| Summary: | 碩士 === 國立交通大學 === 電子物理系 === 91 === We have studied magnetic and electrical properties of granular CoX(SIO2)1-X cosputtered samples near the metal-insulator transition. The composite samples were prepared in a magnetion cosputtering system with Co and SiO2 targets mounted separately on DC and RF guns. The magnetic metal volume fraction was controlled by the relative sputtering rates. Using this method, we can make a series of samples spanning from weakly to strongly disordered regimes.
Samples near the metal-insulator transition demonstrate clear lnT dependent resistivities, consistent with many other 3D granular systems. The physical mechanism is still unclear. However we found that the slope of Dr v.s. lnT increases with disorder and seems has a quadratic dependence on r. Samples with more disorder show hopping behaviors that resistivity increases exponentially with decreasing temperature . A temperature induced crossing behavior from Mott hopping ( v=1/4) at high temperatures to ES hopping ( v=1/2) at low temperatures has been observed. As the sample becomes more disorder, the Mott hopping temperature range shrinks and ES hopping temperature range extends to a higher temperature. Fits to data suggest that the localization length of Co electron is greatly reduced by increasing disorder (decreasing Co concentration).
Magneto-transport behaviors of these granular CoX(SiO2)1-X sample demonstrate hysteresis similarly to the observed GMR in magnetic multilayer systems and TMR in magnetic junction systems. MR cycles through a curve with maximum at the field near the coercive field of the sample and decreases with increasing field intensity until the saturation field. MR for two different field directions, one is parallel to the film plane (H^) and the other is perpendicular to the film plane (H//), were investigated. The resistance was measured by 4-probe measurement with current flowing along the sample plane.
The MR peak occurs at a much bigger field with a broader shape in H^ than that in H//. However the MR ratios are roughly the same for both situations. Incorporated with shape anisotropic model and spin-dependent effects, we suggest that our samples are made of ellipsoidal grains with the long axis lying on the film plane.
For samples near the metal-insulator transition, the magneto-resistance due to spin-dependent scattering effect. The magnitude of MR ratio is 0.05%~0.56% at 10K. For samples in strongly disordered region, the magnetoresistance comes from spin-dependent tunneling effect. The magnitude of MR ratio is 2%~5.5% at 10K.
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