Schlüsselwörter
(Englisch)
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bi-metallic alloys; self-assembly; magnetic nanostructures; superlattices; magnetic recording
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Forschungsprogramme
(Englisch)
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COST-Action MP0903 - Nanoalloys as advanced materials: from structure to properties and applications
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Kurzbeschreibung
(Englisch)
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Present efforts to create magnetic recording media by self-assembly have been hampered by the limitation to two-dimensional structures with one or two atomic layers in height. The order and magnetic properties of these structures are excellent, but due to their limited size the best thermal magnetization stabilities achieved so far are 105 K for a density of 15 Tera units per square inch. The present proposal intends to overcome this limitation in growing ordered superlattices of islands into the third direction. Specifically, we will grow up to 12 atomic layer high pillars out of bi-metallic alloys which will have thermal stability of their magnetization up to room temperature and therefore present the ultimate limit of a magnetic recording medium.
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Weitere Hinweise und Angaben
(Englisch)
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Full name of research-institution/enterprise: EPF Lausanne Institute of Condensed MatterPhysics (ICMP)
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Partner und Internationale Organisationen
(Englisch)
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AT, BE, BG, CH, CZ, DE, DK, ES, FI, FR, IE, IL, IT, LT, PL, RO, RS, SE, SK, TR, UK
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Abstract
(Englisch)
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During the course of this project, that has financed one Post Doc, we have investigated the magnetic properties of nanostructures with sizes all the way from single up to 600 atoms adsorbed onto graphene, and we have made the first steps towards a growth sequence where magnetic nanostructures and graphene are stacked. Our most recent discovery is that the magnetic properties of individual Co atoms adsorbed on graphene can be adjusted by the choice of the underlying substrate on which the graphene is grown. These properties are the magnetic moment of the atom, its preferred orientation (in-plane or out-of-plane), and the magnetic anisotropy energy that keeps it in its preferred orientation. When graphene is grown on Pt(111), the adsorbed Co atoms have a strong magnetic anisotropy favoring in-plane magnetization. In contrast, for graphene on Ru(0001), the easy magnetization axis of Co is out-of-plane, again with a very strong anisotropy, while for Co on graphene on Ir(111) the easy axis is in-plane and the anisotropy, as well as the magnetic moments, are strongly reduced. The reason for these very different behaviors is the varying degree of hybridization between the Co 3d orbitals and graphene ƒÎ bands that can be tailored through the strength of the graphene-substrate coupling. Perfectly ordered Fe cluster superlattices can be grown by Pd-seeding on Al2O3/Ni3Al(111). Pd atoms nucleate in the corner holes in the alumina (ã67 x ã67)R12‹ unit cell, providing a template for the subsequent aggregation of Fe. The substrate has a 20 . 30 nm thick slightly Ni enriched region that is ferromagnetic with a Curie temperature of 80 K. In addition, there is a second much thinner and more strongly Ni enriched region directly at the surface with a significantly larger Curie temperature of 240 K. Thereby, even without adsorbed magnetic clusters, this system exhibits 2 phase transitions. The Fe clusters with a mean size of 400 atoms are magnetically coupled through the 2 monolayer thick oxide to the underlying near surface ferromagnetic layer. This coupling is anti-ferromagnetic and ferromagnetic depending on the distance of the magnetic nanostructures to the Ni clusters in the Ni enhanced near surface region. The Curie temperature of the clusters is with 300 K very large and their easy magnetization axis is out-of-plane. Both are ideal properties for an ultra-high-density (44 Tera per in2) magnetic recording medium. A central aim of this proposal is to grow vertically aligned magnetic nanostructures that are separated by gaphene layers. This gmille-feuilleh structure would permit even higher densities of magnetic nanostructures for magnetic recording while maintaining the magnetic bits stable against thermal reversal up to room temperature. The first step is to cover magnetic nanostructure superlattices self-assembled on a graphene layer by a second graphene layer. This can be done either by repeating the chemical vapor deposition growth of already applied to grow the first layer, however, this technique requires temperatures where the magnetic nanostructures begin to coarsen. The second way is to transfer graphene from a growth wafer to the target sample. We explored both techniques. For the first, we chose to use Ir seed clusters on graphene on Ir(111) since they have been shown to be thermally very stable. On these clusters we grew Fe and Co nanostructure superlattices and managed to grow graphene flakes on top, not yet a full and well-ordered graphene layer. For the graphene transfer we could demonstrate that the transferred layer has low defect concentration. Both results strongly encourage us pursuing our vertical stacking project.
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Datenbankreferenzen
(Englisch)
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Swiss Database: COST-DB of the State Secretariat for Education and Research Hallwylstrasse 4 CH-3003 Berne, Switzerland Tel. +41 31 322 74 82 Swiss Project-Number: C10.0135
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