Ultra-Fast Spin Transfer Torque Switching of Perpendicular Magnetic Tunneling Junctions With Ferrimagnetic Free Layer

Abstract

Magnetic tunneling junctions (MTJs), promising for the development of next-generation information processing and storage devices, consist of two magnetic layers separated by an insulating barrier. Lower resistance, read as 0, occurs when the magnetizations of two layers are parallel, while higher resistance, read as 1, results from antiparallel alignment. Three key requirements of memory devices are: writing data, storing data, and reading data. While reading data, the voltage is applied, and the resistance state of the devices is read. To get the lower read error rate, the magnetoresistance of the devices must be higher. We achieved a magnetoresistance of 257%, the highest value reported in voltage controllable perpendicular MTJ with conventional MgO barrier. We explored MgAl2O4 as an alternative tunneling barrier and demonstrated its superior performance in high voltage operation. For longer data retention, the thermal stability factor of the devices must be higher. The thermal stability factor is directly proportional to the thickness of the free layer. We successfully demonstrated a perpendicular MTJ with multi-interface free layer, showcasing a reasonably high magnetoresistance, 212%. The multi-interface free layer allows a thicker free layer, addressing the thermal stability issue of the memory devices. Writing speed is associated with the switching speed of the recording or free layer. We designed a unique multilayer, [Gd/Co2Fe6B2]N ferrimagnetic free layer structure, achieving the magnetoresistance of above 45% competent of doing spin transfer torque (STT) switching in ferrimagnetic structure. The switching speed of 0.35 ns was demonstrated, at least 5 times faster than that in similar ferromagnetic structures fabricated by us.