🔄 Spintronics — Spin Transport & GMR Effect

GMR is a quantum effect in which the electrical resistance of a ferromagnet/spacer/ferromagnet stack is much lower when the two magnetic layers are aligned parallel than when they are antiparallel. It was discovered in 1988 by Albert Fert and Peter Grünberg, work recognised with the 2007 Nobel Prize in Physics.

Spin-up and spin-down electrons are treated as two independent conduction channels, each with its own resistance depending on whether it is the majority or minority spin in a given ferromagnet. In the parallel state one channel stays low-resistance in both layers; in the antiparallel state every channel meets high resistance in at least one layer, raising the total.

P sets how strongly a layer's magnetisation favours one spin direction, controlling the gap between the majority resistance R↑ = R₀(1 − P) and minority resistance R↓ = R₀(1 + P). A higher P widens that gap, producing a bigger difference between the parallel and antiparallel resistances and hence a larger GMR ratio.

The pinned layer's magnetisation is held fixed, exchange-biased against an antiferromagnet in real devices, while the free layer can be switched by clicking it or pressing Flip Layer. Comparing the free layer's direction with the fixed pinned layer determines whether the stack sits in its low- or high-resistance state.

GMR read heads let hard-disk drives detect much weaker magnetic fields from recorded bits than earlier technologies, driving a huge jump in storage density through the 1990s and 2000s. The same physics underlies modern MRAM memory and a wide range of magnetic field sensors used today.