Unified theoretical framework for spin dynamics excitation by the magnetic field of a high-frequency electromagnetic pulse

Terahertz electromagnetic pulses offer a promising route for the ultrafast manipulation of magnetization in ferromagnetic materials. While previous studies have demonstrated the excitation of spin dynamics using linearly polarized terahertz fields, the role of circular polarization and the effects of rapidly oscillating, time-dependent field profiles remain insufficiently understood. We develop a unified theoretical framework for describing the excitation of spin precession via Zeeman interaction in magnetic materials by high-frequency pulses of arbitrary polarization with a temporal Gaussian profile. In the regime of long pulses (at least several oscillations are within the pulse duration), a circularly polarized magnetic field acts as an effective rectified magnetic field along the pulse propagation, while linear polarized pulses excite no free precession. In the regime of short pulses (less than one oscillation is within the pulse duration), pulses of any polarization, including linear, can excite free spin precession. There is an optimal pulse duration which maximizes the amplitude of the spin precession. It depends on magnetic parameters of the sample and the external magnetic field, as well as on the carrier frequency of the pulse and its amplitude. These findings bridge key gaps in the understanding of terahertz-induced spin dynamics and provide insights into the design of light-controlled magnetization schemes using tailored electromagnetic pulses.