Composition dependence of magnetic and magnetotransport properties in C₆₀-Co granular thin films

Composition dependence of magnetic and magnetotransport properties in C₆₀ Co_x thin films exhibiting large magnetoresistance (MR) effect was investigated in the Co composition range of x=8-20, where x denotes the number of Co atoms per C₆₀ molecule. From the superparamagnetic magnetization curves observed, the average diameter (d_ave) of Co nanoparticles dispersed in the matrix phase was evaluated to be approximately 1 nm for the sample of x=8, and increased with the Co composition, x. By analyzing the temperature (T) dependence of resistivity based on the model by Abeles [Adv. Phys. 24, 407 (1975)], the average charging energies (〈E _C 〉) of Co nanoparticles were evaluated to be 2-9 meV for the samples of x=8-17 while the considerably weak temperature dependence suggested much smaller values of 〈 E _C 〉 for the samples of x>17. The composition dependence of d_ave and 〈 E _C 〉 revealed a structural transition from well-defined granular structures in the range of x=8-17 to magnetically and electronically coupled states of Co nanoparticles over x∼17. As a result of the structural change, the MR behavior became different between the two composition regions separated at x∼17. In particular, for the samples of x=8-17, the bias-voltage (V) dependence of the MR ratio in the low- V region fits well with an unusual exponential form of MR= MR₀ exp (-V/ V_c) at T<20 K, and it is most remarkable that the zero-bias MR ratio (MR₀) is in proportion to 〈 E _C 〉 and also that the fitting parameter V_c is closely correlated with 〈 E _C 〉. These results indicate that the charging effect of Co nanoparticles plays an important role in the anomalously large MR effect of C₆₀ -Co granular films. In addition, the power-law dependence of MR on T (MR T-α,α∼2) was also observed at relatively high temperature range T10 K in the wide range of the composition.