A phenomenological theory of the optical magnetization reversal

Abstract

All-optical switching of the magnetization in magnetic nanostructures by femtosecond circularly polarized laser pulses has been demonstrated in several systems. We present a Landau-Lifshitz-Lambda (LLL) model which describes the magnetization dynamics using three density states: two ferromagnetic grounds states and an excited optical state. One of the ferromagnetic ground states is optically excited by circularly polarized light to a spin reversed state, which is then "Coulomb collapsed" to the magnetization reversed ground state. The time evolution of the optically excited states is described by a Lindblad master equation, in which the optical excitation is introduced via the Hamiltonian. Dissipation terms are introduced via Lindblad operators. The LLL model combines the precessional motion of the magnetization described by the Landau-Lifshitz theory, with the response of the three level Λ system. The optical excitation lasts for the duration of the laser pulse and the system relaxes at a fast rate due to the electron-electron interaction. We study the solution of the eigenvalues problem of the optical equation of motion for the magnetization and identify a coherent and incoherent regimes and derive an LLL model that can be integrated with existing micromagnetic codes to describe optical excitation of magnetic materials.

All-optical spin switching on an ultrafast time scale

“…A single laser pulse drives the electronic spin population from the initial to the final state through an energetically higher spin-mixed intermediate state (the level scheme resembles the Greek letter Λ). Menarini and Lomakin [133] coupled this Λ model to the Landau-Lifshitz formalism, which they call the LLL model. The laser field is taken into account through these three levels and the spin precession through the Landau-Lifshitz equation.…”