mtj

MTJ stands for Magnetic Tunnel Junction, a nanoscale spintronic device comprising two ferromagnetic layers separated by a thin insulating barrier. One layer is magnetically fixed (pinned), while the other is free to switch. Electron tunneling through the barrier is spin dependent, producing a large tunnel magnetoresistance (TMR) effect: the device conducts better when the magnetizations are parallel and less well when they are antiparallel.

Common materials and structure use a MgO barrier with adjacent CoFeB ferromagnetic layers. The interface quality and crystallinity of the barrier govern the magnitude of TMR. The pinned layer is stabilized by exchange bias or a synthetic antiferromagnet, while the free layer can switch its magnetization under applied current or magnetic field.

Operation: In memory applications, the free layer is switched by spin-transfer torque (STT) or spin-orbit torque, allowing electrical writing. The stored state (parallel or antiparallel) is nonvolatile, and a separate sense current reads the resistance. Read and write depend on devices' TMR ratio, switching energy, and magnetic stability.

Applications: MTJs are the core element of STT-MRAM, a nonvolatile memory technology pursued for caching and storage-class memory. They are also used in magnetoresistive sensors for hard-disk drives and in various spintronic circuits and neuromorphic devices.

Performance: Modern MgO-based MTJs can achieve room-temperature TMR ratios well above 100% and often several hundred percent, with ongoing improvements in endurance, retention, and switching energy. Challenges include scaling, variability, and thermal stability for ultra-small cells.