Th. Speliotis

Hard/graded exchange spring composite media based on FePt

Hard-FePt(L10)/graded-FePt(L10 to A1) exchange spring composite media with perpendicular magnetic anisotropy have been studied. Changing the deposition temperature affects the structural transformation of FePt from cubic phase A1, which is magnetically soft, to the tetragonal phase L10, which is magnetically hard. Using this mechanism we were able to fabricate hard/graded composite films. Magnetic measurements revealed a significant reduction of the composite films’ switching fields in comparison with those of the single hard layer.

Polarity of anomalous Hall effect hysteresis loops in [Pt∕Co]15∕AF∕[Co∕Pt]15 (AF=FeMn, NiO) multilayers with perpendicular anisotropy

The effect of the metallic antiferromagnet (AF) γ-FeMn and the AF-semiconductor NiO alloys on the polarity of anomalous Hall resistivity loops is examined in perpendicularly biased [Pt(2nm)∕Co(0.4nm)]15∕AF(3nm)∕[Co(0.4nm)∕Pt(2nm)]15 (AF=NiO, FeMn) multilayers. The Hall resistivity exhibits negative polarity for AF=NiO and positive polarity for AF=FeMn. These differences are explained by the reduced spin-diffusion-length effects and the specular reflection of electrons at FeMn and NiO interfaces, respectively. In addition, it is shown that a sandwiched AF thin layer stabilizes the exchange-bias effect via interlayer coupling between top [Co(0.4nm)∕Pt(2nm)]15 and bottom [Pt(2nm)∕Co(0.4nm)]15 structures in multilayers with strong perpendicular magnetic anisotropy.

Electronic band structure imaging of three layer twisted graphene on single crystal Cu(111)

Few layer graphene (FLG) is grown on single crystal Cu(111) by Chemical Vapor Deposition, and the electronic valence band structure is imaged by Angle-Resolved Photo-Emission Spectroscopy. It is found that graphene essentially grows polycrystalline. Three nearly ideal Dirac cones are observed along the Cu Γ¯K¯ direction in k-space, attributed to the presence of ∼4° twisted three layer graphene with negligible interlayer coupling. The number of layers and the stacking order are compatible with Raman data analysis demonstrating the complementarity of the two techniques for a more accurate characterization of FLG.

Thermo-Mechanical Behaviour of HfO2 Coatings for Aerospace Applications

The use of ceramic composite materials in aerospace applications requires the development of oxidization protection coatings which can withstand very high temperatures. HfO2 is a promising material as a high temperature oxidization protective layer. HfO2 coatings have been deposited by radiation frequency magnetron sputtering all over the surface of SiC substrates and were tested under re-entry conditions. Also their oxidization resistance in air in the temperature range 1100 to 1450°C has been examined. The coatings were found to be stable and well-adhering to the substrate even after 100 re-entry cycles. No oxidization of the underlying SiC structure is observed. Re-entry and oxidization tests result in the formation of HfSiO4 at the HfO2/SiC interface, which further promotes their oxidization resistance.

Deposition of hard magnetic SmCo5 thin films by magnetron sputtering

We report the study of the microstructural and magnetic properties of SmCo5 thin films. Textured SmCo5 films having one or more of the following magnetic properties - high coercivity (1.5 kOe), remanence ratio (0.85), and maximum energy product (15 MGOe) - were prepared by magnetron sputtering on heated Si substrates.

Graphite-Nimonic alloy brazing

A graphite sheet has been successfully brazed to a Nimonic 105 superalloy using a commercial TiCuSil paste. A chromium layer was deposited on the graphite surface by sputtering and controlled heat treatments were employed in order to develop a suitable microstructure. Scanning electron microscopy measurements showed rough, crack-free interfaces between the filler metal and both the graphite and nimonic parts. From metallographic examination a well defined layered structure of the metallic elements close to the filler/graphite interface has been found. The metallic elements transport from the interface to the carbon bulk where they fill all the graphite pores up to a depth up of 50 μm and form a layered structure within the pores.

Ledge-type Co/L10-FePt exchange-coupled composites

FePt-based exchange-coupled composites consisting of a magnetically hard L10-FePt phase exchange-coupled with a soft ferromagnetic material are promising candidates for future ultra-high density (>1 Tbit/in2) perpendicular magnetic recording media, also being of interest for other applications including spin torque oscillators and micro-electro-mechanical systems, among others. In this paper, the effect of the thickness of a soft Co layer (3 < thCo < 20 nm) on the magnetic behavior of ledge-type fcc(100)-Co/L10(001)-FePt composites deposited on an MgO (100) substrate is systematically studied by combining morpho-structural analyses and angular magnetization measurements. Starting from a film consisting of isolated L10(001)–FePt islands, the ledge-type structure was obtained by depositing a Co layer that either covered the FePt islands or filled-up the inter-island region, gradually forming a continuous layer with increasing Co thickness. A perpendicular anisotropy was maintained up to thCo ∼ 9.5 nm and a ...

Current pulse induced toggle switching of dual-vortex magnetization in Ni80Fe20/Cu/Co nanopillar element

The combined effect of the spin-transfer torque and circular Amperian field to the switching of the chiralities of the dual-vortex magnetization in multilayered Ni80Fe20/Cu/Co nanopillar elements (o = 100 nm) is investigated numerically. It is demonstrated that, by passing a current pulse through the nanopillar, the chirality of the Ni80Fe20 layer can be controllably switched while that of the fixed Co layer persists. Consequently, reliable toggle switching between bistable low- and high-MR states can be achieved by applying current pulses with appropriate amplitude, polarity, and duration. It is found that the magnetization dynamics after the onset of a current pulse includes the excitation of spin-wave eigenmodes and that the predominant process throughout is the precessional motion of the free-layer magnetization. A shift of the eigenfrequency is observed when the chirality of the free-layer vortex magnetization switches from parallel to antiparallel relative to that of the fixed layer. This frequency ...

Magnetization Dynamics in Vortex-Imprinted Ni

The dynamic behavior of the vortex magnetization on micrometer-sized, ferromagnetic/antiferromagnetic (FM/AFM) bilayer exchange-coupled square elements is investigated using time-resolved Kerr effect microscopy and micromagnetic modeling.

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