Orbitronics: new torques and magnetoresistance effects

Mathias KLÄUI

While so far the focus has been on spin currents, effects of orbital currents, i.e. the flow of the electrons with finite orbital angular momentum, can outperform conventional spin current effects. As shown, orbital currents can even play a pivotal role in generating spin currents thus leading to torques with unprecedented amplitude to manipulate magnetization. Experimentally, orbital currents for efficient manipulation of magnetization have only recently started to be explored. In order to generate orbital currents, materials with orbital Hall effects can be used that can be light metals and thus cheap, abundant and environmentally friendly.
In our work we studied spin orbit torques generated in TmIG/Pt/(Cu(O)x) heterostructures. We observed that the torques exerted on the TmIG are enhanced by a factor up to 16 if the CuOx is added on top of the Pt compared to the conventional TmIG/Pt stack [1]. Such an enhancement is extremely surprising if one considers only conventional spin-charge interconversion based on spin orbit coupling effects and given the low spin-orbit coupling of Cu and Cu(O)x one does not expect large torques. However the results can be naturally explained as Cu(O)x can generate large orbital currents that are then converted to spin currents in the Pt layer, which then manipulate the TmIG extremely efficiently. More recently we studied magnetoresistance effects in systems with layers that generate orbital currents. We found that the Orbital Rashba-Edelstein Magnetoresistance can be observed in Py/Cu(O)x, which is an orbital magnetoresistance effect related to the conventional spin Hall magnetoresistance [2]. In particular in this work, the length scale of the orbital to spin current conversion in Py could be identified as a key step to harnessing orbital currents efficiently even without a heavy metal based orbital to spin conversion layer [3].

[1] S. Ding et al., Phys. Rev. Lett. 125, 177201 (2020)
[2] S. Ding et al., arxiv:2105.04495; Phys. Rev. Lett. (in press 2021)
[3] D. Go, MK et al., Perspectives Review in EPL 135, 37001 (2021)