605 research outputs found
Surfactant effects in monodisperse magnetite nanoparticles of controlled size
Monodisperse magnetite Fe3O4 nanoparticles of controlled size within 6 and 20
nm in diameter were synthesized by thermal decomposition of an iron organic
precursor in an organic medium. Particles were coated with oleic acid. For all
samples studied, saturation magnetization Ms reaches the expected value for
bulk magnetite, in contrast to results in small particle systems for which Ms
is usually much smaller due to surface spin disorder. The coercive field for
the 6 nm particles is also similar to that of bulk magnetite. Both results
suggest that the oleic acid molecules covalently bonded to the nanoparticle
surface yield a strong reduction in the surface spin disorder. However,
although the saturated state may be similar, the approach to saturation is
different and, in particular, the high-field differential susceptibility is one
order of magnitude larger than in bulk materials. The relevance of these
results in biomedical applications is discussed.Comment: 3 pages, 3 figures. Presented at JEMS 2006 (San Sebastian, Spain).
Submitted to JMM
Modelling exchange bias in core/shell nanoparticles
We present an atomistic model of a single nanoparticle with core/shell
structure that takes into account its lattice strucutre and spherical geometry,
and in which the values of microscopic parameters such as anisotropy and
exchange constants can be tuned in the core, shell and interfacial regions. By
means of Monte Carlo simulations of the hysteresis loops based on this model,
we have determined the range of microscopic parameters for which loop shifts
after field cooling can be observed. The study of the magnetic order of the
interfacial spins for different particles sizes and values of the interfacial
exchange coupling have allowed us to correlate the appearance of loop
asymmetries and vertical displacements to the existence of a fraction of
uncompensated spins at the shell interface that remain pinned during field
cycling, offering new insight on the microscopic origin of the experimental
phenomenology.Comment: 7 pages, 3 figures. Contribution presented at HMM 2007 held at Napoli
4-6 June 2007. To be published in J. Phys. Condens. Matte
Noether's theorem and gauge transformations. Application to the bosonic string and CP(2,n-1) model
New results on the theory of constrained systems are applied to characterize the generators of Noethers symmetry transformations. As a byproduct, an algorithm to construct gauge transformations in Hamiltonian formalism is derived. This is illustrated with two relevant examples
Canonical Noether symmetries and commutativity properties for gauge systems
For a dynamical system defined by a singular Lagrangian, canonical Noether
symmetries are characterized in terms of their commutation relations with the
evolution operators of Lagrangian and Hamiltonian formalisms. Separate
characterizations are given in phase space, in velocity space, and through an
evolution operator that links both spaces.Comment: 22 pages; some references updated, an uncited reference deleted,
minor style change
Gauge transformations in the Lagrangian and Hamiltonian formalisms of generally covariant theories
We study spacetime diffeomorphisms in Hamiltonian and Lagrangian formalisms
of generally covariant systems. We show that the gauge group for such a system
is characterized by having generators which are projectable under the Legendre
map. The gauge group is found to be much larger than the original group of
spacetime diffeomorphisms, since its generators must depend on the lapse
function and shift vector of the spacetime metric in a given coordinate patch.
Our results are generalizations of earlier results by Salisbury and
Sundermeyer. They arise in a natural way from using the requirement of
equivalence between Lagrangian and Hamiltonian formulations of the system, and
they are new in that the symmetries are realized on the full set of phase space
variables. The generators are displayed explicitly and are applied to the
relativistic string and to general relativity.Comment: 12 pages, no figures; REVTeX; uses multicol,fancyheadings,eqsecnum;
to appear in Phys. Rev.
Particle growing mechanisms in Ag-ZrO2 and Au-ZrO2 granular films obtained by pulsed laser deposition
Thin films consisting of Ag and Au nanoparticles embedded in amorphous ZrO2
matrix were grown by pulsed laser deposition in a wide range of metal volume
concentrations in the dielectric regime (0.08<x(Ag)<0.28 and 0.08<x(Au)<0.52).
High resolution transmission electron microscopy (TEM) showed regular
distribution of spherical Au and Ag nanoparticles having very sharp interfaces
with the amorphous matrix. Mean particle size determined from X-ray diffraction
agreed with direct TEM observation. The silver mean diameter increases more
abruptly with metal volume content than that corresponding to gold particles
prepared under the same conditions. Two mechanisms of particle growing are
observed: nucleation and particle coalescence, their relative significance
being different in both granular systems, which yields very different values of
the percolation threshold (xc(Ag)~0.28 and xc(Au)~0.52).Comment: 6 figure
The nanostructural origin of the ac conductance in dielectric granular metals: the case study of Co_20(ZrO_2)_80
We show which is the nanostructure required in granular Co20(ZrO2)80 thin
films to produce an ac response such as the one that is universally observed in
a very wide variety of dielectric materials. A bimodal size distribution of Co
particles yields randomly competing conductance channels which allow both
thermally assisted tunneling through small particles and capacitive conductance
among larger particles that are further apart. A model consisting on a simple
cubic random resistance-capacitor network describes quantitatively the
experimental results as functions of temperature and frequency, and enables the
determination of the microscopic parameters controlling the ac response of the
samples.Comment: Available online at:
http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=APPLAB000091000005052108000001&idtype=cvips&gifs=ye
Magnetic transitions in Pr2NiO4 single crystal
The magnetic properties of a stoichiometric Pr2NiO4 single crystal have been examined by means of the temperature dependence of the complex ac susceptibility and the isothermal magnetization in fields up to 200 kOe at T=4.2 K. Three separate phases have been identified and their anisotropic character has been analyzed. A collinear antiferromagnetic phase appears first between TN = 325 K and Tc1 = 115 K, where the Pr ions are polarized by an internal magnetic field. At Tc1 a first modification of the magnetic structure occurs in parallel with a structural phase transition (Bmab to P42/ncm). This magnetic transition has a first‐order character and involves both the out‐of‐plane and the in‐plane spin components (magnetic modes gx and gxcyfz, respectively). A second magnetic transition having also a first‐order character is also clearly identified at Tc2 = 90 K which corresponds to a spin reorientation process (gxcyfz to cxgyaz magnetic modes). It should be noted as well that the out‐of‐phase component of χac shows a peak around 30 K which reflects the coexistence of both magnetic configurations in a wide temperature interval. Finally, two field‐induced transitions have been observed at 4.2 K when the field is directed along the c axis. We propose that the high‐field anomaly arises from a metamagnetic transition of the weak ferromagnetic component, similarly to La2CuO4
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