Structure Evolution and Corresponding Electrical Properties in Weakly Bound Co-C₆₀ Mixture

Abstract

Mixture films of Co and C60 with various compositions were fabricated by co-evaporation under UHV condition. As for the saturation composition with C60-based phase, Co5C60 is deduced from the systematic Raman analysis. It is reasonably interpreted that the C60-based phase can be realized by less than one electron transfer per a Co atom into C60 and the contribution of a covalent bonding which induces a differently coordinated structure of the C60 molecules from the fcc-C60. The electrical conduction features are dependent on the Co content. In the C60-based phase up to Co5C60, the conduction mechanism seems to be described by the band concept modified with the disorder, followed by the hopping conduction through the precipitated Co clusters or particles and then, by the metallic one with increasing Co content.

Magnetic and Magnetotransport Properties in Nanogranular Co/C<SUB>60</SUB>-Co Film with High Magnetoresistance

“…[19][20][21] It is also noticeable that the A g ðnÞ (n ¼ 1; 2) peaks are strongly damped as compared to pure-C 60 . These spectral features are fully consistent with those for the C 60 -Co compounds 10,12) in which the formation of the covalent C-Co bonds with a bond length (0.201 nm) similar to the %-complexes of transition metals was revealed. 11) This clearly suggests the formation of the similar compound even in the alt-deposited Co/C 60 -Co films.…”

“…The composition of the C 60 -Co compound in the present film is estimated to be close to C 60 Co 4 judging from the peak positions of the A g (2) and H g (8) peaks (1450 cm À1 and 1562 cm À1 ), which were indicated to shift by À4:2 cm À1 /Co atom and À3:6 cm À1 /Co atom from the peak positions for pure-C 60 (1468 cm À1 and 1574 cm À1 ). 10,12,14) The extent of charge transfer from the bonded Co atom to C 60 molecules is estimated to be as small as about 0.2 electrons per a Co atom from the H g (8) peak position which was indicated to shift by À15 cm À1 per a single electron transferred electron to a C 60 molecule. The diameter distribution of Co nanoparticles is estimated from the fitting of the M-H curve at 300 K with the Langevin function assuming the log-normal diameter distribution (the details of the procedure is described elsewhere 24) ).…”

“…The surface of the deposited film was capped with a SiO layer (160 nm thick) to avoid the easy oxidation of the C 60 -Co compound. 10,12) Raman spectroscopic analysis was performed by using a micro-Raman system (NANO-FIND-ER, Tokyo Instruments) with an Ar ion laser (488 nm), where the probing laser (50 W/cm 2 ) was guided through the backside of the MgO substrate. Any spectral changes during the illumination were not detected under the present condition.…”

“…Figure 1 shows the Raman spectrum for the Co/C 60 -Co film together with the spectra for pure polycrystalline C 60 (pure-C 60 ) and the C 60 -Co compound with a composition of C 60 Co 5 (C 60 Co 5 ) which was prepared by the co-deposition method. 10,12) The influences coming from the MgO substrate are approximately subtracted from the respective spectra in this figure. In the pure-C 60 , there appear the 10 Raman-active peaks of C 60 with icosahedral (I h -) symmetry.…”

“…In the previous studies, we have evidenced that the C 60 -Co compounds with C 60 Co x (x 5) are formed in the codeposited mixture films and that the nanogranular films of Co nanoparticles and a matrix of the C 60 -Co compound (C 60 Co 5 ) are generated under the Co-rich conditions. [10][11][12][13][14][15] Very recently, we have shown that the alternately (alt-) deposited C 60 -Co film exhibits the remarkable TMR ranging from 10% to 80%. 16,17) In the present paper, magnetic properties of the altdeposited Co/C 60 -Co film are described, and the structure and the magnetic field/bias voltage dependent magnetotransport behaviors 16,17) are discussed based on them.…”