Spin pumping in magnetic trilayer structures with an MgO barrier.

A A Baker,G Van Der Laan,A I Figueroa,D Pingstone,T Hesjedal,V K Lazarov

doi:10.1038/srep35582

A A Baker, G Van Der Laan + Show 4 more

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https://doi.org/10.1038/srep35582

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Abstract

We present a study of the interaction mechanisms in magnetic trilayer structures with an MgO barrier grown by molecular beam epitaxy. The interlayer exchange coupling, Aex, is determined using SQUID magnetometry and ferromagnetic resonance (FMR), displaying an unexpected oscillatory behaviour as the thickness, tMgO, is increased from 1 to 4 nm. Transmission electron microscopy confirms the continuity and quality of the tunnelling barrier, eliminating the prospect of exchange arising from direct contact between the two ferromagnetic layers. The Gilbert damping is found to be almost independent of the MgO thickness, suggesting the suppression of spin pumping. The element-specific technique of x-ray detected FMR reveals a small dynamic exchange interaction, acting in concert with the static interaction to induce coupled precession across the multilayer stack. These results highlight the potential of spin pumping and spin transfer torque for device applications in magnetic tunnel junctions relying on commonly used MgO barriers.

Highlights

In the classical limit the spin dynamics of a ferromagnet are governed by the damped Landau-Lifshitz-Gilbert (LLG) equation of motion, describing the precession of the magnetisation about an effective field arising from internal and external fields: dm dt
We have presented a study of the coupled magnetodynamics of magnetic heterostructures, demonstrating that both static and dynamic coupling can exist across a suitably thin MgO barrier
SQUID-vibrating sample magnetometer (VSM) and VNA-ferromagnetic resonance (FMR) showed the presence of a static interaction acting to align the two layers, with an apparent oscillatory coupling

Summary

Introduction

In the classical limit the spin dynamics of a ferromagnet are governed by the damped Landau-Lifshitz-Gilbert (LLG) equation of motion, describing the precession of the magnetisation about an effective field arising from internal and external fields: dm dt. The static interlayer exchange coupling is a general term for any interaction that acts to (anti-)align the magnetisations of the two layers in a magnetic trilayer structure such as a spin valve or an MTJ. Examples of such interactions include Ruderman-Kittel-Kasuya-Yosida (RKKY), superexchange, Néel or orange peel coupling, and direct exchange through a discontinuous spacer layer. The sign of Aex indicates whether the interaction favours parallel (positive) or antiparallel (negative) alignment

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