Quantification of the spin-transfer torque effect is a major advance towards the realization of practical non-volatile magnetic memory devices.
Electric currents—as opposed to magnetic fields—will be used to control the operation of next-generation magnetic storage devices. These ‘current-controlled magnetic memories’ will be faster and have a wider range of applications than conventional technology based on the use of external magnetic fields.
Exploitation of the spin-transfer torque effect—where a spin polarized current passing through a ferromagnetic layer rotates the layer’s magnetization—is being studied for the development of non-volatile magnetic devices based on current-control. However, in-spite of the extensive research in this field the spin-transfer torque effect is not fully understood.
Now, researchers at AIST, in collaboration with Canon Anelva Corporation and Osaka University, Japan have quantified the physical mechanisms of the spin-transfer torque effect; their findings are important for the development of practical device structures.1
Hitoshi Kubota’s team focussed on magnetic tunnel junctions consisting of two ferromagnetic layers (CoFeB) separated by a nonmagnetic MgO barrier (Fig. 1). When a bias voltage was applied to the junction, spin-polarized electrons tunnelled from the first to the second ferromagnetic layer, where the transverse component of the electron spins was transferred to the local magnetization, thus producing the spin torque.
A small alternating current component superimposed onto the direct current generated an oscillating torque in the ferromagnetic layer and subsequently induced a measurable voltage, which was directly related to the torque. This method enabled quantification of both the spin-transfer torque, which is parallel to the film plane, as well as the field-like torque, which is perpendicular to the plane.
“Our results show the importance of the bias dependence of the in-plane torque, which against expectations enhances the torque at those values of bias voltage at which magnetization reversal from the parallel state to the anti-parallel state occurs,” says Kubota. “This non-linear bias dependence is directly related to electronic structure of the ferromagnetic layers. Further research on the control of spin-transfer phenomenon will lead to the development of new spintronic technology such as low power consumption magnetic memories and high frequency oscillators.”
References
Kubota, H. et al. Quantitative measurement of voltage dependence of spin-transfer torque in MgO-based magnetic tunnel junctions. Nature Phys. 4, 37–41 (2008).
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Spin-transfer torque: Magnetic enigma. NPG Asia Mater (2008). https://doi.org/10.1038/asiamat.2008.22
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DOI: https://doi.org/10.1038/asiamat.2008.22