J. A. Christodoulides

CoPt and FePt thin films for high density recording media

Highly anisotropic face-centered-tetragonal (fct) CoPt and fct FePt nanoparticles have been prepared and embedded in a C or BN matrix by cosputtering from pure Co50Pt50, BN, and Fe50Pt50, C solid targets using a tandem deposition mode. The as-made films show a disordered face-centered-cubic structure which is magnetically soft and have low coercivity ( 107 erg/cm3. Transmission electron microscope studies showed FePt particles embedded in C matrix with a size increasing from below 3 nm in the as-made state to about 8 nm in the optimum annealed state. These results are very promising and make these materials potential candidates for high-density magnetic recording.

Magnetic, structural and microstructural properties of FePt/M (M=C,BN) granular films

Nanocomposite FePt/C and FePt/BN thin films have been fabricated by dc and rf magnetron sputtering on naturally-oxidized Si[100] substrates. Particles consisting of the highly anisotropic tetragonal L1/sub 0/ FePt phase were obtained by annealing the as-made multilayers by a conventional vacuum furnace in the temperature range of 500 to 800/spl deg/C. By controlling the annealing conditions, samples with grain size in the range of 3 to 20 nm were obtained, with coercivities between 2 and 16 kOe. The evolution of crystal structure during annealing at 600/spl deg/C was studied in FePt/C and the dependence of a, c and c/a on annealing time were determined. These data were used to estimate the degree of ordering which was found to increase with aging, but with a maximum value less than the bulk value, explaining thus the lower H/sub c/ in the nanoparticles.

CoPt and FePt nanoparticles for high density recording media

Highly anisotropic CoPt and FePt nanoparticles have been prepared and embedded in a C and BN matrices using the tandem deposition mode. The as-made multilayer CoPt(FePt)/C(BN) films show a disordered face centered cubic (fcc) structure, which is magnetically soft and have low coercivity ( 10/sup 7/ erg/cm/sup 3/. The coercivities obtained are much below those expected for noninteracting single-domain particles and the difference is attributed mainly to a smaller anisotropy in the nanoparticles associated with a lower degree of atomic ordering of the L1/sub 0/ phase.

Granular CoPt/C films for high-density recording media

Granular CoPt/C films, consisting of nanoparticles of the face-centered-tetragonal (fct) CoPt phase embedded in a carbon matrix, have been made by cosputtering from CoPt and C targets using a tandem deposition mode. The as-made films have a disordered face-centered-cubic structure and low coercivity (below 100 Oe). The granular structure is obtained after an aging heat treatment, which is followed by a drastic increase in coercivity to values up to 9 kOe due to the formation of the highly anisotropic fct phase. Transmission electron microscope studies showed the size of the CoPt particles to increase from 5 nm in the as-made state to 7–12 nm in the optimally annealed state. The measured coercivities are much lower than the value expected for noninteracting, randomly distributed single-domain particles. The large discrepancy is attributed to interparticle interactions, particle-size effects, and mainly to a lower anisotropy of the fct phase because of incomplete ordering.

Preparation and characterization of Dy nanoparticles

A technique of pure nanoparticle preparation has been explored by forming first small metal particles under high pressure sputtering and then embedding them into a sputtered film matrix. The tandem deposition method is used where the sputtered matrix is deposited at an argon pressure of 5 mTorr and the particles are formed within a specially designed gun at a pressure of 1 Torr and then deposited. The particle-matrix compositional limitations of reactivity and miscibility are thus reduced. The new technique was applied to dysprosium metal particles in an aluminum matrix. The absence of the antiferromagnetic transition for particles with sizes of 4–12 nm was observed. The 4 nm particles behaved superparamagnetically, while the larger 12 nm particles showed coercivities over twice as large as that of the bulk material at cryogenic temperatures. The success of the technique is demonstrated by having been able to carry out the study on the dysprosium-aluminum system that normally reacts, preventing the existe...

