Size-dependent magnetic properties ofNi/C60granular films

Abstract

${\mathrm{N}\mathrm{i}/\mathrm{C}}_{60}$ granular films were prepared by magnetron sputtering and thermal evaporation techniques with different ratios of Ni atoms to ${\mathrm{C}}_{60}$ molecules. A high-resolution transmission microscopy analysis shows that Ni nanoparticles are well isolated and embedded in an amorphous ${\mathrm{C}}_{60}$ matrix. X-ray-diffraction and Raman spectra indicate lattice expansion of Ni particles and charge transfer from Ni to ${\mathrm{C}}_{60},$ which provide clear evidence that strong interfacial interactions exist between Ni particles and the ${\mathrm{C}}_{60}$ matrix. Measurements of the surface magneto-optical Kerr effect show that the coercivities ${(H}_C)$ of Ni particles embedded in ${\mathrm{C}}_{60}$ matrix are enhanced significantly. The film with an average Ni particle size of 3.3 nm still presents ferromagnetism with a value of $H_C$ of 45 Oe at room temperature. We suggest that the enhancement of $H_C$ may be attributed to surface spin disorder of Ni particles induced by the strong interfacial interaction between Ni and ${\mathrm{C}}_{60}.$