A Novel Transverse Read Technique for Domain-Wall "Racetrack" Memories

Domain-wall memory (DWM), an extension to spin transfer torque-magnetic random access memory (STT-MRAM), stores multiple bits, each bit in an individual domain within a nanowire. The access mechanism of a DWM is carried out with the help of an access point orthogonal to the nanowire, similar to a conventional magneto-tunnel junction (MTJ) of STT-MRAM. In this context, the MTJ free layer is one of the domains of the nanowire. Prior to an access, the desired bit is brought under the access point by shifting, and then its spin orientation is detected by a traditional sensing circuit or overwritten with a write current. However, the nanowire can also be viewed as a multi-level cell (MLC)-like device when tested with an end-to-end current. Based on this observation, we devised a novel transverse read (TR) technique to detect the number of '1's stored in a DWM without shifting any domains. TR is non-destructive and requires ultra-low power insomuch as the TR current is small compared to the shifting current. The TR has interesting applications in DWM such as a neuron in a neuromorphic engine, a multi-bit digital storage device, or as a fault tolerance tool for shifting reliability in DWM memories. In this work, we experimentally demonstrate the feasibility of TR in both perpendicular and in-plane magnetic anisotropy nanowires and determine the critical requirements of successful TR operation.