Magnetoresistance

Magnetoresistance is the change in electrical resistance of a material in response to an applied magnetic field. It is commonly quantified by the magnetoresistance ratio MR = [R(H) − R(0)] / R(0), expressed as a percentage. The sign and magnitude of MR depend on material, structure, and temperature.

Conventional (classical) magnetoresistance in metals results from the Lorentz force on charge carriers, which alters their trajectories and scattering, typically yielding positive MR that grows with field and mobility.

Anisotropic magnetoresistance (AMR) arises in many ferromagnetic metals due to spin-orbit coupling, with resistance depending on the angle between current flow and the magnetization.

Giant magnetoresistance (GMR) occurs in layered ferromagnetic and nonmagnetic thin films, where the relative alignment of magnetic layers controls spin-dependent scattering. This effect, discovered independently by Albert Fert and Peter Grünberg in 1988, transformed data storage technologies and earned the Nobel Prize in Physics in 2007.

Colossal magnetoresistance (CMR) is observed in certain manganese oxides (perovskites) and related materials, where moderate fields can cause orders-of-magnitude resistance changes near phase transitions. The mechanism involves complex interactions including double exchange and electronic phase separation.

Tunneling magnetoresistance (TMR) occurs in magnetic tunnel junctions, where electrons tunnel through an insulating barrier between ferromagnets, and resistance depends on the relative magnetization orientation. TMR devices use insulating barriers such as MgO or Al2O3 and are central to modern MRAM and read-head technologies.

Applications and outlook: magnetic sensors, hard-disk read heads, memory devices (MRAM), and non-volatile memory. Ongoing research explores new materials and two-dimensional systems to achieve higher MR ratios and lower power.