Abstract
Extensive efforts have been devoted to the study of spin-orbit torque in ferromagnetic metal/heavy metal bilayers and exploitation of it for magnetization switching using an in-plane current. As the spin-orbit torque is inversely proportional to the thickness of the ferromagnetic layer, sizable effect has only been realized in bilayers with an ultrathin ferromagnetic layer. Here we demonstrate that, by stacking ultrathin Pt and FeMn alternately, both ferromagnetic properties and current induced spin-orbit torque can be achieved in FeMn/Pt multilayers without any constraint on its total thickness. The critical behavior of these multilayers follows closely three-dimensional Heisenberg model with a finite Curie temperature distribution. The spin torque effective field is about 4 times larger than that of NiFe/Pt bilayer with a same equivalent NiFe thickness. The self-current generated spin torque is able to switch the magnetization reversibly without the need for an external field or a thick heavy metal layer. The removal of both thickness constraint and necessity of using an adjacent heavy metal layer opens new possibilities for exploiting spin-orbit torque for practical applications.
Highlights
IntroductionAn average effective field strength of 4 × 10−6 Oe/(A/cm2) has been obtained, except for the [Pd/Co]n/Ta multilayer[27] which was reported to exhibit a very large effective field strength to current density ratio in the range of 10−5 Oe/(A/cm[2])
The physical origin of the field-like effective field in FM/HM hetero-structures is still debatable, recent studies suggest that its ratio to charge current density in the HM layer can be written in the following form by taking into account only the spin current generated by SHE in the HM layer[46,47]: H FL/jc θSH 2e μ0MstFM
If we replace NiFe by the multilayer, the spin current from the 1 nm Pt capping layer alone would be too small to account for the effective field obtained experimentally
Summary
An average effective field strength of 4 × 10−6 Oe/(A/cm2) has been obtained, except for the [Pd/Co]n/Ta multilayer[27] which was reported to exhibit a very large effective field strength to current density ratio in the range of 10−5 Oe/(A/cm[2]) In the latter case, the spin Hall current from Ta layer alone is unable to account for the large effective field, indicating possible contributions arising from the Pd/Co interfaces internally, though the exact mechanism is not clear. The spin Hall current from Ta layer alone is unable to account for the large effective field, indicating possible contributions arising from the Pd/Co interfaces internally, though the exact mechanism is not clear Despite these efforts, so far SOT-induced magnetization switching has only be realized in FM/HM structures with ultrathin FM layers. The realization of self-current induced magnetization switching in these standalone and thick magnetic layers will open new possibilities for practical applications of SOT-based devices
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