What Is Giant Magnetoresistance?

Giant magnetoresistance (GMR) is a quantum mechanical phenomenon in which the electrical resistance of a thin-film multilayer structure composed of alternating ferromagnetic and non-magnetic conducting layers changes dramatically depending on the relative orientation of magnetization in adjacent ferromagnetic layers. When the magnetizations are aligned in parallel, resistance is low; when they are antiparallel, resistance rises substantially, with changes of 10 to 80 percent observed experimentally. This sensitivity to magnetic state, far larger than conventional magnetoresistance effects, gave the phenomenon its name.

GMR was discovered independently in 1988 by Albert Fert at the University of Paris-Sud and Peter Grünberg at Forschungszentrum Jülich. Fert's group measured resistance changes of roughly 50 percent in iron-chromium multilayers, while Grünberg's group observed approximately 10 percent in a simpler iron-chromium-iron sandwich structure. The discovery earned both physicists the 2007 Nobel Prize in Physics, with the Nobel Committee noting the rapid transition from laboratory discovery to commercial application as one of the most striking in modern condensed matter physics. Details of the discovery and its significance are documented on the Nobel Prize website.

Physical Mechanism and Thin Film Devices

The GMR effect arises from spin-dependent scattering of conduction electrons. In a ferromagnetic layer, electrons with spin parallel to the local magnetization scatter less frequently than those with antiparallel spin, a consequence of the exchange-split band structure. In a multilayer with parallel magnetizations, one spin channel passes through all layers with low scattering, producing low overall resistance. When adjacent layers are antiparallel, both spin channels encounter high-scattering conditions in at least one layer, increasing total resistance. The magnitude of the effect depends on the thickness and composition of the magnetic and spacer layers, with chromium and copper among the most-studied non-magnetic spacers. Fabrication relies on physical vapor deposition techniques, including magnetron sputtering and molecular beam epitaxy, which can deposit layers with atomic-level precision. These thin film devices represent some of the most precisely engineered layered structures in industrial production.

Hard Disk Read Heads and Magnetoresistive Devices

The most commercially significant application of GMR has been in the read heads of magnetic hard disk drives. Before GMR, inductive read heads set practical limits on the areal density of recorded bits; the resistance change in GMR sensors proved sufficient to detect the weaker magnetic fields from smaller, more closely packed bits. IBM introduced GMR-based spin-valve read heads into commercial disk drives in 1997, and within a few years the technology became the industry standard, enabling storage densities that increased by orders of magnitude over the following decade. Perspectives on this development and subsequent spintronics advances are surveyed in APL Materials work on spintronics technology development. Tunneling magnetoresistance (TMR) devices, which replaced GMR heads in the most advanced drives, use an insulating spacer layer instead of a conducting one and produce even larger resistance changes.

Spintronics and Emerging Applications

GMR established the conceptual and experimental foundation for spintronics, a field that exploits both the charge and spin of electrons to process and store information. Spintronics extends GMR principles to a family of devices including magnetic tunnel junctions, spin-transfer torque oscillators, and magnetic random-access memory (MRAM). MRAM combines non-volatility with the read-write endurance of SRAM, making it attractive for applications ranging from automotive control units to on-chip cache memory. Research into GMR and related effects continues through the IEEE Magnetics Society and associated publications on IEEE Xplore.

Applications

Giant magnetoresistance has applications in a wide range of fields, including:

• Magnetic hard disk drive read heads
• Magnetic field sensors for position and speed detection in automotive and industrial systems
• Magnetic random-access memory (MRAM)
• Biomedical magnetic sensing and diagnostics
• Non-destructive evaluation of materials using magnetic signatures