Unprecedented Single-Digit-Nanometer Magnetic Tunnel Junction Demonstrated

A research group from Tohoku University made up of Professor Hideo Ohno, Associate Professor Shunsuke Fukami, Associate Professor Hideo Sato, Assistant Professor Butsurin Jinnai, and Mr. Kyota Watanabe has revealed ultra-small magnetic tunnel junctions (MTJs) down to a single-digit-nanometer scale that have sufficient retention properties and yet can be switched by a current.

STT-MRAM (spin-transfer torque-magnetoresistive random access memory) has been intensively developed in recent years and commercialization by Mega fab companies is expected in 2018. The STT-MRAM is capable of replacing existing semiconductor-based working memories due to its excellent capabilities in terms of operation speed and read/write endurance. Moreover, it is nonvolatile, i.e., no power supply is required to retain stored information, making it indispensable for future ultralow-power integrated circuits.

MTJs are the heart of STT-MRAM. To continue the journey to increase the performance and capacity of STT-MRAM, it was essential to make the MTJ smaller, while maintaining the capabilities to retain information and be switched by a small current. CoFeB/MgO-based MTJs developed by the same group in 2010, in which an "interfacial anisotropy" at the CoFeB/MgO interface was utilized, paved the way down to around 20-nm generation. However, below 20-nm the desirable retention and switching properties could not have been realized simultaneously. Therefore yet another new approach was required.

The research group at Tohoku University used a "shape anisotropy", which had not been utilized effectively in devices suitable for integration, and developed ultra-small MTJs down to less than 10 nm, or a single-digit-nanometer scale.

The "shape-anisotropy" MTJ has a pillar-shaped magnetic layer, by which the film's normal direction becomes a magnetic easy axis due to the "shape anisotropy". This is in contrast to the "interfacial-anisotropy" MTJs, which were achieved by reducing the thickness of the magnetic layer (Fig. 1 (b)). The smallest diameter of MTJ studied was 3.8 nm, which is an unprecedented scale based on previous research endeavors.

Sufficiently high retention properties, represented by thermal stability factors, were obtained; the obtained value of more than 80 had never been achieved through the conventional scheme. Furthermore, current-induced magnetization switching is observed for the "shape-anisotropy" MTJs with various diameters including below 10 nm devices.

The developed MTJ can work with generations of future semiconductor technologies. The single-digit-nanometer MTJ corresponds to more than 100 Giga-bit capacity, which is about 100-times larger than the current working memory technology.