采用磁控溅射法在硅基片上制备了Co原子分数为13.0%的Co-C纳米复合薄膜. 在真空条件下, 对薄膜进行退火处理, 退火温度从 473 K逐步提高至773 K, 保温时间30 min. 形貌观察表明, 未经退火处理的薄膜中, Co颗粒均匀分布在非晶C基体中, Co颗粒尺寸为1.5-3.0 nm; 673 K退火后, Co颗粒尺寸增大. 磁性能测试表明, 未经退火处理的薄膜磁性较弱, 随着退火温度升高, 薄膜的磁化强度和矫顽力均明显增大; 当退火温度增加至673-773 K时, 薄膜呈现出低温铁磁性、室温超顺磁性的典型颗粒体系磁性特征. 磁输运特性研究表明, 未经退火处理的薄膜在温度为4.2 K, 磁场为3980 kA/m时表现出1.33%的负磁电阻, 随着退火温度升高, 样品磁电阻值下降; 电阻与温度关系在4.2-60 K范围内符合lnR-T-1/4线性关系, 磁输运遵循变程跳跃 (variable range hopping)传导机制.
Co-C nanocomposite thin films with a Co atomic content of 13.0% were fabricated onto Si (100) substrates by magnetron co-sputtering technique. Post annealing was carried out in vacuum at annealing temperature ranging from 473 K to 773 K for 30 min. TEM images indicate that the Co nanoparticles are dispersed uniformly in an amorphous carbon matrix for the as-deposited samples, and Co particle size is in a range of 1.5-3.0 nm. After annealing at 673 K, the average Co particle size is enlarged distinctly. Magnetization hysteresis loops reveal that the as-deposited thin films show low magnetization. As annealing temperature is increased, both magnetization and coercivity are enhanced significantly. The samples annealed at 673 K and 773 K show ferromagnetic behaviors at low temperature, and superparamagnetic behaviors at room temperature, which are characteristic magnetic features for granular system. A negative magnetoresistance (MR) of 1.33% is observed for the as-deposited Co-C thin films at 4.2 K in the applied magnetic field of 3980 kA/m. The MR value decreases with increasing annealing temperature. Resistance (R) versus temperature (T) curves exhibit a good linear relationship of lnR-T-1/4 at a broad low temperature range, suggesting that the conduction in Co-C nanocomposite thin films follows the variable range hopping transport mechanism.
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