Is perpendicular magnetic anisotropy essential to all-optical ultrafast spin reversal in ferromagnets?

TL;DR

This study theoretically compares perpendicular and in-plane magnetic anisotropy in ferromagnets, revealing that perpendicular magnetic anisotropy is generally essential for robust all-optical ultrafast spin reversal, with some exceptions.

Contribution

It demonstrates that perpendicular magnetic anisotropy is crucial for reliable all-optical spin reversal, and explores conditions under which in-plane anisotropy can also enable reversal.

Findings

Spin reversal in IMA systems is possible but requires longer pulses.

PMA systems exhibit robust spin reversal with strong laser fields.

Laser-induced spin-orbit torque is key to the reversal process.

Abstract

All-optical spin reversal presents a new opportunity for spin manipulations, free of a magnetic field. Most of all-optical-spin-reversal ferromagnets are found to have a perpendicular magnetic anisotropy (PMA), but it has been unknown whether PMA is necessary for the spin reversal. Here we theoretically investigate magnetic thin films with either PMA or in-plane magnetic anisotropy (IMA). Our results show that the spin reversal in IMA systems is possible, but only with a longer laser pulse and within a narrow laser parameter region. The spin reversal does not show a strong helicity dependence where the left- and right-circularly polarized light lead to the identical results. By contrast, the spin reversal in PMA systems is robust, provided both the spin angular momentum and laser field are strong enough while the magnetic anisotropy itself is not too strong. This explains why experimentally the majority of all-optical spin-reversal samples are found to have strong PMA and why spins in Fe nanoparticles only cant out of plane. It is the laser-induced spin-orbit torque that plays a key role in the spin reversal. Surprisingly, the same spin-orbit torque results in laser-induced spin rectification in spin-mixed configuration, a prediction that can be tested experimentally. Our results clearly point out that PMA is essential to the spin reversal, though there is an opportunity for in-plane spin reversal.