J. N. Zhou

Adhesion at a granular surface

The adhesion of an ultrathin carbon nitride (CN) coating to the surface of a two-phase CoPt–SiO2 granular film heterogeneous at the 10 nm scale has been studied using nanoscratch techniques. The nanoscratch resistance was found to depend sensitively on the volume fraction of the two phases. Both nanoscratch experiments and complementary electron microscopy observations indicate that CN adheres much more strongly to the SiO2 matrix than to the CoPt granules resulting in enhanced tribological performance in SiO2-rich films. The relative weakness of the CN/CoPt interface is correlated to the absence of interfacial metal nitride formation. The adhesion of the CN coating to the granular surface, the intrinsic mechanical properties of the underlying granular film (nanoindentation hardness and modulus), and the vertical rms surface roughness of the granular layer are all fundamentally changed as the CoPt content reaches the percolation threshold.

Standing spin waves in granular Fe–SiO2 thin films

Magnetically heterogeneous Fe–SiO2 composite thin films (20 nm thick) prepared using standard rf sputtering techniques have been investigated by means of ferromagnetic resonance (FMR). Besides a single FMR absorption line, corresponding to the uniform precession mode, several nearly equally spaced absorption lines could be observed close to the percolation threshold of the Fe granules fp if the applied field H is parallel to the film normal. These extra absorptions have been identified as standing spin waves. A constant separation between lines suggests that volume inhomogeneities are important (as expected in a granular film). We have determined the following characteristic lengths of the standing spin waves for three Fe volume concentrations: L=370 nm, 680 nm, and 270 nm for f=0.63, 0.66, and 0.69, respectively. In continuous thin films L is normally coincident with the film thickness but in this case L is more than an order of magnitude larger than the film thickness. This discrepancy and the absence o...

Thickness dependence of the magnetic percolation threshold in as-deposited and annealed Fe–SiO2 granular thin films

The magnetic properties and microstructure of as-deposited and annealed Fe–SiO2 granular thin films were studied. As-deposited films have a maximum in coercivity at an Fe volume fraction (Fe vol %) ∼62% independent of film thickness. Iron grains in as-deposited films are well defined, nearly equiaxial and ∼5 nm in diameter. From 66 to 90 Fe vol %, some as-deposited films showed an unusual well defined in-plane uniaxial anisotropy. The magnetic percolation threshold, xp, as indicated by the maximum in the Hc vs Fe vol % curve, changed after the films were annealed. The percolation threshold (xp) of films annealed at 420 °C for 30 min shifted to ∼47 Fe vol % except for the 5 nm films, whose xp remained unchanged. After annealing at 510 °C for 3 h, a strong thickness dependence of the percolation threshold was revealed in films thinner than 40 nm, with values ranging from 78 Fe vol % to less than 44 Fe vol %. The shifts of xp in both 420 and 510 °C annealed films can be explained by the effects of reduced di...

Effects of oxidation and abnormal grain growth on the magnetic properties of thin Fe-SiO2 granular films

In the Fe–SiO2 granular system the effects of oxidation and abnormal grain growth on the magnetic behavior are significant. As film thickness is reduced a larger fraction of Fe grains intersect the free surface and are prone to oxidation even in good vacuum conditions. Films coated with protective SiO2 layers were found to be unaffected by oxidation. The presence of a surface during annealing also affects the microstructural evolution. Transmission electron microscopy observations revealed a bimodal distribution of Fe grain sizes in uncoated films due to grain merger assisted by surface diffusion. This bimodal distribution of Fe grain sizes is associated with a coercivity (Hc) vs vol. % Fe curve containing two peaks. By contrast, SiO2 coated films exhibited a uniform microstructure without unusually large grains and a smoother Hc vs vol. % Fe curve with a well defined maximum. Room temperature coercivities of ∼1000 Oe can be routinely achieved in Fe–SiO2 granular films as thin as 10 nm. X-ray photoelectro...

