summary form only given. The speed of magnetization switching is a key feature in next generation magnetic storage devices. The ongoing pursue towards faster magnetization control has triggered the development of laser sources at terahertz frequencies. In fact pulses in this spectral range are more suited than optical laser for coherent magnetization excitation by Zeeman precession [1]. The recent advent of THz pulses with field strength up to several Teslas [2] opens novel opportunities to drive ultrafast magnetization dynamics in the strong-field regime, which is different from the commonly used optical lasers where the magnetization control is mediated by heat deposition [3].Here we report on time-resolved measurements exploring the sub-cycle THz-induced magnetization dynamics in the ferromagnetic thin film samples Co, Fe and Ni [4]. We present the induced magneto-optical Kerr dynamics as function of the THz field strength up to extreme amplitudes of 7 T and 21 MV/cm, respectively. By increasing the THz pump fluence, we find a continuous transition from the regime of purely coherent Zeeman oscillations, to the incoherent regime, where spin oscillations are superimposed by thermal demagnetization. Our observations indicate that while the coherent response is driven only by the magnetic field, the incoherent dynamics are dominated by the associated THz electric field component. The observed magnetization evolution over sub-picosecond time scale is excellently reproduced by simulations based on ab-initio calculations for the Heisenberg spin Hamiltonian and the stochastic Landau-Lifshitz-Gilbert equation to describe the spin dynamics at finite temperature.