SpinTransfer

Spin transfer, in the context of magnetism and spintronics, refers to the transfer of angular momentum from spin-polarized conduction electrons to localized magnetic moments in a material. The most studied form is spin-transfer torque (STT), which occurs when a current containing a net spin polarization passes through a ferromagnet whose magnetization is not aligned with the electron spins. Through exchange interaction and conservation of angular momentum, the spin angular momentum of the electrons is partially absorbed by the magnetic lattice, exerting a torque that can alter the magnetization direction.

The mechanism relies on a fixed reference layer that polarizes the current and a free layer whose magnetization can respond. When the spins of the incoming electrons are non-collinear with the free-layer magnetization, the transverse component of the spin polarization is transferred to the local moments, producing a torque. Depending on current magnitude and duration, this torque can switch the magnetization, drive steady precession, or excite magnetic dynamics. STT effects are commonly described in extended Landau-Lifshitz-Gilbert models that include a Slonczewski torque term.

Spin transfer is central to several technologies, notably STT-based magnetic random-access memory (STT-MRAM), where information is stored in the orientation of magnetic bits and written by current pulses. It also underpins spin-torque nano-oscillators and other nanoscale devices. More broadly, spin-transfer phenomena are complemented by related effects such as spin-orbit torque, which arises from spin-orbit coupling and can enable magnetization control with different material and geometrical configurations.