123 research outputs found
Magnetization in uniaxial spherical nanoparticles: consequence on the interparticle interaction
We investigate the interaction between spherical magnetic nanoparticles which
present either a single domain or a vortex structure. First the magnetic
structure of a uniaxial soft sphere is revisited, and then the interaction
energy is calculated from a micromagnetic simulation. In the vortex regime the
orientation of the vortex relative to the easy axis depends on both the
particle size and the anisotropy constant. We show that the leading term of the
interaction is the dipolar interaction energy between the magnetic moments. For
particles presenting a vortex structure, we show that the polarization due to
the dipolar field must be included. The parameters entering in the dipolar
interaction are deduced from the magnetic behavior of the isolated particle.Comment: 4 pages, proceeding of the JEMS 2008. To be published in the Journal
of Magnetism and Magnetic Materials (available at
http://www.sciencedirect.com/science/journal/03048853
Ferromagnetic order in dipolar systems with anisotropy: application to magnetic nanoparticle supracrystals
Single domain magnetic nanoparticles (MNP) interacting through dipolar
interactions (DDI) in addition to the magnetocrystalline energy may present a
low temperature ferromagnetic (SFM) or spin glass (SSG) phase according to the
underlying structure and the degree of order of the assembly. We study, from
Monte Carlo simulations in the framework of the effective one-spin or macrospin
models, the case of a monodisperse assembly of single domain MNP fixed on the
sites of a perfect lattice with fcc symmetry and randomly distributed easy
axes. We limit ourselves to the case of a low anisotropy, namely the onset of
the disappearance of the dipolar long-range ferromagnetic (FM) phase obtained
in the absence of anisotropy due to the disorder introduced by the latter.Comment: 10 pages, 7 figure
Structure and magnetic properties of nanocrystalline PrCo3
The structure and magnetic properties of nanocrystalline PrCo prepared by
high-energy milling technique have been investigated by means of X-ray
diffraction using the Rietveld method coupled to Curie temperature and magnetic
measurements. The as-milled samples were subsequently annealed in temperature
range from 750 to 1050 {\deg}C for 30 min to optimize the extrinsic properties.
From x-ray studies of magnetic aligned samples, the magnetic anisotropy of this
compounds is found uniaxial. The Curie temperature is 349 {\deg}K and no
saturation reached at room temperature for applied field of 90 kOe. The
coercive field of 55 kOe and 12 kOe measured at 10 and 293 K respectively is
obtained after annealing at 750 {\deg}C for 30 min suggests that
nanocrystalline PrCo are interesting candidates in the field of permanent
magnets. We have completed this experimental study by simulations in the
micromagnetic framework in order to get a qualitative picture of the
microstructure effect on the macroscopic magnetization curve. From this simple
model calculation, we can suggest that the after annealing the system behaves
as magnetically hard crystallites embedded in a weakly magnetized amorphous
matrix. PACS : 75.50.Bb, 75.50.Tt, 76.80.+yComment: Published in Journal of Applied Physics, 107, 083916 (2010). To be
found at: http://jap.aip.or
Magnetic and structural properties of nanocrystalline PrCo
The structure and magnetic properties of nanocrystalline PrCo obtained
from high energy milling technique are investigated by X-ray diffraction, Curie
temperature determination and magnetic properties measurements are reported.
The as-milled samples have been annealed in a temperature range of 1023 K to
1273 K for 30 mn to optimize the extrinsic properties. The Curie temperature is
349\,K and coercive fields of 55\,kOe at 10\,K and 12\,kOe at 293\,K are
obtained on the samples annealed at 1023\,K. A simulation of the magnetic
properties in the framework of micromagnetism has been performed in order to
investigate the influence of the nanoscale structure. A composite model with
hard crystallites embedded in an amorphous matrix, corresponding to the
as-milled material, leads to satisfying agreement with the experimental
magnetization curve. [ K. Younsi, V. Russier and L. Bessais, J. Appl. Phys.
