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

Fig. 1: Schematic of the magnetic tunnel junction used in the experiments.

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.”