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

Abstract

Composition dependence of magnetic and magnetotransport properties in C60Cox 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 (dave) 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 et al. [Adv. Phys. 24, 407 (1975)], the average charging energies (⟨ÊC⟩) 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 ⟨ÊC⟩ for the samples of x>17. The composition dependence of dave and ⟨ÊC⟩ 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/Vc) at T<20 K, and it is most remarkable that the zero-bias MR ratio (MR0) is in proportion to ⟨ÊC⟩ and also that the fitting parameter Vc is closely correlated with ⟨ÊC⟩. 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 T≥10 K in the wide range of the composition.