176 research outputs found
Effects of spin non-collinearities in magnetic nanoparticles
In a many-spin approach that takes account of the internal structure,
microscopic interactions and single-site anisotropies, we investigate the
effect of spin non-collinearities induced by the boundary and surface
anisotropy on the behaviour of individual magnetic nanoparticles.
Through analytical and numerical calculations, we show that there are mainly
two regimes separated by some critical value of the surface anisotropy constant
which controls the intensity of spin non-collinearities: i) the so called
Stoner-Wohlfarth or N\'eel-Brown regime of a macrospin undergoing a coherent
switching, ii) the many-spin regime where the strong spin non-collinearities
invalidate the coherent mechanism, and where the particle's magnetic state and
switching mechanisms can no longer be modeled by a macrospin.
For small-to-intermediate values of , and within two models of surface
anisotropy (transverse and N\'eel), the behaviour of the nanoparticle can be
modeled by that of a macrospin with an effective potential energy containing a
uniaxial and cubic anisotropy terms. This effective spin model provides a
crossover between the two regimes.Comment: 6 pages, 6 figs, Invited Paper at III Joint European Magnetic
Symposia (JEMS), San Sebastian (Spain), 26-30 June 0
Magnetization of nanomagnet assemblies: Effects of anisotropy and dipolar interactions
We investigate the effect of anisotropy and weak dipolar interactions on the
magnetization of an assembly of nanoparticles with distributed magnetic
moments, i.e., assembly of magnetic nanoparticles in the one-spin
approximation, with textured or random anisotropy.
The magnetization of a free particle is obtained either by a numerical
calculation of the partition function or analytically in the low and high field
regimes, using perturbation theory and the steepest-descent approximation,
respectively. The magnetization of an interacting assembly is computed
analytically in the range of low and high field, and numerically using the
Monte Carlo technique.
Approximate analytical expressions for the assembly magnetization are
provided which take account of the dipolar interactions, temperature, magnetic
field, and anisotropy. The effect of anisotropy and dipolar interactions are
discussed and the deviations from the Langevin law they entail are
investigated, and illustrated for realistic assemblies with the lognormal
moment distribution.Comment: 21 pages, 5 eps figure
Ferromagnetic resonance in systems with competing uniaxial and cubic anisotropies
We develop a model for ferromagnetic resonance in systems with competing
uniaxial and cubic anisotropies. This model applies to (i) magnetic materials
with both uniaxial and cubic anisotropies, and (ii) magnetic nanoparticles with
effective core and surface anisotropies; We numerically compute the resonance
frequency as a function of the field and the resonance field as a function of
the direction of the applied field for an arbitrary ratio of cubic-to-uniaxial
anisotropy. We also provide some approximate analytical expressions in the case
of weak cubic anisotropy. We propose a method that uses these expressions for
estimating the uniaxial and cubic anisotropy constants, and for determining the
relative orientation of the cubic anisotropy axes with respect to the crystal
principle axes. This method is applicable to the analysis of experimental data
of resonance type measurements for which we give a worked example of an iron
thin film with mixed anisotropy.Comment: 7 pages, 3 figure
Generalized Ginzburg-Landau theory for non-uniform FFLO superconductors
We derive a generalized Ginzburg-Landau (GL) functional near the tricritical
point in the (T,H)-phase diagram for the Fulde-Ferrell-Larkin-Ovchinnikov
(FFLO) superconducting state, in 1,2, and 3 dimensions. We find that the
transition from the normal to the FFLO state is of second order in 1 and 2
dimensions, and the order parameter with one-coordinate sine modulation
corresponds to the lowest energy near the transition line. We also describe in
the one-dimensional case the transformation of the sine modulation into the
soliton-lattice state as the magnetic field decreases. In 3 dimensions however,
we find that the transition into an FFLO state is of first order, and it is
impossible to obtain an analytic expression for the critical temperature. In
this case the generalized GL functional proposed here provides a suitable basis
for a numerical study of the properties of the FFLO state, and in particular
for computing the critical temperature, and for describing the transition into
a uniform state.Comment: 9 pages of RevTex, 3 figures available upon request at
[email protected]
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