Giant Magneto Resistance (GMR)

Magnetoresistance

Magnetoresistance is the phenomenon in which the electrical resistance of a material changes when an external magnetic field is applied. This change occurs because the magnetic field influences the motion and scattering of conduction electrons.

The term Giant Magnetoresistance (GMR) is used because the change in resistance is exceptionally large compared to ordinary magnetoresistance. In multilayer structures made of alternating ferromagnetic and non-magnetic conductive layers, the electron scattering strongly depends on the relative alignment of magnetic moments.

Principle

GMR devices work based on the Giant Magneto Resistance (GMR) effect. This effect is used in hard disk drives to read data. A low resistance state represents a digital 0 and a high resistance state represents a digital 1.

Construction

A GMR device is built using multiple thin layers of ferromagnetic and non-magnetic films. It contains:

• Free magnetic layer – its magnetization can rotate when an external field is applied.
• Pinned magnetic layer – its magnetization is fixed.

The device resistance changes significantly depending on whether the magnetic layers are parallel (low resistance) or anti-parallel (high resistance).

Working of GMR Devices

• Multilayer Structure: The GMR device consists of two or more ferromagnetic layers separated by a thin non-magnetic conducting layer.
• Spin Polarization: When current flows through the first ferromagnetic layer, electrons become spin-polarized (their spins align in one direction).
• Resistance Change:The electrons pass through the non-magnetic layer and enter the second ferromagnetic layer. The resistance depends on whether the spins in both layers are:
  • Parallel → Low resistance
  • Anti-parallel → High resistance
• Magnetization Alignment: If the free and pinned layers align, resistance is low. When they oppose each other, resistance increases sharply.
• Reading / Retrieving Data: The device measures resistance changes and converts them into digital data (0s and 1s).