Orientation and magnetic properties of FePt and CoPt films grown on MgO(110) single-crystal substrate by electron-beam coevaporation (original) (raw)
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Journal of Applied Physics, 2005
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 Work Supported in part by the Department of Energy Contract DE-AC03-76SF00515 SLAC-PUB-10614 August 2004 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 L1 0 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 occurring more readily in FePt when compared with CoPt. A perpendicular anisotropy developed in CoPt for substrate temperatures above 534 °C and in FePt above 321 °C. The structure and width of the magnetic domains in CoPt and FePt, as seen by magnetic force microscopy, also demonstrated an increase in magnetic anisotropy with increasing temperature. For the films deposited at the highest temperatures (704 °C for CoPt and 620 °C for FePt), the order parameter reached a maximum near the equiatomic composition, whereas the magnetocrystalline anisotropy increased as the concentration of Co or Fe was increased from below to slightly above the equiatomic composition. It is concluded that non-stoichiometric L1 0 CoPt and FePt, with a slight excess of Co or Fe, are preferable for applications requiring the highest anisotropies.
Journal of Applied Physics, 2007
We present structural and magnetic properties of three sets of structures: as-deposited CoPt films cosputtered at 900 K on MgO͑110͒ substrates with a Pt͑110͒ buffer layer and CoPt films deposited by molecular beam epitaxy directly on MgO͑110͒ substrates at 900 K, as prepared and annealed at 900 K. All layers have the L1 0 tetragonal structure. The chemical long-range ordering for the as-deposited CoPt films is incomplete in contrast with the annealed CoPt films, where long-range order is the highest. The structural study of these CoPt films grown on MgO͑110͒ has pointed out that three variants of the L1 0 phase coexist. The proportion of x and y variants, with the concentration modulation along a vector oriented at 45°with respect to the growth direction, is higher than the proportion of the z variant with the concentration modulation within the plane. The magnetic study shows an in-plane easy magnetization axis with a large magnetic anisotropy. This is interesting for the magnetic recording media with classical longitudinal writing and reading heads. The simulation of the magnetization loops confirms that the easy magnetization axis is within the plane and along the ͓110͔ direction, favored by the dominant x and y variants.
Magnetic anisotropy and microstructure in sputtered CoPt(110) films
Catalysis Today, 2004
We compare structural and magnetic properties of CoPt films sputtered at 900 K on MgO(1 1 0) (with a Pt(1 1 0) buffer layer) and MgO(0 0 1) (with a Pt(0 0 1) buffer layer) substrates. We obtain a layer with the L1 0 tetragonal structure. The equiatomic L1 0 phase is a "natural" multilayer, which consists in a stacking along the [0 0 1] direction of pure Co and pure Pt monolayers. At this temperature, the growth of CoPt yields nearly single orientation epitaxial films: CoPt )[0 0 1]//MgO(1 1 0)[0 0 1] and CoPt(0 0 1)[1 1 0]//MgO(0 0 1)[1 1 0] as shown by transmission electron microscopy. On MgO substrates, the long-range chemical ordering is incomplete in contrast with the case of MgO(0 0 1) substrates, where long-range order is nearly perfect. Despite incomplete chemical ordering, a large in-plane magnetic anisotropy is present for the films grown on the MgO(1 1 0) substrate. This is interesting for the magnetic recording writing with a classical recording head and reading with a magnetoresistance head. The structural study of the CoPt films grown on MgO(1 1 0) has pointed out that three variants of the L1 0 phase coexist. The proportion of [1 0 0] and [0 1 0] variants, oriented at 45 • with the ordering growth direction, is much higher than the proportion of the [0 0 1] variant. In fact, the simulation of magnetization loops has shown that the easy magnetization axis is within the plane along the [1,−1,0] direction. This anisotropy is favored for the [1 0 0] and [0 1 0] variants. On MgO(0 0 1), the CoPt films grow as a single variant with the concentration modulation and the magnetic anisotropy along the growth direction.
Nanostructure and magnetic properties of polycrystalline FePdPt/MgO thin films
2002
Atomic ordering, nanostructure and the magnetic properties of polycrystalline FePdPt thin films deposited with MgO underlayers have been investigated. The film thickness was fixed at 5 nm in all films, a thickness where perpendicular anisotropy is dominant. Films deposited at ambient temperature were postannealed at various temperatures to study the kinetics of atomic ordering. The Pd additions to FePt were effective in reducing the temperature needed for atomic ordering. The mean grain size determined by transmission electron microscopy is about 10 nm in annealed films. Within the limits of our observations, FePdPt films do not show phase separation into more than one phase of the L1 0 structure. The maximum coercivity ͑H c ͒ for annealed FePdPt films is about ϳ3000-4000 Oe. In situ ordered FePdPt films at 530°C show no indication of a lower temperature for atomic ordering.
Control of the axis of chemical ordering and magnetic anisotropy in epitaxial FePt films
Journal of Applied Physics, 1996
Growth of epitaxial films of the L1 0 phase of FePt, with the tetragonal c axis along either the film normal or in-plane, is described. Films were grown by coevaporation of Fe and Pt, under ultrahigh vacuum conditions, onto a seed film of Pt grown on MgO or SrTiO 3 substrates. The perpendicular or in-plane orientation of the c axis was controlled by selecting the ͑001͒ or ͑110͒ substrate plane, respectively. Nearly complete chemical ordering was achieved for growth at 500°C for both orientations. Magnetic and magneto-optical characterization of these films confirmed the huge magnetic anisotropy expected for this phase. In the most highly ordered films, anisotropy fields in excess of 120 kOe were measured.
