Interaction between collective coordinates and quasiparticles in spintronic devices

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2006

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Núñez, Álvaro Sebastián

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Abstract

In this dissertation several aspects of the interaction of collective and quasi-particle degrees of freedom are studied. This is done in the context of spin dependent transport effects with applications for spintronics devices. In ferromagnetic metals the effects of quasi-particle currents on spin textures, either domain wall structures or spin waves, are discussed. In nano-magnetic heterostructures, the effects acquire the form of spin transfer torques. The microscopic origin of these effects, as discussed in this work, relies on the relation between exchange fields and spin densities. The presence of the current modifies the spin density. In consequence the exchange fields are also affected by the current. It is these modifi- cations on the exchange fields that are able to alter the dynamics of the collective fields. It is shown how this rather abstract picture of spin transfer reduces to the usual description, that can be found in the extensive literature on the subject, based on a bookkeeping argument and on spin conservation. The most important feature of this picture, as discussed in the text, is that it allows for generalizations of the spin transfer effects to systems were the spin conservation arguments fail or are of little use. We discuss applications of this view to spin transfer torques on systems with spin-orbit interaction and for systems with antiferromagnetic elements. In the latter case, a preliminary model study of spin dependent transport in antiferromagnets is reported, it has revealed that i) giant magnetoresistive effects are possible, and ii) nanostructures containing antiferromagnetic elements will exhibit current-induced magnetization dynamics. In particular it turns out that, contrary to the ferromagnetic case, the spin transfer torques act throughout the entire free antiferromagnet to cooperatively switch it, a result of the special symmetries of the antiferromagnetic state. This implies that the critical current for inducing collective magnetization dynamics is likely to be lower in antiferromagnetic metal nanostructures than in ferromagnetic spin valves.

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