Tunnel Vision: Stopping and looking at AF junctions

Tunnel Vision: Stopping and looking at AF junctions

Pick of the week this week’s tentatively steps out of its usual experimental comfort zone by looking at a purely theoretical paper, in this case APL’s “Spin-dependent transport in antiferromagnetic tunnel junctions” by Merodio et al..

Anti-ferromagnetic spintronics

Anti-ferromagnetic (AF) spintronics is becoming an increasingly popular area of research for reasons such as the insensitivity of AF materials to stray fields.

Spin transfer torque (STT) is an established phenomenon in ferromagnetic (FM) systems that has found practical applications, and it has been predicted that STT in AFs would require lower critical currents than in FM counterparts as the torque would act throughout the layer, unlike in FMs where it is most significant at the interface where the spins are injected, as well as the fact that AFs have no shape anisotropy.

Experimentally, however, AF-STT is hard to detect due to small AF magnetoresistances and difficulties in pinning AF electrodes, and therefore it would need to be inferred from variations in exchange bias at AF-FM interfaces.

Theoretical investigation of STT and TMR

This paper investigates theoretically STT and tunnelling magnetoresistance (TMR) in AF spin-valves where the non-magnetic layer is an insulator to see is the phenomena vary compared with previously investigated AF spin-valves where the non-magnetic layer is metallic.

Keldysh formalisation is used, and the AF electrodes and leads are modelled as alternating localised moments.

Theoretical agreement with experiment

It is found that the computed spatial distributions of both the in- and out-of-plane voltage induced torque in the AF layer are perfectly uniformly staggered.

The in-plane STT distribution is in agreement with previous theoretical results for metallic AF spin-valves, however the out of plane STT distribution in the insulating spin-valve is distinct from metallic spin-valves where it is random.

This is true when using either AF or FM polarising electrodes, and the spatial distribution is also not affected by the orientation of the magnetization of the electrode.

Finite STT, TMR in AF spin-valve

Overall the density of states (DOS) in the AF in unpolarised, however the local interfacial DOS  is polarised, and this allows for the finite STT and TMR effects in the AF spin-valve.

In fact, the TMR effect is found to be interesting as it is small for small voltages, but beyond a critical voltages the TMR massively increases, which is opposite to what is observed for FM TMR.


Image: Tom Olliver Some Rights Reserved