Co/SiO2顆粒狀混合結構樣品在金屬-絕緣相變附近電性與磁性質的研究

Abstract

為了研究顆粒狀結構樣品在金屬-絕緣相變附近的物性，我們成功利用兩支磁控濺鍍槍(DC與RF)分別以Co與SiO2為靶材，製造了一系列Co/SiO2顆粒狀結構樣品。藉由Co與SiO2兩者的相對鍍膜速率，我們可以調變樣品內的金屬體積百分率，使樣品無序程度由弱到強分部。 在電阻量測部分，發現隨著樣品無序程度增加，樣品電阻率與溫度的關係，漸漸從金屬行為(高溫有正TCR)變為非金屬行為(高溫時有負TCR)，在無序程度大時，隨著溫度降低，電阻率增加的越快速。低溫時(55K~1.5K)，在金屬-絕緣相變附近的樣品電阻率與溫度有 的關係，這與其他文獻上的3D金屬/絕緣系統所得結果相同，且樣品無序程度加大，在低溫時電阻率隨著溫度下降增加的越快。而在無序程度更大的樣品，我們發現電阻率與溫度有 的關係。在高溫時，較符合Mott hopping的預測(v=1/4)；在低溫時，則符合Efros與Shklovskii考慮電子-電子作用力的跳躍式傳輸預測(v=1/2)，指出樣品已進入強無序地帶。當樣品的無序程度增加，符合Mott hopping預測的溫度範圍縮減，而符合ES模型預測的溫度範圍則會往高溫延伸。且從數據中我們也可推測侷限長度將隨著樣品無序程度增加 而快速的減小。 在磁性質部分，樣品上所觀察到的磁滯曲線與磁電阻曲線與在多層膜系統中GMR還有磁性接面系統的TMR行為很類似。藉由磁化強度與磁電阻的量測，成功的證明樣品處於矯頑磁場(Hc)時，樣品內的各磁化強度方向剛好互相抵消，將出現最大的磁電阻變化率。我們發現在兩種不同方向的磁場(垂直、平行於膜面)磁電阻曲線形狀並不相同，垂直膜面磁電阻曲線比起平行膜面方向來的平緩，由形狀異向性與自旋相關散射，推測出樣品內磁性顆粒可能形狀為類似壓扁的橄籃球，由於這樣不規則的形狀產生磁異向性，使得垂直磁場方向不易被磁化，其垂直磁場磁電阻率在低磁場時變化較緩慢，曲線較平滑。而平行磁場方向容易被磁化，所以磁電阻率在低磁場時變化快速，曲線較陡峭。 此外，隨著樣品無序程度增加，樣品的磁電阻率變化率有上升的趨勢，在金屬-絕緣相變附近的樣品磁電阻由GMR效應主導，在溫度10K時，樣品最大磁電阻率變化率在0.05%~0.56%之間。隨著樣品無序程度增加，在強無序地帶時，樣品磁電阻由TMR效應主導，其樣品最大磁電阻率變化率在2.0%~5.5%之間。

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.