Fig. 1. Schematics for “shape-anisotropy” MTJ (a) and “interfacial-anisotropy” MTJ (b). The shape-anisotropy MTJ has a structure like a bar magnet standing.
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” (Fig. 1 (a)). 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 (Fig. 2); 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 (Fig. 3).
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.
Fig. 2. Comparison of relation between thermal stability factor and MTJ diameter for the “shape-anisotropy” and “interfacial-anisotropy” MTJs. [1] S. Ikeda et al., Nature Materials 9, 721 (2010). [2] H. Sato et al., Applied Physics Letters 105, 062403 (2014).
Fig. 3. MTJ resistance in response to applied current density for the fabricated “shape-anisotropy” MTJs with diameter D = 8.8 and 10.4 nm. Insets show the MTJ resistance as a function of out-of-plane magnetic field (unit: mT) for the same devices.
