| 1. |
- Pappas, S. D., et al.
(författare)
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Growth and Experimental Evidence of Quantum Confinement Effects in Cu(2)O and CuO Thin Films
- 2011
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Ingår i: Journal of Nano Research. - 1662-5250. ; 15, s. 69-74
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Tidskriftsartikel (refereegranskat)abstract
- Thin Cu films of thickness 0.4 - 150 nm were deposited via radio frequency magnetron sputtering on Si(100) wafers, corning glass and quartz. Subsequently the Cu films were oxidized in ambient air at 230 degrees C and 425 degrees C in order to produce single-phase Cu(2)O and CuO, respectively. Selected samples were measured in the transmission geometry with the help of an ultraviolet - visible spectrophotometer. From the absorption spectra of the films, it was found that the gap E(B) for the dipole allowed transitions showed blue shifts of about 1.2 eV for the Cu(2)O thinnest film (0.75 nm), whereas the E(direct) for the direct gap transitions showed blue shifts of about 0.16 eV for the CuO thinnest film (0.7 mm). The blue shift of the energy gap in the copper-oxide semiconductors is an indication of the presence of strong quantum confinement effects.
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| 2. |
- Pappas, S. D., et al.
(författare)
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Layering and temperature-dependent magnetization and anisotropy of naturally produced Ni/NiO multilayers
- 2012
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 112:5, s. 053918-
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Tidskriftsartikel (refereegranskat)abstract
- Ni/NiO multilayers were grown by magnetron sputtering at room temperature, with the aid of the natural oxidation procedure. That is, at the end of the deposition of each single Ni layer, air is let to flow into the vacuum chamber through a leak valve. Then, a very thin NiO layer (similar to 1.2 nm) is formed. Simulated x-ray reflectivity patterns reveal that layering is excellent for individual Ni-layer thickness larger than 2.5 nm, which is attributed to the intercalation of amorphous NiO between the polycrystalline Ni layers. The magnetization of the films, measured at temperatures 5-300 K, has almost bulk-like value, whereas the films exhibit a trend to perpendicular magnetic anisotropy (PMA) with an unusual significant positive interface anisotropy contribution, which presents a weak temperature dependence. The power-law behavior of the multilayers indicates a non-negligible contribution of higher order anisotropies in the uniaxial anisotropy. Bloch-law fittings for the temperature dependence of the magnetization in the spin-wave regime show that the magnetization in the multilayers decreases faster as a function of temperature than the one of bulk Ni. Finally, when the individual Ni-layer thickness decreases below 2 nm, the multilayer stacking vanishes, resulting in a dramatic decrease of the interface magnetic anisotropy and consequently in a decrease of the perpendicular magnetic anisotropy. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4750026]
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| 3. |
- Pappas, S. D., et al.
(författare)
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Natural Nanomorphous Ni/NiO Magnetic Multilayers : Structure and Magnetism of the High-Ar Pressure Series
- 2014
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Ingår i: Journal of Nanoscience and Nanotechnology. - : American Scientific Publishers. - 1533-4880 .- 1533-4899. ; 14:8, s. 6103-6107
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Tidskriftsartikel (refereegranskat)abstract
- Natural nanomorphous Ni/NiO multilayers have exhibited interesting magnetic properties, such as an unusual positive surface anisotropy and perpendicular magnetic anisotropy. Most attention has been paid to multilayers prepared by radio frequency magnetron sputtering under relatively low (3x10(-3) mbar) Ar pressure. Here we report on the correlation between structural and magnetic properties for a new series of multilayers, prepared under relatively high (3x10(-2) mbar) Ar pressure. The crystalline Ni individual layer thickness ranges between 5-8 nm. The amorphous NiO layer thickness is constant, about 1.1 nm thick. X-ray reflectivity showed that in some of the multilayers the high-order Bragg peaks become broader and diminish quickly. Cross-section transmission electron microscopy reveals that this occurs because the first bilayers are formed in accordance to the growth conditions, while the ones near the top are vanished. Despite the deterioration of the interface quality, all samples show tendency for perpendicular magnetic anisotropy even for large bilayer thickness of about 9 nm. Similar tendency is observed even by a 330 nm thick non-multilayered Ni film grown under the same conditions. This observation reveals the important role of strain and magnetoelastic anisotropy as a source of perpendicular magnetic anisotropy in the Ni/NiO multilayers.
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| 4. |
- Poulopoulos, P., et al.
(författare)
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Growth and Magnetism of Natural Multilayers
- 2011
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Ingår i: Journal of Nano Research. - 1662-5250. ; 15, s. 95-103
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Tidskriftsartikel (refereegranskat)abstract
- In this work, we present a simple method to fabricate high quality Ni/NiO multilayers with the use of a single magnetron sputtering head. Namely, at the end of the deposition of each single Ni layer, air is let to flow into the vacuum chamber through a leak valve. Then, a very thin NiO layer (similar to 1nm) is formed by natural oxidation. The process is reproducible and the result is the formation of a multilayer with excellent layering. Magnetization hysteresis loops recorded at 5 K and room temperature reveal a tendency for perpendicular magnetic anisotropy as the thickness of the individual Ni layers decreases. It is shown that the Ni/NiO interface has sizeable positive surface/interface anisotropy, i.e. it favors the development of perpendicular magnetic anisotropy. This is rather unusual for a Ni-based multilayered system and may render Ni/NiO multilayers useful for magneto-optical recording applications.
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| 5. |
- Vlachos, A., et al.
(författare)
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Magnetic Properties of Textured CoPd Nanocrystalline Thin Films
- 2012
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Ingår i: Journal of Nanoscience and Nanotechnology. - : American Scientific Publishers. - 1533-4880 .- 1533-4899. ; 12:8, s. 6240-6247
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Tidskriftsartikel (refereegranskat)abstract
- CoPd is an important nanomaterial for magnetic and magneto-optic storage of information. In this work, CoPd alloyed thin films are grown via radio frequency magnetron sputtering on silicon, glass and polyimide substrates in a vacuum chamber with base pressure of 5 x 10(-8) mbar. The films are nanocrystalline with grain size between 4 and 80 nm. The magnetic properties of thoroughly textured CoPd alloyed thin films are compared to random polycrystalline ones. Magnetization hysteresis loops recorded under fields up to 12 kOe via a home-made magneto-optic Kerr-effect magnetometer reveal strong tendency for perpendicular magnetic anisotropy for the textured film. This anisotropy leads to the formation of well-defined stripe or labyrinthine ferromagnetic domains with the local spins oriented perpendicular to the film plane. The domain patterns and the hysteresis loops are simulated with micromagnetic calculations. Finally, an induced magnetic moment of 0.44 mu B/atom is measured for Pd via X-ray magnetic circular dichroism and it is separated into spin and orbital magnetic moment contributions.
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