Terahertz-driven magnetization dynamics of bismuth-substituted yttrium iron-gallium garnet thin film near a compensation point

Magnetization dynamics of bismuth-substituted yttrium iron-gallium garnet film $({\mathrm{Y}}_{3\ensuremath{-}z}{\mathrm{Bi}}_z)[{\mathrm{Fe}}_{2\ensuremath{-}x}{\mathrm{Ga}}_x][{\mathrm{Fe}}_{3\ensuremath{-}y}{\mathrm{Ga}}_y]{\mathrm{O}}₁₂$ has been studied using a terahertz pump--optical probe spectroscopy. Two magnetic modes are observed whose frequencies intersect at a magnetization compensation point. The experimental dependence of the excited modes on terahertz pulse polarization, an external magnetic field and the temperature are analyzed. The theoretical description based on symmetry and the Lagrangian formalisms is proposed. Magnetic modes crossing is explained as interplay between exchange and anisotropy energies equally contributing near the compensation point. Simulations based on the Landau-Lifshitz-Gilbert equations show that a difference in magneto-optical susceptibilities of tetrahedral and octahedral iron sublattices can substantially enhance the sensitivity to the magnetic modes.