Magnetoresistive random-access memory (MRAM) is faster than flash memory and more durable than most other forms of non-volatile RAM. The storage bits in MRAM devices are based on magnetic tunnel junctions (MTJs), but attempts to miniaturize MTJ devices below 100 nm — the current level of miniaturization for advanced microcircuits — have struggled to meet all the requirements necessary for technological viability.

A team of researchers led by Hideo Ohno and Shoji Ikeda at the University of Tohoku in Japan1 have now developed 40 nm MTJs that meet these technological challenges. “Our MTJs currently offer the only way to simultaneously satisfy all of the technological requirements for high-performance devices,” says Ikeda.

Fig. 1: Structure of a magnetic tunnel junction. The current emerging from the fixed or free ferromagnetic layers can be used to rotate the magnetization of the free layer and write either a ‘0’ (both ferromagnetic layers aligned in the same direction) or a ‘1’ (layers oppositely aligned).

An MTJ consists of two ferromagnetic layers separated by an insulating barrier layer that is just thin enough to allow electrons to quantum mechanically ‘tunnel’ across it (Fig. 1). Electrons face a lower resistance crossing from one ferromagnetic layer to the other if the layers are magnetized in the same direction. Computing ‘bits’ based on MTJs are designed by having one ferromagnetic layer fixed and the other layer free to rotate into either the ‘0’ (low-resistance) or ‘1’ (high-resistance) state. The record ‘magnetoresistance ratio’ between these two states is 604% at room temperature, which was achieved using MTJs made from cobalt–iron–boron (CoFeB) ferromagnetic layers and a magnesium oxide (MgO) barrier layer.

Compact MTJ devices designed for a sub-100 nm architecture rely on the spin-polarized current flowing out of one electrode to rotate the free layer. Such devices must simultaneously achieve thermal stability at room temperature, operation at a low switching current and a high magnetoresistance ratio. New materials systems explored for MTJs so far, however, only partially satisfy these requirements, and require the use of expensive materials such as platinum.

Ohno and his team utilized the perpendicular anisotropy at the CoFeB/MgO interface to realize a high-performance MTJ that is also stable at the high treatment temperatures used in standard semiconductor manufacturing processes. This approach allowed the team to engineer technologically viable devices smaller than 40 nm in size.

MTJs have the potential to be used as building blocks for compact logic circuits operating at “unprecedentedly low power,” says Ikeda. These findings therefore represent an important contribution in a range of high-density electronic applications.