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Abstract

Ultrashort pulses of intense THz radiation have been shown to represent a powerful and versatile tool for spin control. Here, we employ intense THz pulses in two ways to enter the regime of nonlinear THz-spin interaction. In the first approach, we use the magnetic field of intense THz pulses with amplitudes of up to 0.4 T and frequencies between 0.3 and 2 THz to exert a resonant Zeeman torque onto the spins of nickel oxide (NiO). THz-induced magnetic dynamics is monitored by magneto-optical effects that act on the polarization of ultrashort near-infrared laser pulses. In the second approach, we demonstrate this novel concept of spin excitation in the weak ferromagnet thulium orthoferrite (TmFeO₃). In this material, the magnetic anisotropy for the Fe spins is set by electronic orbitals of the Tm ions. THz transients can resonantly excite transitions between these orbital states and thus modify the anisotropy field. We expose a single crystal of TmFeO₃ to ultrashort THz pulses of variable peak amplitudes and trace the THz-induced magnetization. The oscillatory magnon traces originate from the quasi-ferromagnetic (q-FM, 0.1 THz) and the quasi-antiferromagnetic (q-AFM, 0.8 THz) mode of TmFeO₃. Surprisingly, the relative strength of the q-FM mode is rastically enhanced when the THz amplitude increases.