Abstract-Coherent and incoherent ultrafast magnetization dynamics in 3 d ferromagnets driven by extreme terahertz fields

Mostafa Shalaby, Andreas Donges, Karel Carva, Rolf Allenspach, Peter M. Oppeneer, Ulrich Nowak, and Christoph P. Hauri

https://journals.aps.org/prb/accepted/6a074Y81Tf81f278f2a37c59cec82beaa7ac0c3d7

Ultrafast spin dynamics in magnetic materials is generally associated to ultrafast heating of the electronic system by a near infrared femtosecond laser pulse, thus offering only an indirect and non-selective access to the spin order. Here we explore spin dynamics in ferromagnets by means of extremely intense THz pulses, as at these low frequencies the magnetic field provides a direct and selective route to coherently control the magnetization. We find that, at low fields, the observed off-resonantly excited spin precession is phase-locked to the THz magnetic field. At extreme THz fields, the coherent spin dynamics become convoluted with an ultrafast incoherent magnetic quenching due to the absorbed energy. This demagnetization takes place upon a single shot exposure. The magnetic properties are found to be permanently modified above a THz pump fluence of $\approx \text{SI}100\text{milli}\text{joule}\text{per}\square \text{centi}\text{meter}$. We conclude that magnetization switching cannot be reached. Our atomistic spin-dynamics simulations excellently explain the measured magnetization response. We find that demagnetization driven by THz laser-field coupling to electron charges occurs, suggesting non-conducting materials for achieving coherent THz-magnetization reversal.