Peculiarities of spin dynamics excitation by magnetic field of a high-frequency electromagnetic pulse

TL;DR

This work presents a unified theory for Zeeman-driven spin dynamics excited by high-frequency pulses of arbitrary polarization with Gaussian envelopes. By employing a Lagrangian-based, linearized model for a uniaxial ferromagnet and analyzing long- ($\tau \omega \gg 2\pi$) and short-pulse ($\tau \omega < 2\pi$) regimes, it shows that circular polarization creates a rectified effective field along the propagation axis that drives free precession, while linear polarization does not in the long-pulse limit; in the short-pulse regime, all polarizations can induce free precession with an optimal pulse duration, $\tau_{max} = \sqrt{2}/\omega$. Key results include explicit expressions for spin angles $\phi_1$, $\theta_1$, and the effective field $\mathbf{H}_{eff}$, as well as an FFT-based analysis of the spectral content revealing non-resonant versus resonant excitation characteristics. The findings offer practical guidelines for tailoring THz/IR pulses to maximize magnetization control via the inverse Faraday effect and related photomagnetic phenomena, and they bridge insights across THz, infrared, and visible regimes. This framework provides a roadmap for designing light-controlled magnetization schemes using polarization- and duration-tailored electromagnetic pulses.

Abstract

Terahertz (THz) 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 THz fields, the role of circular polarization and the effects of rapidly oscillating, time-dependent field profiles remained insufficiently understood. We have developed 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 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 one can excite free spin precession. There is an optimal pulse duration which maximizes 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 THz-induced spin dynamics and provide insights into the design of light-controlled magnetization schemes using tailored electromagnetic pulses.

Figures (10)

• Figure 1: (a) Configuration of a magnetic sample with respect to the incident electromagnetic pulse. (b) Polarization state of the electromagnetic pulse with respect to its parameters $\alpha$ and $\Psi$.
• Figure 2: Spin dynamics excited by long electromagnetic pulses ($\tau\omega \gg 2\pi$) with linear (a) and circular (b) polarizations. Spin dynamics is calculated by Eqs. \ref{['phi1_dot']} and \ref{['theta1_dot']}. It is described by spherical-coordinate angles $\phi_1$ and $\theta_1$. A magnetic film with $\omega_r = 17.3~\mathrm{GHz}$ is exemplary considered. Electromagnetic pulse parameters are $\omega = 3~\mathrm{rad~ps^{-1}}$, $\tau = 3~\mathrm{ps}$, $h_0 = 3000~\mathrm{Oe}$, $\psi = 0$, and $\varphi = 0$. Circular polarization refers to $\alpha = 1/\sqrt{2}$, linear polarization (right shown) --- to $\alpha = 1$. Bottom insets display initial part of the excitation process where forced oscillations are present. Top inserts represent $h_x$ and $h_y$ magnetic field components of the electromagnetic pulse.
• Figure 3: Spin dynamics excited by short electromagnetic pulses ($\tau\omega \gg 2\pi$) with linear (a) and circular (b) polarizations. Spin dynamics is calculated by Eqs. \ref{['phi1_dot']} and \ref{['theta1_dot']}. It is described by spherical-coordinate angles $\phi_1$ and $\theta_1$. A magnetic film with $\omega_r = 17.3~\mathrm{GHz}$ is exemplary considered. Electromagnetic pulse parameters are $\omega = 3~\mathrm{rad~ps^{-1}}$, $\tau = 0.5~\mathrm{ps}$, $h_0 = 3000~\mathrm{Oe}$, $\psi = 0$, and $\varphi = 0$. Circular polarization refers to $\alpha = 1/\sqrt{2}$, linear polarization to $\alpha = 1$. Top insets display initial part of the excitation process where forced oscillations are present. Bottom inserts represent $h_x$ and $h_y$ magnetic field components of the electromagnetic pulse.
• Figure 4: Linear polarization
• Figure 5: Circular polarization