193 research outputs found
Ferromagnetic resonance of a two-dimensional array of nanomagnets: Effects of surface anisotropy and dipolar interactions
We develop an analytical approach for studying the FMR frequency shift due to
dipolar interactions and surface effects in two-dimensional arrays of
nanomagnets with (effective) uniaxial anisotropy along the magnetic field. For
this we build a general formalism on the basis of perturbation theory that
applies to dilute assemblies but which goes beyond the point-dipole
approximation as it takes account of the size and shape of the nano-elements,
in addition to their separation and spatial arrangement. The contribution to
the frequency shift due to the shape and size of the nano-elements has been
obtained in terms of their aspect ratio, their separation and the lattice
geometry. We have also varied the size of the array itself and compared the
results with a semi-analytical model and reached an agreement that improves as
the size of the array increases. We find that the red-shift of the
ferromagnetic resonance due to dipolar interactions decreases for smaller
arrays. Surface effects may induce either a blue-shift or a red-shift of the
FMR frequency, depending on the crystal and magnetic properties of the
nano-elements themselves. In particular, some configurations of the
nano-elements assemblies may lead to a full compensation between surface
effects and dipole interactions.Comment: 14 pages, 5 figure
Momentum dependent light scattering in insulating cuprates
We investigate the problem of inelastic x-ray scattering in the spin
Heisenberg model on the square lattice. We first derive a momentum dependent
scattering operator for the and polarization geometries. On
the basis of a spin-wave analysis, including magnon-magnon interactions and
exact-diagonalizations, we determine the qualitative shape of the spectra. We
argue that our results may be relevant to help interpret inelastic x-ray
scattering experiments in the antiferromagnetic phase of the cuprates.Comment: 5 pages, 3 figures, to appear in PR
Orbital degeneracy as a source of frustration in LiNiO
Motivated by the absence of cooperative Jahn-Teller effect and of magnetic
ordering in LiNiO, a layered oxide with triangular planes, we study a
general spin-orbital model on the triangular lattice. A mean-field approach
reveals the presence of several singlet phases between the SU(4) symmetric
point and a ferromagnetic phase, a conclusion supported by exact
diagonalizations of finite clusters. We argue that one of the phases,
characterized by a large number of low-lying singlets associated to dimer
coverings of the triangular lattice, could explain the properties of LiNiO,
while a ferro-orbital phase that lies nearby in parameter space leads to a new
prediction for the magnetic properties of NaNiO.Comment: 18 pages, 17 figure
The Impact of an Oxygen Dopant in an ideal Bi-2212 Crystal
Recent scanning tunneling microscopy studies have shown that local nanoscale
pairing inhomogenities are correlated with interstitial oxygen dopants in
Bi-2212. Combining electrostatic and cluster calculations, in this paper the
impact of a dopant on the local Madelung and charge transfer energies, magnetic
exchange J, Zhang-Rice mobility, and interactions with the lattice is
investigated. It is found that electrostatic modifications locally increases
the charge transfer energy and slightly suppresses J. It is further shown that
coupling to c-axis phonons is strongly modified near the dopant. The combined
effects of electrostatic modifications and coupling to the lattice yield
broadened spectral features, reduced charge gap energies, and a sizable local
increase of J. This implies a strong local interplay between
antiferromagnetism, polarons, and superconducting pairing.Comment: 4 figure
Theory of inelastic light scattering in spin-1 systems: resonant regimes and detection of quadrupolar order
Motivated by the lack of an obvious spectroscopic probe to investigate
non-conventional order such as quadrupolar orders in spin S>1/2 systems, we
present a theoretical approach to inelastic light scattering for spin-1 quantum
magnets in the context of a two-band Hubbard model. In contrast to the S=1/2
case, where the only type of local excited state is a doubly occupied state of
energy , several local excited states with occupation up to 4 electrons are
present. As a consequence, we show that two distinct resonating scattering
regimes can be accessed depending on the incident photon energy. For
, the standard Loudon-Fleury operator remains the
leading term of the expansion as in the spin-1/2 case. For
, a second resonant regime is found with a leading
term that takes the form of a biquadratic coupling \sim({\bf S}_{i}\cdot{\bf
S}_{j)^{2}. Consequences for the Raman spectra of S=1 magnets with magnetic or
quadrupolar order are discussed. Raman scattering appears to be a powerful
probe of quadrupolar order.Comment: 10 Pages, 6 Figures, Submitted to PR
Interplay between surface anisotropy and dipolar interactions in an assembly of nanomagnets
We study the interplay between the effects of surface anisotropy and dipolar interactions in monodisperse assemblies of nanomagnets with oriented anisotropy. We derive asymptotic formulas for the assembly magnetization, taking into account temperature, applied field, core and surface anisotropy, and dipolar interparticle interactions. We find that the interplay between surface anisotropy and dipolar interactions is well described by the analytical expression of the assembly magnetization derived here: the overall sign of the product of the two parameters governing the surface and the dipolar contributions determines whether intrinsic and collective terms compete or have synergistic effects on the magnetization. This is illustrated by the magnetization curves of γ-Fe2O3 nanoparticle assemblies in the low concentration limit
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