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Nanomagnetism at NIST: Magnetotransport
The most famous phenomenon of nanomagnetism is the Giant Magnetoresistance (GMR) effect, which was the basis of the 2007 Nobel prize. GMR occurs in multilayers in which two ultrathin magnetic films are separated by a non-magnetic spacer layer. The resistance of the multilayer depends on the relative orientation of the magnetizations of the two layers. This effect won the 2007 Physics Nobel Prize largely because of the large number of important applications of the effect. GMR and the closely related tunneling magnetoresistance (TMR) are used in read heads in hard disk drives. TMR is used in Magnetic Random Access Memory (MRAM), and both are used in a variety of magnetic field sensors. NIST has been very active is this area, with a program to find the materials and processing conditions that optimize the signal. This research has led to a number of insights and to record values of the GMR. NIST has a theoretical effort to develop models to help guide the experimental effort.
In TMR, the non-magnetic spacer layer separating the two magnetic layers is replaced by an insulator that forms a tunnel barrier. Defects can pin the magnetization as it switches or precesses leading to noise, which degrades the performance of devices incorporating TMR junctions. NIST is studying the dynamic behavior of tunnel junctions as the magnetization switches through Lorentz microscopy. The observed changes in behavior for different preparation conditions suggests that nano-oxide layer layers suggested by materials research at NIST can increase device performance.
There are two big differences between GMR and TMR. Typically, TMR is larger and TMR junctions have a much larger resistance per unit area. An effort is underway to modify tunnel junctions through interactions with highly charged ions in an attempt to reduce the resistance of the tunnel junction while maintaining a large magnetoresistance.
NIST also has a program to explore the possibility of molecular spintronics, the combination of magnetoresistance effects with transport through molecules.