Magnétophone | EPFL Graph Search

Un magnétophone est un appareil permettant d'enregistrer des sons sur une bande magnétique qui peut être enroulée dans une bobine ou une cassette. Le nom Magnetophon était originellement une marque déposée par AEG (Telefunken) et IG Farben et désignait les seuls « enregistreurs à ruban ». Ce terme est passé dans le langage courant, devenant une sorte de nom commun. Le principe est de polariser grâce à un électroaimant (tête magnétique), les particules métalliques magnétiques d'un support souple en ruban, défilant à vitesse constante sur la tête. Historique L'enregistrement magnétique a été imaginé dès 1877 par l'ingénieur américain . Les premières expériences pratiques remontent au Telegraphone de l'ingénieur danois Valdemar Poulsen en 1898. Quatre périodes marquent l'évolution de ces techniques : mécanique, électromécanique, magnétique et enfin électronique. Mécanique Dans une première période, les enregistreurs ne sont pas magnétiques, mais

Growth and Magnetism of Nanostructures Investigated by STM, MOKE, and XMCD

Alberto Cavallin

This thesis presents our studies on nanostructures growth and magnetism, mainly based on scanning tunneling microscopy (STM), magneto-optical Kerr effect (MOKE), and x-ray magnetic circular dichroism (XMCD) measurements. In most of these experiments, nanostructures were grown by molecular beam epitaxy on single crystal substrates under ultra high vacuum conditions. Our combined morphological and magnetic investigations aim at an atomic-level description of the nanostructures magnetic properties. In particular, we are searching for strategies to design the smallest ferromagnet with stable magnetization at room temperature to be used in devices such as magnetic random access memories, storage media, and sensors. A critical factor in pursuing this aim is the determination and control of the magnetization reversal mechanism. Different reversal mechanisms may occur in the nanostructures we considered: either all the spins quasi-coherently rotate or a domain wall is nucleated and propagated along the particle. We prepared monolayer-high Co islands on Pt(111) with compact and ramified shape by adjusting the deposition temperatures and coverages for a sequence of deposition and annealing steps. Combining field-dependent magnetization curves and temperature dependent zero-field susceptibility measurements we identify a size- and shape-dependent transition from quasi-coherent to domain wall propagation reversal regimes. In a second study we investigated a system which according to a recent publication presents many of the characteristics desired in bit patterned magnetic media for ultra-high density storage devices. In particular, it was claimed that Co nanoclusters, growing in registry with the GdAu2/Au(111) moiré periodic substrate, and featuring a density of 52 Tera/in2, present out-of-plane remanent magnetization at T = 300 K. At odds with this claim, no perpendicular magnetic anisotropy was found in our combined MOKE and STM study, which shows an in-plane remanent signal appearing only for coalesced islands. A third study is devoted to FeRh, an alloy material which, thanks to its peculiar magnetic phase diagram, has been proposed coupled to FePt for advanced magnetic recording schemes. We present the first observation of low temperature stabilization of the ferromagnetic (FM) phase in FeRh chemically ordered crystals. Nanocrystals measuring 3.3 nm in diameter have B2 structure with alternating atomic Fe and Rh layers. XMCD demonstrates FM alignment of the Fe and Rh magnetic moments of 3 and 1 μB, respectively. In sharp contrast to FeRh bulk and thin films, which show antiferromagnetic iii Abstract (AFM) order below 300 K, the low-temperature FM 3.3 nm nanocrystals are the hallmark of a size-dependent AFM-FM transition temperature. Finally we present our results towards creation by chemical vapor deposition of a “mille-feuille” structure composed of graphene and magnetic nanoclusters. To realize this objective we optimized the cluster size, composition, and annealing temperature to obtain a compromise between graphene syntesis and cluster coalescence. We demonstrate by STM that annealing to T = 720 K in ethylene stabilizes Ir-seeded clusters against diffusion and coalescence, and by Auger Electron Spectroscopy (AES) that Ni-coated clusters can be covered with up to half of the Carbon contained in a graphene layer.

EPFL2013

Growth and Magnetism of Nanostructures Investigated by STM, MOKE, and XMCD

Alberto Cavallin

EPFL2013