Magneto-impedance properties of thin-film type sensors using CoNbZr/SiO ₂ multilayer films

Thin-film magneto-impedance (MI) sensors have attracted much attention because of their advantages such as high sensitivity at room temperature and ease of miniaturization. However, further improvement in their sensitivity is required for detecting weak biomagnetic signals such as the magnetic fields of the brain. Since the high-frequency permeability of magnetic multilayer films with insulator layers improves due to the reduction in eddy current loss in the films, sensors using such multilayer films are expected to show a larger impedance change. In this paper, we fabricated sensors using Co ₈₅ Nb ₁₂ Zr ₃ /SiO ₂ multilayer films and investigated their properties to evaluate the effect of their use on the sensor properties. The high-frequency permeability of the multilayer films improved as compared with that of CoNbZr single-layer films due to the reduction in eddy current loss. Because of the improvement in the permeability, the maximum impedance change ratio (ΔZ/Z) of the sensors using the multilayer films increased from 140% to 280% as compared with that of the sensor using the single-layer films. Thus, the use of magnetic multilayer films is effective for increasing the ΔZ/Z value of MI sensors.