2007 Nobel Prize in Physics
Reason for Award
Discovery of Giant Magnetoresistance (GMR). Key papers: M.N. Baibich et al., Phys. Rev. Lett. 61 (1988) 2472–2475 (Fert group) and G. Binasch et al., Phys. Rev. B 39 (1989) 4828–4830 (Grünberg group).
Laureates
France
Germany
Explanation
Inside computers and music players there is a tiny metal disk (a hard-disk) that stores information using small magnets. Mr. Fert and Mr. Grünberg made ultra-thin metal layers and noticed that when the direction of the magnets changes, the ease with which electricity flows changes a lot. It is like traffic suddenly speeding up or slowing down when a traffic light turns green or red. They called this effect “Giant Magnetoresistance.” Because GMR lets us read very weak magnets clearly, hard-disks could be made much smaller. The fact that you can carry a pocket music player or a light laptop today is thanks to this discovery.
Related Keywords
Giant Magnetoresistance
Giant Magnetoresistance (GMR) is the large change in electrical resistance that occurs when the magnetizations of two or more ferromagnetic layers switch between parallel and antiparallel alignment. The resistance variation can reach several tens of percent, an order of magnitude larger than traditional anisotropic magnetoresistance. The underlying mechanism is the spin-dependent scattering of electrons at interfaces and within the layers. Because of its high sensitivity, even very weak magnetic fields can be detected reliably. GMR underpins technologies ranging from hard-disk heads to automotive sensors and medical instruments.
Spintronics
Spintronics is a branch of electronics that exploits the electron’s spin in addition to its charge to process and store information. The discovery of GMR provided the first commercially viable spin device and catalyzed the field’s rapid growth. Controlling spin allows the creation of energy-efficient memories and logic elements. Phenomena such as TMR, spin-transfer torque and the spin-Hall effect are active areas of applied research. Potential applications include quantum information processing and neuromorphic computing.
Electron Spin
Electron spin is a quantum-mechanical form of angular momentum that can point either “up” or “down.” It acts like a tiny magnetic dipole, giving the electron its magnetic moment. Inside metals, the probability of scattering and the available conduction channels depend on spin orientation. Spintronics phenomena such as GMR and TMR leverage this spin-dependent transport. Future devices aim to encode information directly in the spin state instead of or in addition to conventional binary charge schemes.
Hard-Disk Read Head
The read head scans the hard-disk surface and converts the magnetization of each bit into an electrical signal. Before GMR, inductive coils and AMR sensors were typically used. Because GMR heads are extremely sensitive, bits can be made smaller while still delivering adequate signal, leading to dramatic increases in recording density. Since the first commercial GMR head shipped in 1997, areal densities have grown at roughly 40 % per year. Modern TMR (magnetic tunnel junction) heads dominate the market, but their conceptual roots lie in GMR.
Magnetic Multilayer
Magnetic multilayers are artificially engineered stacks in which magnetic and non-magnetic layers only a few Ångströms to nanometres thick alternate. Atomic-scale control produces magnetic properties unattainable in bulk materials. Phenomena such as GMR, spin-valves and exchange bias rely on this architecture. Molecular-beam epitaxy and sputtering are common growth techniques. Multilayers are also central to research on spin-wave devices and topological magnetic materials.
Spin Valve
A spin-valve consists of two ferromagnetic layers separated by a thin non-magnetic spacer; one layer’s magnetization is fixed, while the other is free, making the structure a magnetic-field sensor. Exchange bias from an antiferromagnetic layer is often used to pin the reference layer. When a field is applied, the free layer rotates, toggling the parallel or antiparallel alignment and thus the resistance. GMR spin-valves became the core element of modern read heads. The same principle underlies TMR spin-valves and spin-torque oscillators.
Anisotropic Magnetoresistance
Anisotropic Magnetoresistance (AMR), first reported by Lord Kelvin in 1857, is a classical effect in which resistance changes by a few percent depending on the angle between current and magnetization. Before GMR it was the dominant magnetic-sensor technology. AMR occurs even in single-layer films of bulk iron and is easy to manufacture. Because its sensitivity is lower than that of GMR or TMR, it is now mainly used in low-cost applications. It remains an important reference point for appreciating the magnitude of GMR.
Tunneling Magnetoresistance
Tunneling Magnetoresistance (TMR) occurs in a magnetic tunnel junction where two ferromagnetic layers are separated by an insulating barrier. If the barrier is thin, electrons quantum-tunnel through, and the tunneling probability depends strongly on the spin alignment. TMR ratios can exceed 100 % at room temperature and are used in state-of-the-art read heads and MRAM. MgO barriers with CoFeB electrodes are the canonical materials system. Like GMR, TMR employs the spin-valve principle and forms a cornerstone of spintronics.
Nanotechnology
Nanotechnology involves controlling matter on the 1–100 nm scale to elicit new functions and properties. GMR’s discovery relied on the ability to fabricate metal films only a few nanometres thick. Atomic-layer precision is crucial because quantum-mechanical effects dominate at this size scale, enabling novel phenomena. The same principles extend to quantum dots, nanowires and two-dimensional materials. Nanotechnology now plays indispensable roles in medicine, energy and many other fields.
Spin-Dependent Scattering
Spin-dependent scattering refers to the fact that an electron’s probability of scattering from defects or interfaces depends on its spin orientation. In ferromagnets the density of states differs for majority and minority spins, leading to different scattering potentials. The resistance changes in GMR and TMR arise from this contrast. Interface roughness and alloying can be engineered to tune the scattering strength. The concept is also central when discussing spin-relaxation times and spin-current losses.