Structural and magnetic properties of nanocomposite Co:C films

Abstract

The structural and magnetic properties of Co:C nanocomposite films at room temperature were investigated as functions of annealing temperature, Co concentration, and film thickness. The as-deposited films, which were cosputtered from Co and C onto water-cooled glass substrates, are nonmagnetic amorphous Co-C alloys. Hexagonal-close-packed ͑hcp͒ Co grains are formed in 100 nm films annealed at 300°C and most Co takes this structure when annealed at higher temperatures. The sizes of the Co grains range from 10 to 25 nm, with larger grain size resulting from higher annealing temperature and higher Co concentration. With the increase of annealing temperature and Co concentration, the magnetic activation or switching volumes increase faster than the physical grain volumes, suggesting the increased exchange coupling between neighboring Co grains. The films with high coercivities have optimal combinations of large grain size and weak intergrain exchange coupling before a network-like structure is formed and the percolation threshold is reached. Coercivities of about 800 Oe were obtained in 100 nm films with annealing temperature of 400°C and Co concentration of 60 at. %. Higher coercivities, up to 1030 Oe, were obtained in films with reduced thickness and elevated annealing temperature.

Magnetic nanodot arrays produced by direct laser interference lithography

“…Co-C films are chosen in this exploratory study because they form a simple system containing immiscible magnetic ͑Co͒ and nonmagnetic ͑C͒ phases and the metastable Co carbides decompose easily into Co and C upon annealing. [7][8][9][10] Magnetic properties have been studied in laterally homogenous films in both the as-deposited and annealed states. 7,8 One can therefore anticipate that the behavior of laser quenched alloys is similar to that of thermally treated Co-C films.…”

“…[7][8][9][10] Magnetic properties have been studied in laterally homogenous films in both the as-deposited and annealed states. 7,8 One can therefore anticipate that the behavior of laser quenched alloys is similar to that of thermally treated Co-C films. The Co dot arrays are formed by phase separation of Co or Corich clusters under the influence of laser intensity maxima which provide local annealing.…”

“…While the asdeposited films appear amorphous to x-ray diffraction, 8 transmission electron microscopy ͑TEM͒ data show that Co-C ͑Ref. 11͒ and Fe-C ͑Ref.…”

“…12͒ films actually consist of nanocrystalline grains of sizes in the 1-2 nm range. Our earlier work 8 showed that the as-deposited Co-C films were nonmagnetic ͑i.e., nonferromagnetic͒ at room temperature, but upon annealing at temperatures in the range of 350-550°C, magnetic Co nanoparticles are formed with diameters of 10-25 nm. Figure 1 shows a TEM bright-field image of the Co-C films after laser processing.…”

“…The pattern formation is attributed to the phase separation or precipitation of Co under local heating at the laser intensity maxima. [7][8][9][10] The as-sputtered Co-C film contains some Corich nanoclusters that are superparamagnetic at room temperature. After laser patterning, there are local atomic rearrangements that form more strongly interacting magnetic Co or Co-rich clusters and yield the observed magnetic phase at room temperature.…”