Spintronics is a contraction of “spin transport electronics,” also known as magnetoelectronics. This refers both to the science of the magnetic energy of electrons and the technology utilizing this phenomenon for purposes of reading, writing, storing, and processing data. In the last half of the 20th century, conventional electronics used the presence and absence of electron charge to express data in the binary logic of ones and zeroes. In contrast, spintronics expresses data by manipulating two weak magnetic states of electrons in metals known as spin up and spin down. Electrons do not literally spin, but point either “up” or “down” in relation to a magnetic field. Within the electronics community, spintronics is widely seen as a potential revolutionary replacement for electron charge-based equipment, enabling the development of smaller, more efficient, and faster computers.
The first spintronic devices utilized the physical principle of magnetoresistance. This referred to the changed resistance of conductive material when subjected to a magnetic field, a phenomenon that had been observed in the mid-19th century by the British scientist William Thomson (Lord Kelvin). It was not until the 1980s that researchers began to exploit this effect using metal multilayers. Working separately but contemporaneously, the scientists Albert Fert and Peter Grünberg used molecular beam epitaxy (MBE), a device that sprays vaporized materials onto substrates, to create thin film structures with alternating layers of nonmagnetic and permanently magnetized ferromagnetic metals. In 1988, they independently discovered that the electrical resistance of these devices varied from small to large, depending on the orientation of the magnetizations in the magnetic layers, a difference of such magnitude that the French team labeled the phenomenon “giant magnetoresistance” (GMR). For this work, Fert and Grünberg were awarded the Nobel Prize in Physics for 2007.
The first practical application of this research was the “spin valve,” used in a number of GMR devices. An external magnetic field applied to two ferromagnetic layers separated by a nonmagnetic spacer layer several atoms thick functioned as a switch. When magnetization in the layers was parallel or aligned in the same direction, “up” electrons moved easily between the layers. When magnetization was antiparallel or aligned in opposite directions, movement of both “up” and “down” electrons was inhibited. As researchers built and experimented with this device in the early 1990s, they realized its exceptional sensitivity to weak magnetic fields meant it could be used as a means of reading data stored magnetically on computer hard disk drives. Applied in disk drive read/write heads, the spin valve was commercialized in 1997 by IBM, which then licensed the technology. This led to the proliferation of notebook computers and digital audio players capable of storing tens of gigabytes of data. Because thin-film metal multilayers exploited quantum properties of electrons, GMR-based spintronics were heralded as an exemplar of nanotechnology by scientists and National Science Foundation (NSF) program managers lobbying for a national nanotechnology initiative in the 1990s.
Another application of spintronic devices is magnetoresistive random access memory (MRAM). Magnetic charge storage is nonvolatile, radiation resistant, and retrievable even after a computer is shut down, unlike conventional electric charge or current-based random access memory, which is lost when power is cut. Computers equipped with MRAM can have higher storage density and operate faster than incumbent technology, instantaneously activating and deactivating without having to transfer information from hard drive to chip. In first-generation MRAM, a relatively powerful electric current is used to control magnetic properties, much of which is dissipated as heat, impeding further progress in miniaturization and efficiency. Researchers have explored alternate forms of MRAM that use less power, including spin torque transfer (STT), which utilizes the energy of polarizing electrons passing through and interacting with the multilayer structure to write to memory cells. Although MRAM was commercialized around 2007, the market remained relatively limited by 2010, because of developmental delays, high manufacturing costs, and rapid improvements in existing storage technologies.
Over the last decade and a half, spintronics research has focused on improving existing GMR-based technology and developing spin transport in semiconductors. The latter field, much of it supported by the Defense Advanced Research Projects Agency (DARPA), aimed to overcome the inability of metallic multilayers to amplify signals. A spintronic semiconductor would have multifunctional capabilities of valve, storage medium, and processor, possibly all on one chip. However, much remains unknown of the physical principles of spin transport in complex hybrid structures combining semiconducting and conducting materials. As of 2010, this remained an experiment field.
Molecular Motors, Molecular Nanotechnology, Nanomanufacturing, Techno-Optimism.
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