Magneto-transport properties of insoluble Ag-Co films with thin Co concentrations

本研究的目的是了解不溶性AgCo颗粒薄膜中磁输运的机理。用常规超高真空沉积技术制备了Co浓度在10~0.6at%之间的AgCo颗粒合金薄片。从测量的磁化强度，热退火前的样品表现出超顺磁性与阻断温度低于20 K.从Langevin函数拟合推导出的平均直径表明存在的集群，包括Co原子之间的30和200，这取决于Co的浓度和热退火的历史。这些材料的磁阻在55kOe的外加磁场下不饱和，室温下为几个百分点，低温下为20%。MR比随着颗粒尺寸的增加而增加，而电阻变化则与颗粒尺寸呈相反的趋势。考虑到Ag基体电阻的变化，这与Asano等人预测的磁阻变化定性一致。然而，实验得到的比值比理论预测值大。热电功率（TEP）对低Co浓度AgCo颗粒膜的测量表明，在低温下，绝对值急剧增加，在较高温度下，变化逐渐平缓，在热退火之后，具有几乎线性的温度依赖性。这种线性依赖性来源于Co粒子在Ag中的散射。用近藤效应、基体贡献和块状Co粒子的叠加解释了0.6%薄膜的TEP温度依赖性。

The aim of this research project is to understand the mechanism of magneto transport in insoluble AgCo granular thin folms. Thin folms of AgCo granular alloys with Co concentration between 10 and 0.6 at% were made with a conventional ultra high vacuum deposition technique. From the measurement of magnetization, samples before thermal annealing show superparamagnetism with blocking temperature below 20 K.The mean diameter deduced from a Langevin function fitting indicates the presence of clusters including Co atoms between 30 and 200, which depend on the concentration of Co and history of thermal annealing. The magneto-resistance of these materials does not saturate under an applied field of 55kOe and the ratio is several % at room temperatures and 20 % at low temperatures. The MR ratio increases with the increase of particle size while the resistance change has an opposite tendency against the particle sizes. Considering with the change in resistance of Ag matrix, it coincides qualitatively with the magneto-resistance change predicted by Asano et al. However, the ratio experimentally obtained has larger values of those theoretically predicted.Thermo-Electric Power (TEP) measurements of low Co concentration AgCo granular films show a steep increase of absolute value at low temperature and the change is getting gradual at higher temperatures while the samples after thermal annealing have almost linear temperature dependence. The linear depemdence originates form the scattering with the bulk like Co particles in Ag. The temperature dependence of TEP of 0.6% thin films is satisfactory explained by the superposition of Kondo effect, matrix contribution and bulk like Co particles.