Spin-dependent transport in antiferro and ferrimagnetic nanostructures

Abstract : Spin transfer torque (STT) and tunnelling magnetoresistance (TMR) in magnetic tunnel junctions with ferromagnetic (F) leads are two essential underlying phenomena of modern spintronics. We present here a theoretical study of STT in antiferromagnet (AF) based tunnel junctions, where two AF metal electrodes are separated by a thin nonmagnetic insulating barrier. In particular, the behaviour of STT and TMR in epitaxial AF-based tunnel junctions is investigated using tight binding calculations in the framework of the Keldysh formalism. The spatial distribution of the STT out-of-plane component is found to be staggered, similar to the in-plane component. This behaviour is specic to the use of a tunnel barrier and signicantly diers from the out-of-plane torques reported in previous works using a metallic spacer. Additionally, we show that unlike conventional ferromagnetic-based tunnel junctions, the TMR can increase with applied bias and reach values comparable to typical magnetoresistances found for usual spin valves. Next, the analysis carried out for AFs is extended to ferrimagnets (FI), for which AFs constitute simpler limiting cases. The additional magnetic complexity inherent to FI materials yields to a richer physics concerning the STT spatial behaviour in FI based tunnel junctions. Electronic structure parameters such as band widths and exchange splittings of the FI are shown to have a strong inuence on STT. In particular, the STT spatial distribution within the leads exhibits a striking spin-modulated wave-like behaviour resulting from the interplay between the exchange splittings of the two FI sublattices. This wave-like behaviour can also be tuned via the applied voltage across the junction. Furthermore, the fundamental intrinsic parameter for quantifying STT characteristic lengths in FI metals is identied. This fundamental parameter can be considered as an eective exchange eld in FIs, similar to the homogeneous exchange eld in the F case. Finally, the STT characteristic lengths in AF materials are investigated experimentally. Here, room temperature critical depths and absorption mechanisms of spin currents in Ir20Mn80 and Fe50Mn50 are determined by F-resonance and spin pumping. In particular, room temperature critical depths are observed to be originated from dierent absorption mechanisms: dephasing for Ir20Mn80 and spin ipping for Fe50Mn50.
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Contributor : Pablo Merodio <>
Submitted on : Friday, June 12, 2015 - 11:27:31 AM
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  • HAL Id : tel-01163162, version 1



Pablo Merodio. Spin-dependent transport in antiferro and ferrimagnetic nanostructures. Physics [physics]. Université de Grenoble, 2014. English. ⟨tel-01163162⟩



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