Adhesion of cn/sub x/ overcoat to copt-sio/sub 2/ granular magnetic film

The tribological properties of the carbon nitride (CN/sub x/) coated CoPt-SiO/sub 2/ granular films, especially the adhesion at the overcoat/magnetic layer interface, have been studied. Nanoscratch experiments and complementary scanning electron microscope (SEM) observations indicate that CN/sub x/ adheres more strongly to the SiO/sub 2/ matrix than to the CoPt granules resulting in enhanced adhesion at the interface as a function of the SiO/sub 2/ content in the CoPt-SiO/sub 2/ layer.

Surface characterization of Co/CNx granular films fabricated by nanolamination

Cobalt–carbon thin films proposed for use as granular magnetic media are generally prepared by co-deposition sputtering. An alternative method is nanolamination of the component layers (media and matrix) followed by annealing. We have produced and characterized thin-film granular structures prepared from nanolaminate layers of Co and CNx. For the as-deposited samples, only metallic cobalt is observed with XPS and a constant nitrogen concentration of x ∼ 0.15 was measured for the carbon nitride layers. The annealed films have oxidized cobalt in the very near surface region. Atomic force microscopy measurements show that the surface of the film roughens significantly upon annealing, with the RMS roughness increasing from 0.2 to 1.0 nm. Thus, it appears that the nanostructural evolution caused by the annealing process, which gives rise to grain formation, also degrades the smooth surface of the CNx capping layer and exposes some of the cobalt to ambient. The potential implications of these observations on tribological performance are explored. Copyright

Structure and magnetic properties of multilayer Co/CN granular films

The magnetic and structural properties of a series of granular Co/CN films prepared by annealing multilayers of Co and CN have been studied. The films contained both hcp and fcc phases, and grain sizes were in the range 18–27 nm. The coercivity varied with cobalt fraction and with bilayer thickness, and a maximum room temperature coercivity of 1230 Oe was obtained for 50% cobalt and 2.7 nm bilayer thickness. Measurements of the time and temperature dependence of the highest coercivity films suggest that their coercivity is due primarily to the hcp phase and to the grain size. All films had relatively low values of magnetic viscosity (2%–3%/decade).

X-ray reflectivity and nanotribological study of deposition-energy-dependent thin CNx overcoats on CoCr magnetic films

Using a combination of high-resolution x-ray reflectivity (XRR), nanotribological, and x-ray photoelectron spectroscopy (XPS) techniques, a thorough description of the structural parameters of ultrathin CNx overcoats on CoCr magnetic films is obtained. In addition, the tribological performance as a function of deposition energy is shown to correlate well with trends found by XRR and XPS.

Interparticle interactions and time effects in very thin Fe–SiO2 granular films

Time decay and remanence measurements have been made on a series of very thin Fe–SiO2 granular thin films of varying composition. The susceptibility and the magnetic viscosity decreases drastically when the film thickness is thin enough to reach a two-dimensional arrangement of the granules. The measured values of the magnetic viscosity are very low (2% loss per decade) in the thinnest films so that further investigations in granular magnetic systems with higher anisotropy promise greater advantages for recording media.

Thermal stability of Co/sub 80/Pt/sub 20/-SiO/sub 2/ granular films sputtered on DC pulse biased substrates

Granular Co/sub 80/Pt/sub 20/-SiO/sub 2/ films have been prepared with thicknesses of /spl sim/15 nm using a novel pulsed dc bias technique. This enabled the growth of thermally stable films with coercivities as high as 2.3 kOe without annealing. The CoPt granules are randomly oriented with a slight (1010) texture with diameters of 5-10 nm at 51% CoPt volume fraction. Measurement of the time-dependent coercivity was used to extract the value of intrinsic switching field Ho and the thermal stability factor KV/kT by fitting to Sharrocks formula. The value of H/sub 0/ decreases from 3.5 kOe at 41% CoPt to 2.5 kOe at 58% CoPt while KV/kT increases from 78 to a maximum of 236 at 56% CoPt before decreasing to 160 at 58% CoPt. The coercivity was relatively flat from 45% to 58% CoPt, presumably because of the offsetting effects of the increase in H/sub 0/ and the decrease in KV/kT over this range.