Abstract

Understanding the complete light-spin interactions in magnetic systems is the key to manipulating the magnetization using optical means at ultrafast timescales. The selective addressing of spins by terahertz (THz) electromagnetic fields via Zeeman torque is, by far, one of the most successful ultrafast means of controlling magnetic excitations. Here we show that this traditional Zeeman torque on the spins is not sufficient, rather an additional relativistic field-derivative torque is essential to realize the observed magnetization dynamics [1,2]. We accomplish this by exploring the ultrafast nonlinear magnetization dynamics of rare-earth, Bi-doped iron garnet when excited by two co-propagating THz pulses. First, by exciting the sample with an intense THz pulse and probing the magnetization dynamics using the magneto-optical Faraday effect, we find the collective exchange resonance mode between rare-earth and transition metal sublattices at 0.48 THz [3]. We further explore the magnetization dynamics via a rather direct and efficient THz time-domain spectroscopic means. We find that the observed nonlinear trace of the magnetic response cannot be mapped to the magnetization precession induced by the Zeeman torque, while the Zeeman torque supplemented by an additional field-derivative torque (FDT) follows the experimental evidence [4]. This breakthrough not only enhances our comprehension of ultra-relativistic effects but also paves the way for the development of novel technologies harnessing light-induced control over magnetic systems. Towards the end, the effects of FDT has been compared in several magnetic classes e.g., ferromagnets and ferrimagnets together with the magnetization reversal in these systems.

[1] R Mondal, M Berritta, and PM Oppeneer Physical Review B 94 (14), 144419 (2016)
[2] R Mondal, A Donges, U Ritzmann, PM Oppeneer, and U Nowak Physical Review B 100 (6), 060409 (R) (2019)
[3] A. Dutta, C. Tzschaschel, D. Priyadarshi, K. Mikuni,T. Satoh, R. Mondal, and S. Pal, Experimental observation of relativistic field-derivative torque in nonlinear THz response of magnetization dynamics Advanced Functional Materials 35, 2414582 (2025)
[4] A Dutta, P Mukherjee, SP Sarangi, S Bhattacharjee, S Pal, and R Mondal, Role of material-dependent properties in THz field-derivative-torque-induced nonlinear magnetization dynamics Physical Review Materials 8, 114404 (2024)