GIANT MAGNETO-IMPEDANCE EFFECT IN AMORPHOUS WIRES

摘要

The GMI effect, which represents the strong dependence of AC impedance of a soft magnetic conductor on applied magnetic field, can be understood in the frame of classical electrodynamics. The strong dependence of permeability in soft magnetic materials on applied field is in fact the origin of GMI effect, because the permeability together with the resistivity and with the frequency of the AC current de ermine the skin depth of the electromagnetic field in the magnetic conductor. The experimental results and theoretical considerations show that large GMI effect is achieved for magnetic anisotropy with the easy direction transversal to the longitudinal axis of conductor. The most important problem of the theory is to find the most appropriate formula for the effective circumferential permeability which best describes the response of the special domain structure to the axial driving AC current. The change of the permeability is related to the domain walls movement up to 1 MHz when they are completely motionless and to rotation of the magnetic moments for higher frequencies. The GMI effect is very important in amorphous materials as ribbons, wires, glass-covered wires and thin films. Values of ΔZ/Z up to 600 % were obtained. Big effort is consumed now to find new materials as soft as possible from magnetic point of view by the control of magnetostriction via composition or by annealing, and to induce controlled transverse/ circumferential anisotropies by controlling the induced mechanical stresses in the preparation process or by special mechanicla or magnetic annealing. A large area of applications of GMI effect was developed and is currently in progress based on the high value of the magneto-impedance and al; o on the small length of the sample that is required.