All-optical magnetization reversal by circularly polarized laser pulses: Experiment and multiscale modeling

TLDR

The study investigates all‑optical magnetization reversal using single circularly‑polarized laser pulses in Gd_xFe_{90‑x}Co_{10} ferrimagnetic alloys (20 % < x < 28 %). The reversal is driven by ultrafast laser heating that brings the material into a highly nonequilibrium state and by the inverse Faraday effect, where the circularly polarized pulse generates an effective magnetic field up to ~20 T; the required pulse duration ranges from ~40 fs to ~1700 fs and optimal reversal occurs just below the ferrimagnetic compensation temperature. The experiments and simulations reveal that the magnetization reverses linearly via longitudinal relaxation without precession, with polarization‑dependent reversal only at fluences below 10 %, and that optimal reversal occurs just below the compensation temperature, enabling record‑speed write‑read cycles within 30 ps.

Abstract

We present results of detailed experimental and theoretical studies of all-optical magnetization reversal by single circularly-polarized laser pulses in ferrimagnetic rare earth---transition metal (RE--TM) alloys Gd${}_{x}$Fe${}_{90\ensuremath{-}x}$Co${}_{10}$ ($20%<x<28%$). Using single-shot time-resolved magneto-optical microscopy and multiscale simulations, we identified and described the unconventional path followed by the magnetization during the reversal process. This reversal does not involve precessional motion of magnetization but is governed by the longitudinal relaxation and thus has a linear character. We demonstrate that this all-optically driven linear reversal can be modeled as a result of a two-fold impact of the laser pulse on the medium. First, due to absorption of the light and ultrafast laser-induced heating, the medium is brought to a highly nonequilibrium state. Simultaneously, due to the ultrafast inverse Faraday effect the circularly polarized laser pulse acts as an effective magnetic field of the amplitude up to $\ensuremath{\sim}$20 T. We show that the polarization-dependent reversal triggered by the circularly polarized light is feasible only in a narrow range (below 10%) of laser fluences. The duration of the laser pulse required for the reversal can be varied from $\ensuremath{\sim}$40 fs up to at least $\ensuremath{\sim}$1700 fs. We also investigate experimentally the role of the ferrimagnetic properties of GdFeCo in the all-optical reversal. In particular, the optimal conditions for the all-optical reversal are achieved just below the ferrimagnetic compensation temperature, where the magnetic information can be all-optically written by a laser pulse of minimal fluence and read out within just 30 ps. We argue that this is the fastest write-read event demonstrated for magnetic recording so far.