Structure magnetic memory device uses magnetisation switching

Since the magnetisation can, be in general, be reversed at high speed unlimitedly, the MRAMs are regarded as a promising replacement for currently-used semiconductor-based working memories such as static random access memories (SRAMs) and dynamic random access memories (DRAMs), which are now facing several serious issues.

The central issue of the MRAM development is how to achieve magnetisation reversal efficiently.

Recently, spin-orbit-torque (SOT)-induced magnetisation switching - where torques brought about by an in-plane current through the spin-orbit interactions are utilised - was demonstrated and intensively studied. In principle, the SOT-induced switching allows for an ultrafast magnetisation reversal in a nanosecond timescale.

The research group of Tohoku University showed a new scheme of SOT-induced magnetisation switching. Whereas there had been two kinds of switching schemes where the magnetisation is directed orthogonally to the applied write current, the present structure has the magnetisation directing collinear with the current.

The group fabricated three-terminal devices with the new structure, where a Ta/CoFeB/MgO-based magnetic tunnel junction is used, and successfully demonstrated the switching operation.

Experimental result of the resistance versus applied current density under the perpendicular field of -15 mT (a), and +15 mT (b). The fact that the switching direction depends on the sign of the perpendicular field indicates that the spin-orbit torque drives the magnetisation reversal. Credit: Shunsuke Fukami

The required current density to induce the magnetisation switching was reasonably small and the resistance difference between "0" and "1" states was reasonably large, indicating that the new structure is a promising candidate for the MRAM applications.

In addition, the group showed that the new structure has the potential to serve as a useful tool to go deeply into the physics of SOT-induced switching, in which a number of unrevealed issues remain.

The magnetic memory device can store the information without power supply, allowing a drastic reduction of the power consumption of integrated circuits. In particular, this benefit becomes significant for applications that have relatively long standby times, such as sensor nodes which are likely to perform important roles in future IoT societies.

In this regard, the present work is expected to pave the way toward the realisation of ultralow-power and high-performance integrated circuits and IoT societies.