Spin electronics (see also the tutorial)
Spin electronics refer to systems where not only the charge properties of the transport electrons are taken into account, but their magnetic moment -or spin- as well. Though transport properties can be changed by acting on the spin by changes in the magnetization. The above phenomenon can only be observed in very small structures, and in particular in very thin films, because the spin coherence is conserved on very short distances (nm scale). Based on this principle, systems exhibiting giant magnetoresistance (GMR) and tunnel magnetoresistance (TMR) have been proposed in the 1970s. Their experimental demonstration, realized in 1988 both in France and Germany, has been awarded by the Nobel Prize in Physics to Albert Fert and Peter Grünberg in 2007. The GMR effect is widely exploited in hard disk drive of computers in which the information coded on a magnetic medium is read by μm² surface GMR elements. As GMRs and TMRs sense the magnetic field, unlike SQUIDS, their sensitivity does not depend on their size and they can be miniaturized down to micron size. Schematic of a MR device used in sensing application and typical resistance-vs-field response is given in Figure 1.Typical GMR noise level is around hundred pT/sqrt(Hz) to few nT/sqrt(Hz) at RT. Moreover, MR technology can be used into biological tissues, in fluidic environments upon proper surface passivation.
Figure 1: Left: Typical configuration of a GMR or TMR element is the spin valve, where one part of the layers has a fixed magnetization (pinned layer-in red) and the other a magnetization which follows the in-plane applied field (free layer-pink). These two parts are separated by a thin non-magnetic layer (copper for the GMR, insulator like MgO for TMR). Right: Overall resistance change in a GMR as a function of the field experienced by the free layer, with two saturation levels corresponding to parallel or anti-parallel state. The linear-variation part can be used for magnetic field detection.
Figure 2: Comparison of different magnetic sensors. Sensitivity in field is given for a bandwidth of 1Hz.
