The ability to control the magnetization of a solid-state component is fundamental for a number of efficient memory devices. One such case is that of magnetic random access memory based on magnetic tunneling junctions (MTJs).

Fig. 1: Sketch of the magnetic configuration in a magnetic tunneling junction.

In an MTJ, two magnetic electrodes are placed on either side of a tunneling barrier, and the tunneling resistance is relatively low or high depending on whether the magnetization of the electrodes are oriented parallel or antiparallel (Fig. 1). The magnetic configuration of the electrodes can be switched by spin transfer torque (STT), that is, the action of the spins of tunneling electrons on the magnetization of the electrodes. As the STT depends on the applied voltage, the resistance between the two values can be switched electrically.

Kyung-Jin Lee and colleagues in Korea and France1 have now demonstrated how to improve control of the magnetization switching in asymmetric MTJs. STT has in-plane and perpendicular vector components, and it is mainly the in-plane component of the STT that is believed to affect the magnetization switching. However, the team also found the perpendicular STT to have a strong influence. More importantly, because the voltage dependence of the two components is different, the perpendicular STT may be the cause of magnetization switching at undesired voltage values, resulting in ‘anomalous switching’.

In general, the perpendicular STT increases quadratically with applied voltage. However, as Lee says, “A recent theory predicted that the perpendicular STT can have an extra linear dependence when the MTJ symmetry is broken.” The team investigated two asymmetric MTJs, monitoring the magnetization switching as a function of applied voltage and external magnetic field. Not only did they confirm the extra linear dependence of the perpendicular STT, but also that this can be changed by varying the asymmetry of the MTJ.

Apart from providing experimental confirmation of the theoretical prediction about STT, the study may have important practical implications. “This linear bias dependence can be used to resolve engineering issues, which is quite important for applications,” says Lee. For example, it may be possible to design MTJs in which the sign of the linear dependence of the perpendicular STT can be properly selected, thereby eliminating anomalous back-switching.