Atomic ordering and coercivity mechanism in FePt and CoPt polycrystalline thin films
IEEE Transactions on Magnetics, 2001
40 nm thick CoPt and FePt films were prepared on oxidized Si substrates with 10 nm MgO underlayers. The maximum coercivity ( ) for CoPt films was found to be 10 kOe after annealing at 700 C for 20-30 minutes (min). Structural analysis showed a significant amount of FCC phase as well as the ordered L1 0 phase in these films. FePt films showed an abrupt increase of ordered volume fraction and in the initial stage of annealing and predominance of the tetragonal L1 0 phase after 10 min. at 700 C. The maximum reached 16 kOe after annealing at 700 C for more than 20 minutes. Dark field (DF) images of the annealed CoPt films showed individual grains which exhibited a possibility of several variants or disordered phase with dimensions similar to the exchange correlation length, . The temperature dependence of seems to indicate a weak pinning mechanism in the highly ordered FePt films. Magnetic force microscopy indicated a complex domain structure consisting of clusters with dimensions of several hundred nanometers.
L1o-CoPt/Co bilayer ferromagnetic films: interdiffusion, structure and microstructure
Acta Materialia, 2003
Detailed understanding of the connections between nanostructure and magnetic properties is key to the realization of optimal exchange-spring magnets. To this end, x-ray and electron diffraction and transmission electron microscopy were used to study the evolution of phases, microstructure and texture of model exchange-spring bilayers of L1 0-CoPt/Co upon annealing at temperatures ranging from 300-550°C. The work was motivated by previously detailed changes in the magnetic exchange coupling of the system induced by the annealing treatments [1]. Unannealed CoPt/Co bilayers comprised a Ͻ111Ͼ fiber-textured L1 0 CoPt layer and a Ͻ0001Ͼ fiber-textured Co layer. The Co layer predominantly consisted of the hcp (A3) form, but contained a minority fcc (A1) phase. Annealing the bilayers at low temperatures (300-450°C) or for short times resulted in a strengthening of the hcp Co Ͻ0001Ͼ fiber texture and disappearance of the minor fcc phase. By contrast, annealing at higher temperatures (450-550°C) or for longer times resulted in the interdiffusion of the Co and CoPt layers and the formation of new Co-Pt solid solutions. One of these solid solutions was found to be hcp in structure, while the other was fcc. The amount of the fcc solid solution increased at the expense of the L1 0 CoPt, hcp Co and hcp Co-Pt solid solution phases as annealing time and temperature were increased.
Controlling magnetic anisotropy in epitaxial FePt(001) films
Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 2009
Epitaxial equiatomic Fe50Pt50 thin films with a variable order parameter ranging from 0 to 0.9 and Fe100−xPtx thin films with x ranging from 33 to 50 were deposited on MgO (001) substrates by dc sputtering. A seed layer consisting of nonmagnetic Cr (4nm)∕Pt (12nm) was used to promote the crystallinity of the magnetic films. The crystal structure and magnetic properties were gauged using x-ray diffraction and magnetometry. The magnetic anisotropy can be controlled by changing the order parameter. For Fe100−xPtx films, the increase in Fe composition leads to an increase in coercivity in the hard axis loop and causes a loss of perpendicular anisotropy.
Perpendicular magnetic anisotropy in CoPtsub 3 films grown on a low energy surface at room temperature
Journal of Applied Physics, 2002
Epitaxial ͑111͒-oriented CoPt 3 films were deposited on WSe 2 (0001) substrates at room temperature using molecular beam epitaxy. We observed strong growth induced uniaxial perpendicular magnetic anisotropy which has a maximum of 3.2ϫ10 6 erg/cm 3 and coercivity of about 200 Oe for films with thicknesses Ͻ6 nm. At a thickness larger than 6 nm the easy magnetization axis progressively rotates into the plane of the film as the film thickness is increased. The magnetic domain structure in films with perpendicular magnetic anisotropy was investigated by photoemission electron microscopy revealing a characteristic thickness dependence near the reorientation transition.
Surface and Coatings Technology, 2005
By co-sputtering Cu and FePt onto Cr 90 Mo 10 underlayers, the preferred orientation and easy axis of magnetic anisotropy of FePt films were successfully changed from the perpendicular to the in-plane direction. The pure FePt film grown on the Cr 90 Mo 10 underlayer showed a (001) preferred orientation with out-of-plane magnetic anisotropy. As 20 and 40 vol.% Cu were doped, the FePt films showed a (200) preferred orientation with in-plane magnetic anisotropy. The pure FePt film showed a continuous microstructure, while the Cu-doped FePt films showed a mixture of particle-like and continuous microstructures. The change of the preferred orientation in the FePt films by Cu doping might be due to the competition of grain-boundary energy, surface free energy and epitaxial-strain energy. The experimental results suggest that the Cu doping be a promising method for the fabrication of the in-plane oriented FePt films. The Cu-doped FePt films had a lower Ms than the pure FePt film. D