66 research outputs found
Confined step-flow growth of Cu intercalated between graphene and a Ru(0001) surface
By comparing the growth of Cu thin films on bare and graphene-covered
Ru(0001) surfaces, we demonstrate the role of graphene as a surfactant allowing
the formation of flat Cu films. Low-energy electron microscopy, X-ray
photoemission electron microscopy and X-ray absorption spectroscopy reveal that
depositing Cu at 580 K leads to distinct behaviors on both types of surfaces.
On bare Ru, a Stranski-Krastanov growth is observed, with first the formation
of an atomically flat and monolayer-thick wetting layer, followed by the
nucleation of three-dimensional islands. In sharp contrast, when Cu is
deposited on a graphene-covered Ru surface under the very same conditions, Cu
intercalates below graphene and grows in a step-flow manner: atomically-high
growth fronts of intercalated Cu form at the graphene edges, and extend towards
the center of the flakes. Our findings suggest potential routes in metal
heteroepitaxy for the control of thin film morphology.Comment: 9 pages, 4 figure
Proposal to recover an extensive ground state degeneracy in a two-dimensional square array of nanomagnets
We investigate numerically the micromagnetic properties and the low-energy
physics of an artificial square spin system in which the nanomagnets are
physically connected at the lattice vertices. Micromagnetic simulations reveal
that the energy stored at the vertex sites strongly depends on the type of
magnetic domain wall formed by the four connected nanomagnets. As a
consequence, the energy gap between the vertex types can be partially modified
by varying the geometrical parameters of the nanomagnets, such as their width
and thickness. Based on the energy levels given by the micromagnetic
simulations, we compute the thermodynamic properties of the corresponding spin
models using Monte Carlo simulations. We found two regimes, both being
characterized by an extensive ground state manifold, in sharp contrast with
similar lattices with disconnected nanomagnets. For narrow and thin
nanomagnets, low-energy spin configurations consist of independent
ferromagnetic straight lines crossing the whole lattice. The ground state
manifold is thus highly degenerate, although this degeneracy is subdominant. In
the limit of thick and wide nanomagnets, our findings suggest that the
celebrated square ice model may be fabricated experimentally from a simple
square lattice of connected elements. These results show that the micromagnetic
nature of artificial spin systems involves another degree of freedom that can
be finely tuned to explore strongly correlated disordered magnetic states of
matter.Comment: 6 pages, 5 figure
The skyrmion-bubble transition in a ferromagnetic thin film
Magnetic skyrmions and bubbles, observed in ferromagnetic thin films with
perpendicular magnetic anisotropy, are topological solitons which differ by
their characteristic size and the balance in the energies at the origin of
their stabilisation. However, these two spin textures have the same topology
and a continuous transformation between them is allowed. In the present work,
we derive an analytical model to explore the skyrmion-bubble transition. We
evidence a region in the parameter space where both topological soliton
solutions coexist and close to which transformations between skyrmion and
bubbles are observed as a function of the magnetic field. Above a critical
point, at which the energy barrier separating both solutions vanishes, only one
topological soliton solution remains, which size can be continuously tuned from
micrometer to nanometer with applied magnetic field
Intercalating cobalt between graphene and iridium (111): a spatially-dependent kinetics from the edges
Using low-energy electron microscopy, we image in real time the intercalation
of a cobalt monolayer between graphene and the (111) surface of iridium. Our
measurements reveal that the edges of a graphene flake represent an energy
barrier to intercalation. Based on a simple description of the growth kinetics,
we estimate this energy barrier and find small, but substantial, local
variations. These local variations suggest a possible influence of the graphene
orientation with respect to its substrate and of the graphene edge termination
on the energy value of the barrier height. Besides, our measurements show that
intercalated cobalt is energetically more favorable than cobalt on bare
iridium, indicating a surfactant role of graphene
Head-to-head domain walls in one-dimensional nanostructures: an extended phase diagram ranging from strips to cylindrical wires
International audienceSo far magnetic domain walls in one-dimensional structures have been described theoretically only in the cases of flat strips, or cylindrical structures with a compact cross-section, either square or disk. Here we describe an extended phase diagram unifying the two pictures, extensively covering the (width,thickness) space. It is derived on the basis of symmetry and phase-transition arguments, and micromagnetic simulations. A simple classification of all domain walls in two varieties is proposed on the basis of their topology: either with a combined transverse/vortex character, or of the Bloch-point type. The exact arrangement of magnetization within each variety results mostly from the need to decrease dipolar energy, giving rise to asymmetric and curling structures. Numerical evaluators are introduced to quantify curling, and scaling laws are derived analytically for some of the iso-energy lines of the phase diagram
Anatomy and giant enhancement of the perpendicular magnetic anisotropy of cobalt-graphene heterostructures
We report strongly enhanced perpendicular magnetic anisotropy (PMA) of Co
films by graphene coating from both first-principles and experiments. Our
calculations show that graphene can dramatically boost the surface anisotropy
of Co films up to twice the value of its pristine counterpart and can extend
the out-of-plane effective anisotropy up to unprecedented thickness of 25~\AA.
These findings are supported by our experiments on graphene coating on Co films
grown on Ir substrate. Furthermore, we report layer-resolved and
orbital-hybridization-resolved anisotropy analysis which help understanding the
physical mechanisms of PMA and more practically can help design structures with
giant PMA. As an example, we propose super-exchange stabilized Co-graphene
heterostructures with a robust out-of-plane constant effective PMA and linearly
increasing interfacial anisotropy as a function of film thickness. These
findings point towards possibilities to engineer graphene/ferromagnetic metal
heterostructures with giant magnetic anisotropy more than 20 times larger
compared to conventional multilayers, which constitutes a hallmark for future
graphene and traditional spintronic technologies.Comment: 17 pages, 4 figure
Indirect localization of a magnetic domain wall mediated by quasi walls
International audienceThe manipulation of magnetic domain walls in thin films and nanostructures opens new opportunities for fundamental and applied research. But controlling reliably the position of a moving domain wall still remains challenging. So far, most of the studies aimed at understanding the physics of pinning and depinning processes in the magnetic layer in which the wall moves (active layer). In these studies, the role of other magnetic layers in the stack has been often ignored. Here, we report an indirect localization process of 180° domain walls that occurs in magnetic tunnel junctions, commonly used in spintronics. Combining Scanning Transmission X-Ray Microscopy and micromagnetic simulations, magnetic configurations in both layers are resolved. When nucleating a 180° domain wall in the active layer, a quasi wall is created in the reference layer, atop the wall. The wall and its quasi wall must then be moved or positioned together, as a unique object. As a mutual effect, a localized change of the magnetic properties in the reference layer induces a localized quasi wall in the active layer. The two types of quasi walls are shown to be responsible for an indirect localization process of the 180° domain wall in the active layer
Asymmetric hysteresis of N\'eel caps in flux-closure magnetic dots
We investigated with XMCD-PEEM magnetic imaging the magnetization reversal
processes of N\'eel caps inside Bloch walls in self-assembled, micron-sized
Fe(110) dots with flux-closure magnetic state. In most cases the
magnetic-dependent processes are symmetric in field, as expected. However, some
dots show pronounced asymmetric behaviors. Micromagnetic simulations suggest
that the geometrical features (and their asymmetry) of the dots strongly affect
the switching mechanism of the N\'eel caps.Comment: Proceeding for MMM-Intermag 2010 (Washington
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