Magneto-impedance in multilayer films

This paper reviews results on magneto-impedance (MI) in multilayers consisting essentially of two soft ferromagnetic films (F) and a conductive inner lead (M): F/M/F. Multilayer film MI has a potential to be utilized for developing highly sensitive and quick-responding micro-magnetic sensors for advanced intelligent measurement and control systems and high-density magnetic recording. In materials with a uniform conductivity, a large change in impedance is accountable to a strong skin-effect, requiring the operation frequencies to be of the order of several gigahertz, for a submicron MI element. In the multilayer films F/M/F with sufficiently large difference between the conductivity values of layers M and F, the impedance can vary considerably at relatively low frequencies when the skin effect is not essential. The theory predicts that the MI ratio in multilayers 1-mu m thick with a Cu inner lead and soft magnetic amorphous films (CoSiB) or permalloy (NiFe) films is as much as 400-500% at frequencies of 10-100 MHz; in the case of a sandwich 0.1-mu m thick, the MI ratio, although drops considerably, is still large being 15-20% at frequencies of 300-400 MHz, which is in good agreement with the experimental results. On the other hand, in narrow films, the flux leakage across the inner conductor may result in a considerable drop in MI ratio if the film width is smaller than some critical value depending on the transverse permeability and the thickness of the M and F layers. (C) 2000 Elsevier Science S.A. All rights reserved.