ABSTRACT
This chapter discusses magnetoelectric properties of magnetic tunnel junctions (MTJs), memory operations of spin-transfer-torque magnetoresistive random access memory (STT-MRAM) bitcells, and advances and prospects of STT-MRAM technology. In 1990s, semiconductor industry started MRAM development. It was Freescale Semiconductor that shipped the first 4 Mb MRAM product in 2006. MRAM defines binary states by two discrete resistance values of an MTJ. Physical origins of the damping torque have been attributed to energy relaxations due to interactions with s-electrons, spin–orbit interactions, etc. Damping can be characterized from ferromagnetic resonance measurements, and α values reported with typical free-layer materials is ∼0.01 or less. When the MTJ film is patterned into an elliptical shape, shape anisotropy allows the free-layer moment to have only two energetically favorable directions along the easy-axis. To program MTJ cells, one can apply external magnetic fields as illustrated in a resistance–magnetic field hysteresis loop.
