397 research outputs found
On the reorientation transition of ultra-thin Ni/Cu(001) films
The reorientation transition of the magnetization of ferromagnetic films is
studied on a microscopic basis within a Heisenberg spin model. Using a modified
mean field formulation it is possible to calculate properties of magnetic thin
films with non-integer thicknesses. This is especially important for the
reorientation transition in Ni/Cu(001), as there the magnetic properties are a
sensitive function of the film thickness. Detailed phase diagrams in the
thickness-temperature plane are calculated using experimental parameters and
are compared with experimental measurements by Baberschke and Farle (J. Appl.
Phys. 81, 5038 (1997)).Comment: 7 pages(LaTeX2e) with one figure(eps), accepted for publication in
JMMM. See also http://www.thp.Uni-Duisburg.DE/Publikationen/Publist_Us_R.htm
Anisotropy of ultra-thin ferromagnetic films and the spin reorientation transition
The influence of uniaxial anisotropy and the dipole interaction on the
direction of the magnetization of ultra-thin ferromagnetic films in the
ground-state is studied. The ground-state energy can be expressed in terms of
anisotropy constants which are calculated in detail as function of the system
parameters and the film thickness. In particular non-collinear spin
arrangements are taken into account. Conditions for the appearance of a spin
reorientation transition are given and analytic results for the width of the
canted phase and its shift in applied magnetic fields associated with this
transition are derived.Comment: 6 pages, RevTeX
Theory of the Spin Reorientation Transition of Ultra-Thin Ferromagnetic Films
The reorientation transition of the magnetization of ferromagnetic films is
studied on a microscopic basis within Heisenberg spin models. Analytic
expressions for the temperature dependent anisotropy are derived from which it
is seen that the reduced magnetization in the film surface at finite
temperatures plays a crucial role for this transition. Detailed phase diagrams
in the temperature-thickness plane are calculated.Comment: 6 pages(LaTeX2e), one figure(eps), accepted for publication in JMM
Influence of magnetic fields on the spin reorientation transition in ultra-thin films
The dependence of the spin reorientation transition in ultra-thin
ferromagnetic films on external magnetic fields is studied. For different
orientations of the applied field with respect to the film, phase diagrams are
calculated within a mean field theory for the classical Heisenberg model. In
particular we find that the spin reorientation transition present in this model
is not suppressed completely by an applied field, as the magnetization
component perpendicular to the field may show spontaneous order in a certain
temperature interval.Comment: 11 pages(LaTeX2e), 6 figures(eps), submitted to Phil. Mag. B. See
also http://www.thp.Uni-Duisburg.DE/Publikationen/Publist_Us_R.htm
Reorientation transition of ultrathin ferromagnetic films
We demonstrate that the reorientation transition from out-of-plane to
in-plane magnetization with decreasing temperature as observed experimentally
in Ni-films on Cu(001) can be explained on a microscopic basis. Using a
combination of mean field theory and perturbation theory, we derive an analytic
expression for the temperature dependent anisotropy. The reduced magnetization
in the film surface at finite temperatures plays a crucial role for this
transition as with increasing temperature the influence of the uniaxial
anisotropies is reduced at the surface and is enhanced inside the film.Comment: 4 pages(RevTeX), 3 figures (EPS
Are WC9 Wolf-Rayet stars in colliding-wind binaries?
We present results from a spectroscopic search for massive companions to
dust-making Galactic WC9 stars as a step to testing the paradigm that dust
formation in these systems requires colliding winds to produce over densities.
We find evidence for OB companions to the WC9 stars WR 59 and WR 65, but not WR
121 or WR 117. We identify lines of N III-V and possibly N II in the spectrum
of WR 88, one of the few Galactic WC9 stars which do not make circumstellar
dust, and suggest that WR 88 is a transitional WN-WC9 object and less evolved
than the other WC9 stars. On the other hand, the possible identification of a
strong emission line at 4176A in the spectrum of WR 117 with Ne I suggests that
this star is more evolved than other WC9 stars studied.Comment: 4 pages, 3 figures, contribution to "Massive Stars and High-Energy
Emission in OB Associations"; JENAM 2005, held in Liege (Belgium
Sheared Ising models in three dimensions
The nonequilibrium phase transition in sheared three-dimensional Ising models
is investigated using Monte Carlo simulations in two different geometries
corresponding to different shear normals. We demonstrate that in the high shear
limit both systems undergo a strongly anisotropic phase transition at exactly
known critical temperatures T_c which depend on the direction of the shear
normal. Using dimensional analysis, we determine the anisotropy exponent
theta=2 as well as the correlation length exponents nu_parallel=1 and
nu_perp=1/2. These results are verified by simulations, though considerable
corrections to scaling are found. The correlation functions perpendicular to
the shear direction can be calculated exactly and show Ornstein-Zernike
behavior.Comment: 6 pages, 3 figure
Monte Carlo Simulation of Ising Models with Dipole Interaction
Recently, a new memory effect was found in the metamagnetic domain structure
of the diluted Ising antiferromagnet by domain imaging
with Faraday contrast. Essential for this effect is the dipole interaction. We
simulate the low temperature behavior of diluted Ising-antiferromagnets by a
Monte Carlo simulation considering long range interaction. The metamagnetic
domain structure occurring due to the dipole interaction is investigated by
graphical representation. In the model considered the antiferromagnetic state
is stable for an external magnetic field smaller than a lower boundary
while for fields larger than an upper boundary the system is in the
saturated paramagnetic phase, where the spins are ferromagnetically polarized.
For magnetic fields in between these two boundaries a mixed phase occurs
consisting of ferromagnetic domains in an antiferromagnetic background. The
position of these ferromagnetic domains is stored in the system: after a cycle
in which the field is first removed and afterwards applied again the domains
reappear at their original positions. The reason for this effect can be found
in the frozen antiferromagnetic domain state which occurs after removing the
field.Comment: Latex, 10 pages; 3 postsript-figures, compressed tar-file, uuencoded,
report 10109
Critical behavior of the Random-Field Ising model at and beyond the Upper Critical Dimension
The disorder-driven phase transition of the RFIM is observed using exact
ground-state computer simulations for hyper cubic lattices in d=5,6,7
dimensions. Finite-size scaling analyses are used to calculate the critical
point and the critical exponents of the specific heat, magnetization,
susceptibility and of the correlation length. For dimensions d=6,7 which are
larger or equal to the assumed upper critical dimension, d_u=6, mean-field
behaviour is found, i.e. alpha=0, beta=1/2, gamma=1, nu=1/2. For the analysis
of the numerical data, it appears to be necessary to include recently proposed
corrections to scaling at and beyond the upper critical dimension.Comment: 8 pages and 13 figures; A consise summary of this work can be found
in the papercore database at http://www.papercore.org/Ahrens201
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