A. J. Kellock

Epitaxial growth and properties of ferromagnetic co-doped TiO2 anatase

We have used oxygen-plasma-assisted molecular-beam epitaxy (OPA-MBE) to grow CoxTi1−xO2 anatase on SrTiO3(001) for x=∼0.01–0.10, and have measured the structural, compositional, and magnetic properties of the resulting films. Whether epitaxial or polycrystalline, these CoxTi1−xO2 films are ferromagnetic semiconductors at and above room temperature. However, the magnetic and structural properties depend critically on the Co distribution, which varies widely with growth conditions. Co is substitutional in the anatase lattice and in the +2 formal oxidation state in ferromagnetic CoxTi1−xO2. The magnetic properties of OPA-MBE grown material are significantly better than those of analogous pulsed laser deposition-grown material.

Temperature dependent magnetic properties of highly chemically ordered Fe55−xNixPt45L10 films

Magnetic media using materials with high uniaxial magneto-crystalline anisotropy, KU, combined with a thermal assist to overcome write field limitations have been proposed as one of the potential technologies to extend the areal density of magnetic disk recording beyond the limitations of current technology. Here we present an investigation on structural and temperature dependent magnetic properties of chemically ordered epitaxial Fe55−xNixPt45 thin films. Increasing Ni additions result in a steady reduction of magneto-crystalline anisotropy, saturation magnetization, and Curie temperature. The ability to control thermomagnetic properties over a wide range makes Fe55−xNixPt45 and similar FePt-based pseudo-binary alloys attractive base materials for media applications in thermally assisted magnetic recording.

Ion-beam patterning of magnetic films using stencil masks

Previously, ion-beam irradiation has been shown to locally alter the magnetic properties of thin Co/Pt multilayer films. In this work, we have used ion-beam irradiation through a silicon stencil mask having 1-μm-diam holes to pattern a magnetic film. Regularly spaced micrometer-sized regions of magnetically altered material have been produced over areas of a square millimeter in Co/Pt multilayers. These magnetic structures have been observed by magnetic force microscopy. The patterning technique is demonstrated with mask–sample spacing as large as 0.5 mm. In addition, smaller regions of magnetic contrast, down to 100 nm, were created by using two masks with partially overlapping micrometer holes. Such patterned magnetic films are of interest for application in high-density magnetic recording.

On the relationship of magnetocrystalline anisotropy and stoichiometry in epitaxial L10 CoPt (001) and FePt (001) thin films

Two series of epitaxial CoPt and FePt films, with nominal thicknesses of 42 or 50 nm, were prepared by sputtering onto single-crystal MgO(001) substrates in order to investigate the chemical ordering and the resultant magnetic properties as a function of alloy composition. In the first series, the film composition was kept constant, while the substrate temperature was increased from 144 to 704 °C. In the second series the substrate temperature was kept constant at 704 °C for CoPt and 620 °C for FePt, while the alloy stoichiometry was varied in the nominal range of 40–60-at. % Co(Fe). Film compositions and thicknesses were measured via Rutherford backscattering spectrometry. The lattice and long-range order parameter for the L10 phase were obtained for both sets of films using x-ray diffraction. The room-temperature magnetocrystalline anisotropy constants were determined for a subset of the films using torque magnetometry. The order parameter was found to increase with increasing temperature, with ordering...

Crystallization properties of ultrathin phase change films

The crystallization behavior of ultrathin phase change films was studied using time-resolved x-ray diffraction (XRD). Thin films of variable thickness between 1 and 50nm of the phase change materials Ge2Sb2Te5 (GST), N-doped GST, Ge15Sb85, Sb2Te, and Ag- and In-doped Sb2Te were heated in a He atmosphere, and the intensity of the diffracted x-ray peaks was recorded. It was found for all materials that the crystallization temperature increases as the film thickness is reduced below 10nm. The increase depends on the material and can be as high as 200°C for the thinnest films. The thinnest films that show XRD peaks are 2nm for GST and N-GST, 1.5nm for Sb2Te and AgIn-Sb2Te, and 1.3nm for GeSb. This scaling behavior is very promising for the application of phase change materials to solid-state memory technology.

