Big data analysis is an important part of Society5.0, which is the future vision that the Japanese society is aiming for, and power saving of IT equipment is required for the big data analysis. MRAM with high energy efficiency is attracting much attention as one potential solution. MRAM is a memory that stores information as the magnetization (magnet orientation: upward or downward) of magnetic tunnel junction (MTJ) elements, and it has features such as non-volatility that does not require standby power, high-speed operation, and high durability. Current-writing type MRAM (STT-MRAM) that write and read information by directly applying current to MTJ elements have already been commercialized as system LSI mixed memory.

Meanwhile, spin-orbit torque MRAM (SOT-MRAM) has attracted much attention as one of the candidate technologies for next-generation MRAM. In SOT-MRAM, current is applied to the spin source layer adjacent to the MTJ elements, and information is written by using the spin current generated by charge-to-spin conversion from the current to control the magnetization of MTJ element. Information is read by applying micro currents to the MTJ elements in the same manner as an STT-MRAM. In this memory structure, current is not applied to the MTJ elements when writing, so in principle there are no problems such as breakdown of MTJ elements, which is an issue during high-speed STT-MRAM operation. For this reason, SOT-MRAM has the advantage that it is easier to achieve both high-speed operation and high reliability compared to STT-MRAM, so application as ultra-high-speed memory is expected. Research and development of SOT-MRAM has so far used nonmagnetic material as the spin source layer. In nonmagnetic materials, a spin current with the spin polarized in the in-plane direction of the thin film is generated, which enables information writing by in-plane magnetized MTJ elements. However, when this material is integrated to perpendicularly magnetized MTJ elements capable of high memory integration, many problems such as miswrite occur. Therefore, realization of new charge-to-spin conversion technology suitable for writing perpendicularly magnetized MTJ elements is highly demanded.

Researchers in AIST were focusing on ferromagnetic materials as a new candidate spin source layer material to improve the performance of SOT-MRAM (left figure). They have significantly improved spin conversion efficiency by clarifying the origin of the conversion mechanism.

Researchers in AIST were focusing on charge-to-spin conversion phenomena in ferromagnetic materials which could potentially improve the performance of SOT-MRAM. However, the detailed mechanism of charge-to-spin conversion in ferromagnetic materials has not been clarified. Thus, there was no valid guidelines to realize the high efficiency of charge-to-spin conversion (spin conversion efficiency) essential for application. This work developed a magnetic multilayer structure enabling to accurately detect the charge-to-spin conversion in ferromagnetic material and succeed on systematical examination of its spin conversion efficiency. As a result, it was clarified that there were two different spin conversion mechanisms originating from the interface and interior (bulk) of the ferromagnetic material. They also improved the spin conversion efficiency by controlling the magnetic material at the interface (right figure). These achievements contribute to next-generation memory SOT-MRAM, which combines ultra-fast operation with power savings, and it will lead to energy saving and higher performance in mobile terminals and data centers in the future.