All-optical switching in granular ferromagnets caused by magnetic circular dichroism.

Matthew O A Ellis,Roy W Chantrell,Eric E Fullerton

doi:10.1038/srep30522

Matthew O A Ellis, Roy W Chantrell + Show 1 more

Open Access

https://doi.org/10.1038/srep30522

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Abstract

Magnetic recording using circularly polarised femto-second laser pulses is an emerging technology that would allow write speeds much faster than existing field driven methods. However, the mechanism that drives the magnetisation switching in ferromagnets is unclear. Recent theories suggest that the interaction of the light with the magnetised media induces an opto-magnetic field within the media, known as the inverse Faraday effect. Here we show that an alternative mechanism, driven by thermal excitation over the anisotropy energy barrier and a difference in the energy absorption depending on polarisation, can create a net magnetisation over a series of laser pulses in an ensemble of single domain grains. Only a small difference in the absorption is required to reach magnetisation levels observed experimentally and the model does not preclude the role of the inverse Faraday effect but removes the necessity that the opto-magnetic field is 10 s of Tesla in strength.

Highlights

Granular ones and the larger uniaxial anisotropy in FePt would suggest that the all-optical switching mechanism remains unclear in these types of ferromagnets
The opto-magnetic field is assumed to couple into the spin dynamics in the same manner as an applied field, that is initially zero but triggers with the laser pulse
By using atomistic spin dynamics parameterised from ab initio calculations the switching window is seen to require fields that are either of magnitude in excess of 60 T or a duration greater than 5 times that of the laser pulse

Summary

Introduction

Granular ones and the larger uniaxial anisotropy in FePt would suggest that the all-optical switching mechanism remains unclear in these types of ferromagnets. We utilise a 2 state Master equation model, using the switching probabilities calculated from the atomistic spin dynamics, to predict the evolution of the magnetisation as a function of increasing laser pulses. To understand the role that thermal effects play we investigate the switching probability during a single laser pulse without any opto-magnetic field as a function of the laser fluence.

Results

Conclusion