What is the effect where a material's electrical resistance changes in a magnetic field?

The effect in which a material’s electrical resistance changes in a magnetic field is called magnetoresistance.

Magnetoresistance occurs because a magnetic field can alter how charge carriers move through a material. The Lorentz force bends the paths of moving electrons, while in magnetic materials the field can also change magnetization and electron scattering. Depending on the material and mechanism, resistance may increase or decrease.

William Thomson, later known as Lord Kelvin, discovered ordinary magnetoresistance in iron and nickel in 1856. His observations led to the study of anisotropic magnetoresistance, in which resistance depends on the angle between the current and the material’s magnetization. Later varieties included giant magnetoresistance, tunnel magnetoresistance, colossal magnetoresistance, and extraordinary magnetoresistance.

Magnetoresistance is important technologically. Giant magnetoresistance helped enable sensitive read heads for hard-disk drives, and related effects are used in magnetic sensors, biosensors, and magnetoresistive random-access memory. The effect is often summarized with the ratio [R(B) − R(0)]/R(0), comparing resistance in a field with resistance at zero field.

It is commonly confused with the Hall effect, which produces a transverse voltage across a current-carrying material in a magnetic field rather than describing resistance change itself.