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Spin-Orbit Torque MRAM (SOT-MRAM)

A type of MRAM with separate paths for write and read.


Spin-orbit torque MRAM (SOT-MRAM) is a type of non-volatile magnetoresistive RAM that has two paths, one for write and one for read.

SOT-MRAM devices are comprised of a magnetic tunnel junction (MTJ)  that has a thin dielectric layer between a magnetic fixed layer and a magnetic free layer. Writing is performed by switching magnetization of the free magnetic layer, which is done by injecting an in-plane current in an adjacent SOT layer — typically made of a heavy metal.

Since the write current in SOT-MRAM passes parallel to, or across, the layers, current can be set arbitrarily high (within reason) without worrying about wear-out. For this reason, SOT-MRAM cells are expected to have much faster write times than STT cells. (In spin-transfer torque MRAM, or STT-MRAM, current runs through the MTJ.)

STT MRAM (left) uses current through the MTJ for writing the state. SOT MRAM (right) uses current parallel to the layers in the stack to write the state. Source: Bryon Moyer/Semiconductor Engineering

Read current can pass through the MTJ. However, if read current is too high, it begins to act like a write current and may change the value of the state of the bit, so improved read speeds over STT are not expected.

While still in development, SOT-MRAM has been proposed as an L1/L2 cache SRAM replacement thanks to its high endurance and sub-ns switching speed.