Theoretical Studies On Giant Magneto-Impedance Effect For Sandwich Structured Films

A model of Giant Magneto-Impedance (GMI) effect for sandwich structured films consisting of two soft magnetic layers and an inner nonmagnetic metal layer was proposed, the GMI effect was investigated theoretically. The main innovative results were as follows:1. A model of sandwich-structured film was proposed based on the principle of field superposition, the Maxwell equations were resolved under certain boundary conditions and the electromagnetic field in the sandwich-structured film was derived. From the relation between the energy transfer and the impedance for the film, the expression of the AC impedance was derived, the well accepted impedance expression for monofilm could be deduced from the expression for the sandwich structured film.2. Considering the motion of the magnetization under the external magnetic field, we used Landau-Lifshitz equation to describe the motion and we got the relation between the AC magnetization and AC magnetic field, so did the magnetic permeability tenser, then we obtained the effective AC transverse magnetic permeability.3. Utilizing the expression of the impedance and magnetic permeability, we calculated the dependence of the GMI effects on various parameters of the sandwich structured films. The results of the simulation showed that the GMI ratio for a sandwich structured film sandwiching high conductive metal layer was several times lager than that for a monofilm, and the maximum impedance change ratio occurred at the frequency of about 10 MHz which was lower than that of single layer film. If the electric conductivity of the inner layer was very small (such as metal Ti), as though it was a sandwich structured film, its GMI effect was weak, the result was in agreement with experimental results. The calculated dependences of GMI effects on other parameters were similar to the experimental results; these results verified the theoretical model.4. The GMI effects for sandwich structured film and monofilm were discussed from the calculated electromagnetic field distribution, the interaction between the inner metal layer and the outer magnetic layer was investigated, and skin effect in magnetic layer was still the important factor of large GMI effect for sandwich structured films.