Effect of preparation conditions on the hysteresis behavior of granular Fe-SiO2

Abstract The hysteresis behavior of Fe χ (SiO 2 ) 1- χ granular films was studied as a function of preparation conditions. The films were prepared by dc and rf magnetron sputtering over the composition range χ v = 0.1–0.9 volume fraction of Fe. The Tandem deposition method and deposition from a composite target both with and without titanium sublimation, were used to prepare the films. A typical granular structure was observed, with grain size in the range 2–20 rim. X-ray diffraction and selected area diffraction showed an α-Fe (bcc) type structure. Magnetic properties showed that by varying the deposition method and some of the sputtering parameters (sputtering rate, argon flow, film thickness and substrate temperature), it is possible to switch from a relativeIy magnetically hard sample ( H c ∼ 700 Oe) to a soft sample ( H c ∼ 20 Oe). A dramatic increase in H c has been observed at cryogenic temperatures. This result, along with the Mossbauer data, suggests a shell/core granule morphology with an Fe and/or (Fe-Si) core surrounded by Fe-Si-O.

Effects of preparation conditions on the hysteresis behavior of granular Fe–SiO2

The hysteresis behavior of Fex(SiO2)1−x granular films was studied as a function of preparation conditions. The films were prepared by dc and rf magnetron sputtering over the composition range of xv=0.1–0.9, where xv is the volume fraction of Fe. The tandem and composite target deposition methods, both with and without titanium presputtering, were used to prepare the films. Typical granular structure was observed in these films, with grain size in the range of 2–20 nm. X-ray diffraction and selected area diffraction showed a bcc α-Fe-type structure. Magnetic measurements showed that by varying the deposition method and some of the sputtering parameters including sputtering rate, argon flow, film thickness, and substrate temperature, it is possible to switch from a relatively magnetically hard sample (Hc∼650 Oe) to a soft sample (Hc∼20 Oe). For the former sample, a dramatic increase in Hc has been observed at cryogenic temperatures. This result, along with the Mossbauer data, suggest a shell/core granule m...

Granular thin film deposition by simultaneous spark erosion and sputtering

We have investigated a new method of preparing granular thin films, which are composed of fine metallic particles embedded in a uniform matrix material. By our method, fine metallic particles are created by spark erosion in an inert gas environment and directly deposited onto a cooled substrate. Magnetron sputtering is employed concurrently thereby building up the matrix. Using such a deposition method, granular thin films are created without the need of any heat treatments and can be made with virtually any material and composition. Furthermore it is even possible to create granular thin films from miscible phases. The size of the particles can be controlled by varying the pressure under which the spark erosion is conducted up to a few Torr of Ar. We have applied our method to the case of magnetic particles in a nonmagnetic matrix. Preliminary transmission electron microscope measurements on Fe particles in a Cu matrix showed 10-nm-diam. Fe particles. Selected area diffraction patterns show that the film...

Preparation of Dy and Mn nanoparticles

Abstract We have recently extended our studies on nanoparticles to rare earth and Mn particles. The nanometer scale Dy and Mn particles have been prepared using a new method in which particles are first formed from high purity constituent materials in an inert gas atmosphere and then embedded into a matrix by planar magnetron sputtering. The Dycontaining samples consisted of particles with diameters on the order of 10 nm, which were separated by the matrix material. A strong relationship was observed between the particle size and the magnetic ordering temperature with the absence of antiferromagnetic transition observed at 176 K in the bulk. This method allows for the synthesis of a wider variety of materials by reducing the problems of miscibility and reactivity between the particles and the matrix material.

Formation of Sm–Co nanoparticles in a W matrix using multilayered precursors

Sm–Co particles have been produced in a W matrix by annealing sputtered Sm–Co/W multilayers with layer thicknesses of 5–28 nm at temperatures of 600–850 °C. The Sm–Co particles had the intermetallic 1:7 and 2:17 structures with sizes below 10 nm and coercivities in the range of 2–10 kOe. A reduction of magnetization with prolonged annealing has been observed and attributed to alloying of Co with both W and Si by interdiffusion through the W layer.