Tools and devices
Magnetic nanostructures and spintronics
Conventional electronics uses only one property of the electron, its charge. Electrons also carry spin, a quantum property that makes each one a tiny magnet. Spintronics is the effort to use spin as well as charge, and its first great success was built from metal layers only a few atoms thick.
Spin as a second degree of freedom
The Wikipedia article on spintronics describes the field, whose name is short for spin transport electronics, as the study of the electron's intrinsic spin and its associated magnetic moment, in addition to its charge, in solid-state devices. Using spin as an extra degree of freedom, the article explains, has implications for how efficiently data can be stored and transferred. The article traces the field's emergence to discoveries in the 1980s about how spin affects the transport of electrons in solids, including the injection of spin-polarised electrons from a magnetic metal into a normal metal in 1985 and, most importantly, giant magnetoresistance in 1988.
Giant magnetoresistance
According to the Wikipedia article on giant magnetoresistance, the effect is observed in multilayers built from alternating magnetic and non-magnetic metal layers. The electrical resistance of such a stack changes significantly depending on whether the magnetisations of neighbouring magnetic layers point the same way or opposite ways: low when they are parallel, high when they are antiparallel. Because an external magnetic field can switch the layers between those two states, a tiny field produces a large change in resistance. The physical basis, the article explains, is that the scattering of electrons depends on the orientation of their spin relative to the magnetisation of the layer they pass through.
The Wikipedia article on the effect records that Albert Fert and Peter Grünberg discovered it independently in 1988, and that the 2007 Nobel Prize in Physics was awarded to them for the discovery, which also laid the foundation for the study of spintronics. The article also warns about a common confusion in the literature: giant magnetoresistance in layered metals is unrelated to colossal magnetoresistance, a different effect found in certain magnetic oxides.
From physics to hard disks
What made giant magnetoresistance famous was how quickly it reached everyday technology. The Nobel Prize popular information for the 2007 physics prize explains that applications of the effect revolutionised the techniques used to read data from hard disks, that it plays a major role in various magnetic sensors, and that it can be regarded as one of the first major applications of nanotechnology. The prize text frames the story around read-out heads: as hard disks shrank and stored more data, each stored bit produced a weaker magnetic field, and a more sensitive detector was needed to sense it. A stack of nanometre-thin layers whose resistance jumps in a weak field was exactly that detector.
The Wikipedia article adds that the main application of the effect is in magnetic field sensors used in hard-disk drives, biosensors and microelectromechanical systems, and that multilayer structures based on it are also used in magnetoresistive random-access memory, where each cell stores one bit.
Why this counts as nanoscience
Giant magnetoresistance is a good reminder that nanotechnology does not always mean particles or tubes. The structures involved are flat and can be many micrometres wide, but the crucial dimension, the thickness of each layer, is only a few nanometres. The effect depends on electrons crossing from one magnetic layer to the next before their spin orientation is scrambled, which only happens if the layers are thin enough. Only techniques able to deposit smooth metal films a few atoms thick could make such stacks at all.
Beyond metal layers
The spintronics article describes several later directions: magnetic tunnel junctions, in which the non-magnetic spacer is a thin insulator and electrons tunnel across it; a 1990 proposal for a spin field-effect transistor that brought spintronics into semiconductors; and materials such as dilute magnetic semiconductors and Heusler alloys. It also notes interest in spintronics for quantum computing. In the literature of the early 2000s these subjects appear side by side with work on magnetic nanoparticles and patterned magnetic dots, often in the same physics journals.
Reading spintronics papers
Spintronics papers are dense with acronyms, and the same device may be described in terms of its layers, its effect or its application. Three questions help. What is the structure, layer by layer, and how thick is each layer? At what temperature was the effect measured, since many effects that are large at a few kelvin shrink at room temperature? And is the reported number a change in resistance, a ratio, or a figure of merit defined by the authors? Comparing papers without checking these points is the most common way readers are misled.
For neighbouring device subjects, see molecular electronics and nanowires; for the probe microscopes used to map magnetic domains, see seeing and touching atoms.