{\bf 107}, 083916 (2010)]. The microscopic scale will also be considered from
DFT based calculations of the electronic structure of Co compounds,
where = (Y, Pr) and = 2,3 and 5.Comment: To be published in J. Phys.: Conference Series in the JEMS 2010
special issue. To be found once published at
http://iopscience.iop.org/1742-659
Spherical magnetic nanoparticles: magnetic structure and interparticle interaction
The interaction between spherical magnetic nanoparticles is investigated from
micromagnetic simulations and ananlysed in terms of the leading dipolar
interaction energy between magnetic dipoles. We focus mainly on the case where
the particles present a vortex structure. In a first step the local magnetic
structure in the isolated particle is revisited. For particles bearing a
uniaxial magnetocrystaline anisotropy, it is shown that the vortex core
orientation relative to the easy axis depends on both the particle size and the
anisotropy constant. When the particles magnetization present a vortex
structure, it is shown that the polarization of the particles by the dipolar
field of the other one must be taken into account in the interaction. An
analytic form is deduced for the interaction which involves the vortex core
magnetization and the magnetic susceptibility which are obtained from the
magnetic properties of the isolated particle.Comment: 20 pages, 10 figures Published in Journal of Applied Physics. To be
found at: http://link.aip.org/link/?jap/105/07391
Phase diagram for ensembles of random close packed Ising-like dipoles as a function of texturation
International audienceWe study random close packed systems of magnetic spheres by Monte Carlo simulations in order to estimate their phase diagram. The uniaxial anisotropy of the spheres makes each of them behave as a single Ising dipole along a fixed easy axis. We explore the phase diagram in terms of the temperature and the degree of alignment (or texturation) among the easy axes of all spheres. This degree of alignment ranges from the textured case (all easy axes pointing along a common direction) to the non-textured case (randomly distributed easy axes). In the former case we find long-range ferromagnetic order at low temperature but, as the degree of alignment is diminished below a certain threshold, the ferromagnetic phase gives way to a spin-glass phase. This spin-glass phase is similar to the one previously found in other dipolar systems with strong frozen disorder. The transition between ferromagnetism and spin-glass passes through a narrow intermediate phase with quasi-long-range ferromagnetic order
Magnetic ordering of random dense packings of freely rotating dipoles
We study random dense packings of Heisenberg dipoles by numerical simulation.
The dipoles are at the centers of identical spheres that occupy fixed random
positions in space and fill a fraction of the spatial volume. The
parameter ranges from rather low values, typical of amorphous ensembles,
to the maximum =0.64 that occurs in the random-close-packed limit. We
assume that the dipoles can freely rotate and have no local anisotropies. As
well as the usual thermodynamical variables, the physics of such systems
depends on . Concretely, we explore the magnetic ordering of these
systems in order to depict the phase diagram in the temperature- plane.
For we find quasi-long-range ferromagnetic order coexisting
with strong long-range spin-glass order. For the
ferromagnetic order disappears giving way to a spin-glass phase similar to the
ones found for Ising dipolar systems with strong frozen disorder.Comment: 12 pages, 16 figures, 1 tabl
Structure of sticky-hard-sphere random aggregates: The viewpoint of contact coordination and tetrahedra
International audienceWe study more than 10 4 random aggregates of 10 6 monodisperse sticky hard spheres each, generated by various static algorithms. Their packing fraction varies from 0.370 up to 0.593. These aggregates are shown to be based on two types of disordered structures: random regular polytetrahedra and random aggregates, the former giving rise to δ peaks on pair distribution functions. Distortion of structural (Delaunay) tetrahedra is studied by two parameters, which show some similarities and some differences in terms of overall tendencies. Isotropy of aggregates is characterized by the nematic order parameter. The overall structure is then studied by distinguishing spheres in function of their contact coordination number (CCN). Distributions of average CCN around spheres of a given CCN value show trends that depend on packing fraction and building algorithms. The radial dependence of the average CCN turns out to be dependent upon the CCN of the central sphere and shows discontinuities that resemble those of the pair distribution function. Moreover, it is shown that structural details appear when the CCN is used as pseudochemical parameter, such as various angular distribution of bond angles, partial pair distribution functions, Ashcroft-Langreth and Bhatia-Thornton partial structure factors. These allow distinguishing aggregates with the same values of packing fraction or average tetrahedral distortion or even similar global pair distribution function, indicative of the great interest of paying attention to contact coordination numbers to study more precisely the structure of random aggregates
Statistical field theory for liquid vapor interface
A statistical field theory for an inhomogeneous liquid, a planar liquid/vapor interface, is devised from first principles. The grand canonical partition function is represented via a Hubbard-Stratonovitch transformation leading, close to the critical point, to the usual φ4 scalar field theory which is then rigorously considered at the one-loop level. When further simplified it yields the well-known capillary wave theory without any ad hoc phenomenological parameter. Internal coherence of the one-loop approximation is discussed and good overall qualitative agreement with recent numerical simulations is stressed
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