Study of tunnel transport and novel giant magnetoresistance effect in metal-nonmetal granular alloys

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Metal-nonmetal granular alloy thin films consist of magnetic metal granules and insulating matrix, and they exhibit a novel giant magnetoresistance due to spin dependent tunneling. In this research project, we have investigated mechanism of the tunnel-type giant magnetoresistance in metal-nonmetal granular alloy thin films in order to understand (i) the general aspect of tunneling magnetoresistance and (ii) magnetotransport phenomena in microscopic magnetic materials. In addition, we have attempted to apply this tunnel-type giant magnetoresistance to practical magnetic sensing devices.Metal-nonmetal granular alloy thin films of Co-Al-O etc.were prepared by reactive sputtering. Microstructural analysis and magnetic and transport measurement were performed for the films. From these results, it is understood that the basic mechanism of the magnetoresistance is the same as that of ferromagnetic tunnel junctions. However, anomalous enhancement of magnetoresistance at low temperatures and an … More omalous bias voltage dependence of resistivity and magnetoresistance have also found for the granular alloy films. For example, the magnetoresistance value of Fe-Al-O films reaches more than 30% at 4.2K.From systematic measurements of magnetoresistance, we conclude that the anomalous behavior is attributed to a higher-order tunneling effect to which a charging effect of nanometer-sized granules gives rise. The charging effect is characteristic for granular systems and is quite interesting in the research field of magnetotransport and its application. The enhanced magnetoresistance observed in this research may be used for magnetic sensors with large magnetoresistive response.In addition to above results, we have succeeded in improvement of the field sensitivity of magnetoresistance of metal-nonmetal granular alloy thin films. As metal-nonmetal granular alloy films were hybridized with soft magnetic alloy film possessing a lateral gap (we call it Granular-In-Gap structure), the effective magnetic field applying metal-nonmetal granular alloy was enforced and large magnetoresistance appeared at very low external magnetic field. In the case of permalloy used for soft magnetic alloy, only a few Oe of external field is required to obtain a large magnetoresistive response. This hybrid structure containing metal-nonmetal granular alloy is considered to be useful for practical applocations of magnetic sensing materials Less