Tunnel Magnetoresistance
Introduction
Tunnel magnetoresistance (TMR) is a magnetoresistive effect that occurs in a magnetic tunnel junction (MTJ), which is a component consisting of two ferromagnets separated by a thin insulator. The insulator, typically only a few nanometers thick, is thin enough to allow quantum tunnelling of the electrons through the insulator barrier. The magnitude of the tunnelling current depends on the relative alignment of the magnetic moments (or spins) in the ferromagnetic layers.
Principle of Operation
The operation of a magnetic tunnel junction, and hence the tunnel magnetoresistance effect, is based on the principle of spin-dependent tunnelling. This is a quantum mechanical effect where the probability of an electron tunnelling through a barrier depends on the relative alignment of the electron's spin and the magnetic moment of the ferromagnets.
When the magnetic moments of the two ferromagnets are aligned (parallel configuration), the tunnelling probability is higher, resulting in a lower resistance. Conversely, when the magnetic moments are anti-aligned (antiparallel configuration), the tunnelling probability is lower, leading to a higher resistance. This change in resistance due to the relative alignment of the magnetic moments is the tunnel magnetoresistance effect.
History
The tunnel magnetoresistance effect was first predicted theoretically in 1975 by M. Julliere, a French physicist. However, the experimental confirmation of this effect was not achieved until the 1990s, due to the technical difficulties in fabricating high-quality magnetic tunnel junctions.
The first significant observation of the TMR effect was reported in 1995 by a research group led by J. S. Moodera at the MIT. They observed a TMR ratio of about 11.8% at low temperatures in junctions composed of iron and chromium oxide.
Since then, the TMR effect has been extensively studied and has found applications in various fields, particularly in spintronics, where it is used for information storage and readout.
Applications
One of the main applications of the tunnel magnetoresistance effect is in MRAM (Magnetoresistive Random-Access Memory). In MRAM devices, information is stored in the form of the relative alignment of the magnetic moments in magnetic tunnel junctions. The state of each memory cell (0 or 1) is determined by the resistance of the MTJ, which is read out using the TMR effect.
Another application of the TMR effect is in magnetic field sensors, particularly in read heads for hard disk drives. The high sensitivity of the TMR effect to the magnetic field makes it suitable for this application.
Future Directions
Research is currently ongoing to further improve the performance of devices based on the tunnel magnetoresistance effect. One of the main areas of focus is on increasing the TMR ratio, which is the percentage change in resistance due to the relative alignment of the magnetic moments. Higher TMR ratios would lead to better performance in applications such as MRAM and magnetic field sensors.
Another area of research is on understanding and controlling the spin-dependent tunnelling process at a fundamental level. This could potentially lead to the development of new devices and technologies based on the TMR effect.