Giant magneto-impedance effect in layered thin films

Giant Magneto-Impedance (GMI) of films with a layered structure has been studied. They are Co-Si-B/Cu/Co-Si-B, Co-Si-B/Ag/Co-Si-B, and Fe-Co-Si-B/Cu/Fe-Co-Si-B with a magnetic closed-loop structure. They also have a certain magnetic configuration, for which the uniaxial anisotropy is perpendicular to both the driving current and the external field. Consequently, both reactance X and resistance R of the films change remarkably due to the external field in the frequency range from 100 kHz to 10 MHz, at which the GMI effect hardly appears in the single layer films of the same thickness. The conductivity difference between the outer and inner layers is important in order to achieve a high impedance change ratio in this frequency range. As a result, the ratios /spl Delta/Z/|Z/sub 0/|=(Z/sub maximum/-Z/sub [Hext=0]/)/Z/sub [Hext=0]/ of Co-Si-B/Ag/Co-Si-B films are 440% for a field of 9 Oe at 10 MHz, and the average sensitivity is 49%/Oe. Furthermore, /spl Delta/Z/|Z/sub 0/| of Co-Si-B/Cu/Co-Si-B and Co-Si-B/Ag/Co-Si-B films at 1 MHz is as much as 140%, and the average sensitivity reaches 15%/Oe. The sensitivity at 1 MHz is higher than that of single-layer magneto-impedance films of the same thickness by three orders of magnitude.