Thickness and Growth Temperature Dependence of Structure and Magnetism in FePt Thin Films

We describe structural and magnetic measurements of polycrystalline, L10 chemically ordered Fe(55–60)Pt(45–40) films as a function of film thickness (from 3 to 13 nm) and growth temperature (270–370 °C). With increasing film thickness, the coercivity increases from about 1 kOe up to 11 kOe (growth at 400 °C), while for increasing growth temperature, the coercivity grows from 0.2 to 6 kOe for 4.3 nm thick films and 1.6 to 10 kOe for 8.5 nm thick films. There is a strong, nearly linear correlation between coercivity and the extent of L10 chemical order. In all the films there is a mixture of L10 and chemically disordered, fcc phases. The grain size in the L10 phase increases with both film thickness and growth temperature (increasing chemical order), while in the fcc phase the grain size remains nearly constant and is smaller than in the L10 phase. The films all contain twins and stacking faults. The relationship between the coercivity and the film structure is discussed and we give a possible mechanism for...

Absolute cross section for hydrogen forward scattering

Abstract The differential cross section for hydrogen recoil scattering from helium ions has been determined for energies in the range 1–3 MeV and forward scattering angles of 20, 25, 30 and 35°. Polystyrene (C 8 H 8 ) n thin films were used as a reference standard, enabling the recoil cross section to be determined with reference to the backscattering cross section for carbon, which was itself newly determined in terms of the simple Rutherford cross section.

Ion induced magnetization reorientation in Co/Pt multilayers for patterned media

Co/Pt multilayer films with perpendicular magnetic anisotropy and large out-of-plane coercivities of 3.9 - 8.5 kOe have been found to undergo a spin reorientation transition from out-of-plane to in-plane upon irradiation with 700 keV nitrogen ions. X-ray reflectivity experiments show that the multilayer structure gets progressively disrupted with increasing ion dose, providing direct evidence for local atomic displacements at the Co/Pt interfaces. This effectively destroys the magnetic interface anisotropy, which was varied by about a factor of two, between KS@ 0.4 erg/cm2 and KS@ 0.85 erg/cm2 for two particular films. The dose required to initiate spin-reorientation, 6x1014 N+/cm2 and 1.5x1015 N+/cm2, respectively, scales with KS. It is roughly equal to the number of Co interface atoms per unit interface area contributing to KS.

Direct observation of amorphous to crystalline phase transitions in nanoparticle arrays of phase change materials

We have used time-resolved x-ray diffraction to study the amorphous-crystalline phase transition in 20–80nm particles of the phase change materials Ge2Sb2Te5, nitrogen-doped Ge2Sb2Te5, Ge15Sb85, Sb2Te, and Sb2Te doped with Ag and In. We find that all samples undergo the phase transition with crystallization temperatures close to those of similarly prepared blanket films of the same materials with the exception of Sb2Te that shows the transition at a temperature that is about 40°C higher than that of blanket films. Some of the nanoparticles show a difference in crystallographic texture compared to thick films. Large area arrays of these nanoparticles were fabricated using electron-beam lithography, keeping the sample temperatures well below the crystallization temperatures so as to produce particles that were entirely in the amorphous phase. The observation that particles with diameters as small as 20nm can still undergo this phase transition indicates that phase change solid-state memory technology should...

Growth of cubic-TaN thin films by plasma-enhanced atomic layer deposition

Low resistivity cubic-TaN thin films were grown by plasma-enhanced-atomic layer deposition using TaCl5 as the metal precursor and hydrogen/nitrogen plasma. The deposition has been performed by alternate exposures of TaCl5 and the plasma of hydrogen and nitrogen mixture. X-ray diffraction analyses show that the film is composed of cubic TaN grains, in contrast to the previously reported highly resistive Ta3N5 films grown by Ta3N5 grown by TaCl5 and NH3 as precursors. The composition and thickness were measured by Rutherford backscattering and hydrogen concentrations were obtained by forward recoil elastic spectrometry as a function of growth parameters. The N content of the cubic TaN films was controlled from N/Ta=0.7 up to 1.3 by changing nitrogen partial pressure. The resistivity and growth rate increase with increasing N concentration in the film. The Cl and H content were found to be strong functions of plasma exposure time and growth temperatures, and TaN films with resistivity as low as 350 μΩ cm wer...