In recent years, the demand for low-power and high-speed memory has been increasing as big data and 5G develop rapidly. Silicon-based memory is hard to sustain Moore’s law due to its limits of capacity, power consumption and write speed. Magnetic random-access memory (MRAM) is expected to overcome these bottlenecks.
The first generation MRAM writes data by magnetic field and stores data by in-plane magnetic anisotropy, which is used in aeronautics and astronautics such as airbus A350. The second generation MRAM writes data by spin-transfer torque (STT) and stores data by interfacial perpendicular magnetic anisotropy, which is used in various Internet-of-Thing (IoT) such as Huawei GT2 smartwatch. The third generation MRAM is intended to write data by spin-orbit torque (SOT), but mechanism for storing data remains controversial. For example, TSMC/Toshiba/Stanford/Tohoku insist on the storage method of first generation MRAM, which has low power consumption but low capacity. IMEC/Samsung/Intel/Beihang adhere to the storage method of second generation MRAM, which has high capacity but the power consumption is relatively high. At present, there is no existing MRAM that meets both low power consumption and high capacity.
Researchers at Beihang University announced that the exchange bias in IrMn/CoFeB thin films can be manipulated by SOT. “It is found that a critical current density for exchange bias switching exists which is larger than that for magnetization reversal of the CoFeB layer.” Thus, they proposed the third method of storing data based on the exchange bias characteristics, which is expected to be the most suitable mechanism for SOT considering power consumption and capacity. It is an important discovery in spintronic field and the research is published in Nature Electronics.
The researchers explored the underlaying mechanism of the exchange bias with the X-ray magnetic circular dichroism (XMCD) and polarized neutron reflectometry (PNR) measurements. The exchange bias originates from the pinned spins in IrMn layer which possess a higher effective anisotropy than the rotatable spins. Furthermore, “By manipulating the current direction and amplitude, independent switching of the magnetization and exchange bias field can be repeatably achieved below the blocking temperature”.
AFM material does not demonstrate macroscopic magnetization because the spins are aligned in antiparallel direction. This research revealed that the spins in AFM layer can be manipulated by SOT current, which enable novel functions for AFM devices. As a result, the direction of pinned spins, shown in hysteresis loops as exchange bias, can be used to store information “0” or “1”. This storage method may offer a way to improve the performance of MRAMs, such as storage capacity and power dissipation.
This manuscript is published in Nature Electronics at the following URL: https://www.nature.com/articles/s41928-020-00504-6