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

金属-非金属颗粒合金薄膜由磁性金属颗粒和绝缘基体组成，由于自旋相关的隧道效应而表现出一种新的巨磁电阻。在本研究项目中，我们研究了金属-非金属颗粒合金薄膜中隧道型巨磁电阻的机理，以了解(i)隧道型磁电阻的一般方面和（ii）微观磁性材料中的磁输运现象。此外，我们还尝试将这种隧道式巨磁电阻应用于实际的磁敏器件中。采用反应溅射法制备了Co-Al-O等金属-非金属颗粒合金薄膜。对薄膜进行了显微结构分析、磁性和输运测试。从这些结果可以了解到，磁电阻的基本机制与铁磁隧道结相同。然而，在低温下，颗粒状合金薄膜的磁电阻率和磁电阻的偏置电压依赖性也出现了异常。例如，Fe-Al-O薄膜在4.2K时磁阻值达到30%以上。从磁电阻的系统测量中，我们得出结论，这种异常行为归因于纳米颗粒的充电效应引起的高阶隧道效应。电荷效应是颗粒体系的特征，是磁输运及其应用研究领域中一个非常有趣的问题。本研究所观察到的磁阻增强可用于具有大磁阻响应的磁传感器。除上述结果外，我们还成功地提高了金属-非金属颗粒合金薄膜的磁电阻场灵敏度。金属非金属颗粒合金薄膜与具有侧隙的软磁合金薄膜（我们称之为隙中颗粒结构）杂化后，施加在金属非金属颗粒合金上的有效磁场被增强，在极低的外加磁场下出现较大的磁阻。对于用于软磁合金的坡莫合金，只需少量的外场即可获得较大的磁阻响应。这种含有金属-非金属颗粒合金的杂化结构被认为对磁敏材料的实际应用有很大的帮助

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