Spintransfertorque

Spin transfer torque (STT) is a phenomenon in which the magnetization of a ferromagnetic layer experiences a torque due to a spin-polarized electric current. As spin-polarized electrons pass through a magnetic material, their spins tend to align with the local magnetic moments, transferring angular momentum and exerting a torque that can reorient the magnetization. The concept was developed in the 1990s by theorists Slonczewski and Berger, and has since become central to current-driven spintronics.

In many devices, a fixed 'pinned' magnetic layer polarizes the current while a second 'free' layer absorbs the torque. The resulting STT can compete with intrinsic damping and, if strong enough, reverse or oscillate the magnetization. The torque is often described as comprising an in-plane (adiabatic) component that lies in the plane of the magnetization and a field-like (non-adiabatic) component that acts perpendicular to it; the non-adiabatic term is characterized by a beta parameter and plays a crucial role in switching efficiency. In theoretical treatments, STT enters the Landau-Lifshitz-Gilbert equation as an additional torque term.

STT is a key mechanism in magnetic tunnel junctions (MTJs) and read/write memory devices. Spin transfer torque MRAM uses current-induced switching of the free layer to store information, offering nonvolatility, scalability, and fast operation. STT is also exploited in spin-torque nano-oscillators and in controlled domain-wall motion for racetrack memory. These devices rely on materials with high spin polarization, low damping, and high-quality interfaces, such as CoFeB/MgO stacks.

Current densities required for switching are typically on the order of 10^6 to 10^7 A/cm^2, though materials and geometry can reduce this. Ongoing research aims to reduce switching energy, increase endurance, and distinguish STT from related mechanisms such as spin-orbit torque, which uses spin-orbit coupling in heavy metals to generate torque without a separate spin-polarizing layer.