Spintronics is the study and exploitation in solid-state devices of electron spin and its associated magnetic moment, along with electric charge. Some consider the topic esoteric, given the conceptually challenging quantum physics and chemistry that underpins it, but the same was once said of what today is mainstream electronics. The reality is that spintronics is a maturing field of applied science and engineering, as well as fascinating pure science in its own right.
Electronics is based on the manipulation of electrons and other charge carriers, but in addition to charge, electrons possess a property known as spin. When spin is manipulated with magnetic and electric fields, the result is a spin-polarised current that carries more information than is possible with charge alone. Spin-transport electronics, or spintronics, is a subject of active investigation within Europe's Graphene Flagship.
Electron spin and quantum logic
Before looking at spintronics in graphene, it is worth noting that spintronics is already established in one critical area of digital electronics, namely data storage.
Spin can be thought of as the rotation of the electron around its own axis. It is a form of intrinsic angular momentum, and can be detected as a magnetic field with one of two orientations: up and down. Combine these magnetic orientations with the on/off current states in binary logic, and we have a system of four states, with the two magnetic orientations forming a quantum bit, or qubit.
In computing technology terms, four states rather than two provides for higher data transfer speeds, increased processing power and memory density, and added storage capacity. Electron spin provides an additional degree of freedom to store and manipulate information.
The read heads of modern magnetic hard drives exploit the spin-related effects known as Giant Magnetoresistance (GMR) and Tunnel Magnetoresistance (TMR). In GMR devices, two or more layers of ferromagnetic materials are separated by a spacer. When the magnetisation vectors of the magnetic layers are aligned, the electrical resistance is lower than when the vectors are in the opposite sense. A device based on such a configuration is known as a spin valve. In TMR, electron transport is achieved by quantum mechanical tunnelling of the particles through an insulator separating ferromagnetic layers.