Spin-orbit torque (SOT) is a fundamental concept in the field of magnetic memory devices, specifically in spintronic devices. Spintronics is a branch of electronics that utilizes the spin of electrons, in addition to their charge, to manipulate and store information. Unlike conventional electronics, which relies solely on the charge of electrons, spintronics offers unique advantages in terms of lower energy consumption and non-volatility.
In magnetic memory devices, such as magnetic random-access memory (MRAM), the information is stored in the magnetic orientation of nanoscale magnets. These magnets can have two stable states, typically represented as "up" and "down," corresponding to the binary states 0 and 1.
The spin-orbit torque phenomenon occurs when a current of electrons passes through a heavy metal layer and exerts a torque on adjacent ferromagnetic material, leading to a change in its magnetic orientation. This effect arises due to the interaction between the electrons' spin (intrinsic angular momentum) and their orbital motion (movement around the atomic nucleus) in the presence of spin-orbit coupling, which is significant in heavy metals.
There are two types of spin-orbit torques:
Spin-Orbit Torque (SOT) in the SHE (Spin Hall Effect) geometry:
In this geometry, when a charge current passes through a heavy metal layer with strong spin-orbit coupling, it leads to the accumulation of spins at the material's edges.
These spin accumulations, known as spin Hall currents, then exert torques on the adjacent ferromagnetic layer, causing it to switch its magnetic orientation.
The direction and magnitude of the spin-orbit torque can be controlled by the direction and amplitude of the charge current, enabling non-volatile information storage.
Spin-Orbit Torque (SOT) in the TME (Topological Magnon Effect) geometry:
In this geometry, the spin-orbit torque is generated by the interaction between the spins of the electrons and the magnons (collective excitations of spins) in the ferromagnetic material.
This effect can also be used to switch the magnetic orientation of the adjacent ferromagnetic layer in magnetic memory devices.
Spin-orbit torque has several advantages over traditional magnetic manipulation methods, such as spin-transfer torque (STT). It can lead to more energy-efficient and faster operation of spintronic devices, making them promising candidates for next-generation memory and logic applications.
It's worth noting that research in the field of spintronics is ongoing, and new discoveries and developments may have occurred beyond my last update in September 2021.