Prediction of Deterministic All-Optical Switching of Ferromagnetic Thin Film by Ultrafast Optothermal and Optomagnetic Couplings

Yongmin Liu,Chen Chen,Feng Cheng,Zhidong Du,Liang Pan

doi:10.1038/s41598-017-13568-w

Yongmin Liu, Chen Chen + Show 3 more

Open Access

https://doi.org/10.1038/s41598-017-13568-w

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Abstract

All-optical switching (AOS) of magnetization induced by ultrafast laser pulses is fundamentally interesting and promises unprecedented speed for magnetic data storage that is three orders of magnitudes faster than the current techniques. For ferrimagnetic material, the AOS is attributed to magnetic circular dichroism and angular momentum transfer between sublattices. Recently, ferromagnetic material is demonstrated in AOS under multiple pulses. Since the magnetic field needed to flip the ferromagnetic magnetization within femtosecond timescale is unphysically high, some theories hypothesized that there exists a prolonged magnetic field beyond the pulse duration in the switching process. This is intuitively inconsistent with the phenomenological explanation based on the light-induced magnetic field arising from the inverse Faraday effect (IFE). Here, we numerically study the AOS process and provide new insights into the long-standing paradox of the duration of the induced magnetic field. We show that the prolonged magnetic field duration originates from the ultrafast optothermal and optomagnetic coupling. Moreover, we numerically studied both single- and multiple-pulse AOS under different coupling strength between spins and the thermal bath in the macroscopic Fockker-Planck and Landau-Lifshitz-Bloch model. This numerical model may provide a guide to find suitable ferromagnetic materials for AOS.

Highlights

Despite the successful demonstrations of All-optical switching (AOS), the role of ultrafast laser pulses in AOS is still under debate
We numerically study the ultrafast deterministic AOS process for ferromagnetic material under a single laser pulse taking into account the realistic experimental conditions
We confirmed the dependence of AOS on the helicity of the single laser pulse and the existence and role of a prolonged B field which is induced by optothermal and optomagnetic couplings

Summary

Introduction

Despite the successful demonstrations of AOS, the role of ultrafast laser pulses in AOS is still under debate. When the AOS first came out, it was generally believed that circularly polarized laser pulses have two-fold effects in the AOS process It energizes the electrons in the magnetic material and the demagnetization happens because the electron temperature increases above the www.nature.com/scientificreports/. Several attempts have been made to explain the flipping process using phenomenological assumptions to prolong the induced magnetic field duration[13,23,24,34] Such assumptions are seemingly inconsistent with the fact that inverse Faraday effect should disappear together with the laser pulse. The simulation results show that the single laser pulse AOS in ferromagnetic system is possible under some optical, thermal and magnetic parameters, but it hasn’t been demonstrated experimentally. This work will allow prediction of parameter regions where the AOS process could happen and help development of ultrafast magnetic data recording and nanomagnetic devices

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