Olivier Fruchart

Epitaxial graphene prepared by chemical vapor deposition on single crystal thin iridium films on sapphire

Uniform single layer graphene was grown on single-crystal Ir films a few nanometers thick which were prepared by pulsed laser deposition on sapphire wafers. These graphene layers have a single crystallographic orientation and a very low density of defects, as shown by diffraction, scanning tunnelling microscopy, and Raman spectroscopy. Their structural quality is as high as that of graphene produced on Ir bulk single crystals, i.e., much higher than on metal thin films used so far.

Ultrathin epitaxial cobalt films on graphene for spintronic investigations and applications

Graphene is an attractive candidate in spintronics for a number of reasons, among which are its electric-field-controlled conductivity, its expected long spin lifetime and its two-dimensional nature. A number of recent proposals call for the development of high-quality ferromagnetic thin films in contact with graphene, whereas only thick polycrystalline or three-dimensional (nanoclusters) morphologies have been demonstrated so far. We report on the growth of flat, epitaxial ultrathin Co films on graphene using pulsed laser deposition. These display perpendicular magnetic anisotropy (PMA) in the thickness range 0.5–1 nm, in agreement with our first-principles calculations. PMA, epitaxy and ultra-small thickness bring new perspectives on graphene-based spintronic devices making use of the zero-field control of an arbitrary magnetization direction, band matching between electrodes and graphene, and interface phenomena such as the Rashba effect and electric field control of magnetism.

Observation of Bloch-point domain walls in cylindrical magnetic nanowires

Topological protection is an elegant way of warranting the integrity of quantum and nanosized systems. In magnetism one example is the Bloch-point, a peculiar object implying the local vanishing of magnetization within a ferromagnet. Its existence had been postulated and described theoretically since several decades, however it has never been observed. We con rm experimentally the existence of Bloch points, imaged within domain walls in cylindrical magnetic nanowires, combining surface and transmission XMCD-PEEM magnetic microscopy. This opens the way to the experimental search for peculiar phenomena predicted during the motion of Bloch-point-based domain walls.

Three-dimensional magnetic flux-closure patterns in mesoscopic Fe islands

We have investigated three-dimensional magnetization structures in numerous mesoscopic Fe/Mo(110) islands by means of x-ray magnetic circular dichroism combined with photoemission electron microscopy (XMCD-PEEM). The particles are epitaxial islands with an elongated hexagonal shape with length of up to 2.5 micrometer and thickness of up to 250 nm. The XMCD-PEEM studies reveal asymmetric magnetization distributions at the surface of these particles. Micromagnetic simulations are in excellent agreement with the observed magnetic structures and provide information on the internal structure of the magnetization which is not accessible in the experiment. It is shown that the magnetization is influenced mostly by the particle size and thickness rather than by the details of its shape. Hence, these hexagonal samples can be regarded as model systems for the study of the magnetization in thick, mesoscopic ferromagnets.

Perpendicular magnetic anisotropy of cobalt films intercalated under graphene

Magnetic properties of nanometer-thick Co films intercalated at the graphene/Ir(111) interface are investigated using spin-polarized low-energy electron microscopy and Auger electron spectroscopy. We show that the graphene top layer promotes perpendicular magnetic anisotropy in the Co film underneath, even for relatively thick intercalated deposits. The magnetic anisotropy energy is significantly larger for the graphene/Co interface than for the free Co surface. Hybridization of the graphene and Co electron orbitals is believed to be at the origin of the observed perpendicular magnetic anisotropy.

Growth modes of Fe(110) revisited: a contribution of self-assembly to magnetic materials

We have revisited the epitaxial growth modes of Fe on W(110) and Mo(110), and propose an overview or our contribution to the field. We show that the Stranski?Krastanov growth mode, acknowledged for a long time in these systems, is in fact characterized by a bimodal distribution of islands for a growth temperature in the range ~250?700??C. We observe firstly compact islands whose shape is determined by Wulff?Kaischevs theorem, and secondly thin and flat islands that display a preferred height, i.e.?are independent of nominal thickness and deposition procedures?(1.4?nm for Mo, and 5.5?nm for W on the average). We used this effect to fabricate self-organized arrays of nanometres-thick stripes by step decoration. Self-assembled nanoties are also obtained for nucleation of the flat islands on Mo at fairly high temperature, i.e.?~800??C. Finally, using interfacial layers and solid solutions we separate two effects on the preferred height, first that of the interfacial energy, and second that of the continuously varying lattice parameter of the growth surface.

Growth modes of W and Mo thin epitaxial (110) films on (112̄0) sapphire

Abstract Growth modes of W(110) and Mo(110) epitaxial thin films grown with pulsed laser deposition (PLD) on (1120) sapphire were studied. The films properties were studied both in reciprocal space using reflection high energy electron diffraction (RHEED) and X-ray diffraction and in real space using atomic force microscope (AFM). The conventionally used high temperature growth process, usually considered to be the best process – according to reciprocal space characterization – to minimize in-plane crystallites twinning and mosaicity, is proved to yield three-dimensional growth and rough surfaces. A sample continuously wedged up to a thickness of 600 A was studied and only partial islands coalescence was observed. An alternative optimized growth process is proposed, based on 200°C deposition, subsequent annealing, and further high temperature deposition, that enables the elimination of in-plane twinned-crystallites while yielding to nearly perfect W(110) or Mo(110) surfaces, from both crystallographically and geometrically points of view. Throughout the paper we discuss information gained respectively from RHEED and AFM characterization, and analyse their relationships and complementarity. The comparison with MBE grown films suggests that what we report on is not a PLD-specific behavior. Eventually we report on a R(15×6) Mo surface reconstruction, which had never been observed before to our knowledge.

Magnetism of cobalt nanoclusters on graphene on iridium

The structure and magnetic properties of Co clusters, comprising 26–2700 atoms, self-organized or not on the graphene/Ir(111) moire, were studied in situ with the help of scanning tunneling microscopy and x-ray magnetic circular dichroism. Surprisingly, the small clusters have almost no magnetic anisotropy. We find indication for a magnetic coupling between the clusters.

Magnetism in reduced dimensions

Abstract We propose a short overview of a few selected issues of magnetism in reduced dimensions, which are the most relevant to set the background for more specialized contributions to the present Special Issue. Magnetic anisotropy in reduced dimensions is discussed first, on a theoretical basis, then with experimental reports and views from surface to single-atom anisotropy. Then, conventional magnetization states are reviewed, including macrospins, single domains, multidomains, and domain walls in stripes. Dipolar coupling is examined for lateral interactions in arrays, and for interlayer interactions in films and dots. Finally thermally-assisted magnetization reversal and superparamagnetism are presented. For each topic we have sought a balance between well established knowledge and recent developments. To cite this article: O. Fruchart, A. Thiaville, C. R. Physique 6 (2005).

Reduction of magnetostatic interactions in self-organized arrays of nickel nanowires using atomic layer deposition

Ordered arrays of magnetic nanowires are commonly synthesized by electrodeposition in nanoporous alumina templates. Due to their dense packing, strong magnetostatic interactions prevent the manipulation of wires individually. Using atomic layer deposition we reduce the diameter of the pores prior to electrodeposition. This reduces magnetostatic interactions, yielding fully remanent hysteresis loops. This is a first step toward the use of such arrays for magnetic racetrack memories.