Giant magneto-impedance in soft magnetic “Wires”

Abstract

The giant magneto-impedance, GMI, effect consists in the huge change of the impedance, both real and imaginary components, up to around 600% relative change in the presence of static magnetic fields (a few Oe) or applied stresses. Sensitivity of up to 500%/Oe has been observed in the very low (less than 1 Oe) field region. This effect was detected a few years ago in extremely soft amorphous magnetic wire-shaped materials. In the low frequency region, up to around 50–100 kHz, changes of impedance may be ascribed to the magnetoinductive effect due to the circular magnetization process. At higher frequencies (0.1–10 MHz) although lower than for on-magnetic conductors, the skin effect becomes dominant because of the large permeability. At the microwave regime, ferromagnetic resonance induces absorption phenomena. Variations of permeability originated by the static applied field result in changes in the skin depth and finally in the impedance. The main characteristics of GMI as single-peak or two-peak behaviors are described considering the magnetic anisotropies, the magnetization processes and the domain structure according to existing models. The influence on the GMI of the permeability relaxation with frequency is also discussed. Finally, attention is paid to the recent studies in the microwave regime, on novel materials as heterogeneous microwires and thin film layered structures. The main technological applications for magnetic field sensors devices based on GMI are outlined. In summary, GMI opens a new branch of research by combining classical electrodynamics and micromagnetism of soft magnets.