Giant magneto-impedance (GMI) in amorphous wire, single layer film and sandwich film

We present a review on giant magneto-impedance (GMI) in amorphous wires, single layer films and sandwich films, in which the effects with sensitivity up to 120%/Oe, 8%/Oe and 50%/Oe, respectively, have been found at MHz frequencies, even for tiny samples of 0.1-1 mm in length. GMI utilizes the skin effect (or the inductive effect) at which both the skin depth and the transverse permeability can be controlled with an external applied field. Consequently, GMI depends considerably upon a particular magnetic structure, a frequency range and an excitation method, varying which various types of GMI characteristics can be obtained: having a maximum or a minimum at zero external field, without a hysteresis or exhibiting a sharp bistable hysteresis, symmetrical or asymmetrical with respect to the field. The comparative analysis of GMI in a uniform material in the form of a single magnetic layer and a composite structure in the form of a sandwich film with the inner high conductive non-magnetic material is given as well, demonstrating that the sandwich structure can show an increase in GMI of about a factor of 10 over a single magnetic layer and the maximum GMI effect occurs at considerably lower frequencies than that for a single layer. GMI has made it possible to construct new micro-sized magnetic field sensors with a high sensitivity and quick response, which are expected in the near future to replace giant magnetoresistive elements and fluxgate sensors.