194 research outputs found
Effect of uniaxial strain on the site occupancy of hydrogen in vanadium from density-functional calculations
We investigate the influence of uniaxial strain on site occupancy of hydrogen
vanadium, using density functional theory. The site occupancy is found to be
strongly influenced by the strain state of the lattice. The results provide the
conceptual framework of the atomistic description of the observed hysteresis in
the alpha to beta phase transition in bulk, as well as the preferred octahedral
occupancy of hydrogen in strained V layers
Proximity effect of vanadium on spin-density-wave magnetism in Cr films
The spin-density wave (SDW) state in thin chromium films is well known to be
strongly affected by proximity effects from neighboring layers. To date the
main attention has been given to effects arising from exchange interactions at
interfaces. In the present work we report on combined neutron and synchrotron
scattering studies of proximity effects in Cr/V films where the boundary
condition is due to the hybridization of Cr with paramagnetic V at the
interface. We find that the V/Cr interface has a strong and long-range effect
on the polarization, period, and the N\'{e}el temperature of the SDW in rather
thick Cr films. This unusually strong effect is unexpected and not predicted by
theory.Comment: 7 figure
Hydrogen site occupancy and strength of forces in nano-sized metal hydrides
The dipole force components in nano-sized metal hydrides are quantitatively
determined with curvature and x-ray diffraction measurements. Ab-initio density
functional theory is used to calculate the dipole components and the symmetry
of the strain field. The hydrogen occupancy in a 100 nm thick V film is shown
to be tetrahedral with a slight asymmetry at low concentration and a transition
to octahedral occupancy is shown to take place at around 0.07 [H/V] at 360 K.
When the thickness of the V layer is reduced to 3 nm and biaxially strained, in
a Fe_0.5V_0.5/V superlattice, the hydrogen unequivocally occupies octahedral
z-like sites, even at and below concentrations of 0.02 [H/V]
Magnon softening in a ferromagnetic monolayer: a first-principles spin dynamics study
We study the Fe/W(110) monolayer system through a combination of first
principles calculations and atomistic spin dynamics simulations. We focus on
the dispersion of the spin waves parallel to the [001] direction. Our results
compare favorably with the experimental data of Prokop et al. [Phys. Rev. Lett.
102, 177206], and correctly capture a drastic softening of the magnon spectrum,
with respect to bulk bcc Fe. The suggested shortcoming of the itinerant
electron model, in particular that given by density functional theory, is
refuted. We also demonstrate that finite temperature effects are significant,
and that atomistic spin dynamics simulations represent a powerful tool with
which to include these.Comment: v1: 11 pages, 3 figures. v2: double column, 5 pages, 3 figures, typos
corrected, references adde
Thermally induced magnetic relaxation in square artificial spin ice
The properties of natural and artificial assemblies of interacting elements,
ranging from Quarks to Galaxies, are at the heart of Physics. The collective
response and dynamics of such assemblies are dictated by the intrinsic
dynamical properties of the building blocks, the nature of their interactions
and topological constraints. Here we report on the relaxation dynamics of the
magnetization of artificial assemblies of mesoscopic spins. In our model
nano-magnetic system - square artificial spin ice - we are able to control the
geometrical arrangement and interaction strength between the magnetically
interacting building blocks by means of nano-lithography. Using time resolved
magnetometry we show that the relaxation process can be described using the
Kohlrausch law and that the extracted temperature dependent relaxation times of
the assemblies follow the Vogel-Fulcher law. The results provide insight into
the relaxation dynamics of mesoscopic nano-magnetic model systems, with
adjustable energy and time scales, and demonstrates that these can serve as an
ideal playground for the studies of collective dynamics and relaxations.Comment: 15 pages, 5 figure
Finite-size effects in amorphous Fe90Zr10/Al75Zr25 multilayers
The thickness dependence of the magnetic properties of amorphous Fe90Zr10
layers has been explored using Fe90Zr10/Al75Zr25 multilayers. The Al75Zr25
layer thickness is kept at 40 \AA, while the thickness of the Fe90Zr10 layers
is varied between 5 and 20 \AA. The thickness of the Al75Zr25 layers is
sufficiently large to suppress any significant interlayer coupling. Both the
Curie temperature and the spontaneous magnetization decrease non-linearly with
decreasing thickness of the Fe90Zr10 layers. No ferromagnetic order is observed
in the multilayer with 5 {\AA} Fe90Zr10 layers. The variation of the Curie
temperature with the Fe90Zr10 layer thickness is fitted with a
finite-size scaling formula [1-\Tc(t)/\Tc(\infty)]=[(t-t')/t_0]^{-\lambda},
yielding , and a critical thickness \AA, below which the
Curie temperature is zero.Comment: 8 pages, 8 figure
Magnetic order and energy-scale hierarchy in artificial spin ice
In order to explain and predict the properties of many physical systems, it
is essential to understand the interplay of different energy-scales. Here we
present investigations of the magnetic order in thermalised artificial spin ice
structures, with different activation energies of the interacting Ising-like
elements. We image the thermally equilibrated magnetic states of the
nano-structures using synchrotron-based magnetic microscopy. By comparing
results obtained from structures with one or two different activation energies,
we demonstrate a clear impact on the resulting magnetic order. The differences
are obtained by the analysis of the magnetic spin structure factors, in which
the role of the activation energies is manifested by distinct short-range
order. This demonstrates that artificial spin systems can serve as model
systems, allowing the definition of energy-scales by geometrical design and
providing the backdrop for understanding their interplay.Comment: 8 pages, 5 figures (+ supplementary 6 pages, 4 figures
The impact of nanoscale compositional variation on the properties of amorphous alloys
The atomic distribution in amorphous FeZr alloys is found to be close to random, nevertheless, the composition can not be viewed as being homogenous at the nm-scale. The spatial variation of the local composition is identified as the root of the unusual magnetic properties in amorphous Fe1-xZr alloys. The findings are discussed and generalised with respect to the physical properties of amorphous and crystalline materials
Emergent tri-criticality in magnetic metamaterials
Metallic discs engineered on the 100 nm scale have an internal magnetic
texture which varies from a fully magnetized state to a vortex state with zero
moment. The interplay between this internal structure and the inter-disc
interactions is studied in magnetic metamaterials made of square arrays of the
magnetic discs. The texture is modeled by a mesospin of varying length with
O(2) symmetry and the inter-disc interaction by a nearest neighbour coupling
between mesospins. The thermodynamic properties of the model are studied
numerically and an ordering transition is found which varies from
Kosterlitz-Thouless to first order via an apparent tri-critical point. The
effective critical exponent characterising the finite size magnetization
evolves from the value for the 2D-XY model to less than half this value at the
tri-critical point. The consequences for future experiments both in and out of
equilibrium are discussed.Comment: 10 pages, 9 figure
Dynamic Critical Behavior of the Heisenberg Model with Strong Easy Plane Anisotropy
The dynamic critical behavior of the Heisenberg model with a strong
anisotropy of the exchange constant in the z direction is investigated. The
main features of the time evolution of this model are revealed. The static and
dynamic critical behavior of planar magnetic models is shown to be described
well by the Heisenberg model with strong easy plane anisotropy.Comment: 5 pages, 4 figures, 1 tabl
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