All-optical spin switching: A new frontier in femtomagnetism — A short review and a simple theory

Abstract

Using an ultrafast laser pulse to manipulate the spin degree of freedom has broad technological appeal. It allows one to control the spin dynamics on a femtosecond time scale. The discipline, commonly called femtomagnetism, started with the pioneering experiment by Beaurepaire and coworkers in 1996, who showed subpicosecond demagnetization occurs in magnetic Ni thin films. This finding has motivated extensive research worldwide. All-optical helicity-dependent spin switching (AOS) represents a new frontier in femtomagnetism, where a single ultrafast laser pulse or multiple pulses can permanently switch spin without any assistance from a magnetic field. This review summarizes some of the crucial aspects of this new discipline: key experimental findings, leading mechanisms, controversial issues, and possible future directions. The emphasis is on our latest investigation. We first develop the all-optical spin switching rule that determines how the switchability depends on the light helicity. This rule allows one to understand microscopically how the spin is reversed and why the circularly polarized light appears more powerful than the linearly polarized light. Then we invoke our latest spin-orbit coupled harmonic oscillator model to simulate single spin reversal. We consider both cw excitation and pulsed laser excitation. The results are in a good agreement with the experimental result. a We then extend the code to include the exchange interaction among different spin sites. We show where the "inverse Faraday field" comes from and how the laser affects the spin reversal nonlinearly. Our hope is that this review will motivate new experimental and theoretical investigations and discussions.

First-principles and model simulation of all-optical spin reversal

“…To start with, we note that all-optical spin reversal is an optical process and must follow the dipole selection rule. Consider a laser field propagating along the −z axis toward a sample surface [28,32] (see Fig. 1),…”

“…Over the years, the explanation progresses from the inverse Faraday effect [10,19], Raman scattering [20,21], magnetic circular dichroism [22], pure heating [16], and sublattice spin exchange [23], to ultrafast exchange scattering [24], with new theories emerging in ferromagnets [25][26][27]. This raises a serious question whether a big picture is missing from the existing theories [28]. Furthermore, no theory ever addresses a design protocol for future photospintronic devices based on AOS technology [29].…”

Understanding all-optical spin switching: Comparison between experiment and theory

“…Most simulations have been phenomenological, 6 where laser fields are treated as an effective magnetic field. 13 Ostler et al 6 and Mentink et al 53 showed that in GdFeCo, HID-AOS and HD-AOS depend on the laser intensity (electric field squared), not the field helicity. As shown in our recent study, 54 caution must be taken if the system has two spin sublattices.…”

“…As discussed above, the majority of theoretical research has been phenomenological. 13 This calls for a systematic experimental investigation by tuning both system-and laser-specific parameters. Before one can pin down the origin of AOS, it is necessary to develop a many-to-one correspondence between the proposed mechanisms (see Table 1) and more fundamental interactions.…”

Spin-orbit torque-mediated spin-wave excitation as an alternative paradigm for femtomagnetism

“…In our model, we add a twist by replacing the tight-binding term by a real-space kinetic energy and potential energy term, so we can easily include one extra term, spin-orbit interaction. With these conditions in mind, we have the Hamiltonian as [37][38][39][40][41],…”