776 research outputs found
Magnetic relaxation in terms of microscopic energy barriers in a model of dipolar interacting nanoparticles
The magnetic relaxation and hysteresis of a system of single domain particles
with dipolar interactions are studied by Monte Carlo simulations. We model the
system by a chain of Heisenberg classical spins with randomly oriented
easy-axis and log-normal distribution of anisotropy constants interacting
through dipole-dipole interactions. Extending the so-called
method to interacting systems, we show how to relate the simulated relaxation
curves to the effective energy barrier distributions responsible for the
long-time relaxation. We find that the relaxation law changes from
quasi-logarithmic to power-law when increasing the interaction strength. This
fact is shown to be due to the appearence of an increasing number of small
energy barriers caused by the reduction of the anisotropy energy barriers as
the local dipolar fields increase.Comment: 11 pages, 10 figure
A temperature and magnetic field dependence Mössbauer study of ɛ-Fe2O3
ɛ-Fe2O3 was synthesized as nanoparticles by a pre-vacuum heat treatment of yttrium iron garnet (Y3Fe5O12) in a silica matrix at 300-C followed by sintering in air at 1,000-C for up to 10 h. It displays complex magnetic properties that are characterized by two transitions, one at 480 K from a paramagnet (P) to canted antiferromagnet (CAF1) and the second at ca. 120 K from the canted antiferromagnet (CAF1) to another canted antiferromagnet (CAF2). CAF2 has a smaller resultant magnetic moment (i.e. smaller canting angle) than CAF1. Analysis of the zero-field Mossbauer spectra at different temperatures shows an associated discontinuity of the hyperfine field around 120 K. In an applied field, the different magnetic sublattices were identified and the directions of their moments were assigned. The moments of the two sublattices are antiparallel and collinear at 160 K but are at right angle to each other at 4.2 K
Monte Carlo simulation study of exchange biased hysteresis loops in nanoparticles
We present the results of Monte Carlo simulations of the magnetic properties
of a model for a single nanoparticle consisting in a ferromagnetic core
surrounded by an antiferromagnetic shell. The simulations of hysteresis loops
after cooling in a magnetic field display exchange bias effects. In order to
understand the origin of the loop shifts, we have studied the thermal
dependence of the shell and interface magnetizations under field cooling. These
results, together with inspection of the snapshots of the configurations
attained at low temperature, show the existence of a net magnetization at the
interface which is responsible for the bias of the hysteresis loops.Comment: 9 pages, 3 figures embedded. To be published in Physica
Finite-size versus Surface effects in nanoparticles
We study the finite-size and surface effects on the thermal and spatial
behaviors of the magnetisation of a small magnetic particle. We consider two
systems: 1) A box-shaped isotropic particle of simple cubic structure with
either periodic or free boundary conditions. This case is treated analytically
using the isotropic model of D-component spin vectors in the limit , including the magnetic field. 2) A more realistic particle (-FeO) of ellipsoidal (or spherical) shape with open boundaries.
The magnetic state in this particle is described by the anisotropic classical
Dirac-Heisenberg model including exchange and dipolar interactions, and bulk
and surface anisotropy. This case is dealt with by the classical Monte Carlo
technique. It is shown that in both systems finite-size effects yield a
positive contribution to the magnetisation while surface effects render a
larger and negative contribution, leading to a net decrease of the
magnetisation of the small particle with respect to the bulk system. In the
system 2) the difference between the two contributions is enhanced by surface
anisotropy. The latter also leads to non saturation of the magnetisation at low
temperatures, showing that the magnetic order in the core of the particle is
perturbed by the magnetic disorder on the surface. This is confirmed by the
profile of the magnetisation.Comment: 6 pages of RevTex including 4 Figures, invited paper to 3rd
EuroConference on Magnetic Properties of Fine Nanoparticles, Barcelona,
October 9
Surface effects in nanoparticles: application to maghemite -Fe_{2}O_{3}
We present a microscopic model for nanoparticles, of the maghemite (% -FeO) type, and perform classical Monte Carlo simulations of
their magnetic properties. On account of M\"{o}ssbauer spectroscopy and
high-field magnetisation results, we consider a particle as composed of a core
and a surface shell of constant thickness. The magnetic state in the particle
is described by the anisotropic classical Dirac-Heisenberg model including
exchange and dipolar interactions and bulk and surface anisotropy. We consider
the case of ellipsoidal (or spherical) particles with free boundaries at the
surface. Using a surface shell of constant thickness ( nm) we vary
the particle size and study the effect of surface magnetic disorder on the
thermal and spatial behaviors of the net magnetisation of the particle. We
study the shift in the surface ``critical region'' for different
surface-to-core ratios of the exchange coupling constants. It is also shown
that the profile of the local magnetisation exhibits strong temperature
dependence, and that surface anisotropy is reponsible for the non saturation of
the magnetisation at low temperatures.Comment: 15 pages, 7 figures, to appear in Eur. Phys. J.
Ferromagnetism in Co-doped ZnO films grown by molecular beam epitaxy: magnetic, electrical and microstructural studies
We studied structural, optical and magnetic properties of high-quality 5 and
15% Co-doped ZnO films grown by plasma-assisted molecular beam epitaxy (MBE) on
(0001)-sapphire substrates. Magnetic force microscopy (MFM) and magnetic
measurements with SQUID magnetometer show clear ferromagnetic behavior of the
films up to room temperature whereas they are antiferromagnetic below 200 K
approximately. Temperature dependence of the carrier mobility was determined
using Raman line shape analysis of the longitudinal-optical-phonon-plasmon
coupled modes. It shows that the microscopic mechanism for ferromagnetic
ordering is coupling mediated by free electrons between spins of Co atoms.
These results bring insight into a subtle interplay between charge carriers and
magnetism in MBE-grown Zn(1-x)CoxO films.Comment: 10 pages, 9 figures, 2 table
Petite histoire de l’École polytechnique sur la montagne Sainte-Geneviève
En 1794, lorsque les Conventionnels, sur la proposition de Gaspard Monge, approuvèrent avec enthousiasme le projet de créer une École centrale des travaux publics, ils savaient pourquoi la Nation avait un besoin urgent d’une telle institution et sur quelles bases la construire. La question de sa localisation se posa très vite : la nouvelle école devait être proche du pouvoir révolutionnaire. Les locaux qui lui furent initialement affectés étaient des dépendances du Palais-Bourbon, là où s’éta..
Composition Dependence of Structural Parameters and Properties of Gallium Ferrite
We show the effect of composition on structural and magnetic characteristics
of pure phase polycrystalline GaFeO (GFO) for compositions between
0.8 <= x <= 1.3. X-ray analysis reveals that lattice parameters of GFO exhibit
a linear dependence on Fe content in single phase region indicating
manifestation of Vegard's law. Increasing Fe content of the samples also leads
to stretching of bonds as indicated by the Raman peak shifts. Further, low
temperature magnetic measurements show that the coercivity of the samples is
maximum for Ga:Fe ratio of 1:1 driven by a competition between decreasing
crystallite size and increasing magnetic anisotropy.Comment: 15 pages with 4 figure
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