1,076 research outputs found
Defining block character
In this paper I propose a clear, efficient, and accurate method for determining if a block of contiguous buildings has an overall character. The work is needed because most contemporary design reviews presuppose the existence of visual character, but existing design principles are often too vague to make the required determination. Clarity is achieved by shifting from vague notions to a definite concept for block character: a design feature will be perceived as part of the overall character of that block if the frequency of the feature is greater than a critical threshold. An experiment suggested that the critical frequency was quite high: over 80%. A case history illustrates how the new concept of visual character could greatly increase the efficiency and accuracy of actual planning decisions.
Effect of weak disorder on the ground state of uniaxial dipolar spin systems in the upper critical dimension
Extensive Monte Carlo simulations are used to investigate the stability of
the ferromagnetic ground state in three-dimensional systems of Ising dipoles
with added quenched disorder. These systems model the collective ferromagnetic
order observed in various systems with dipolar long-range interactions. The
uniaxial dipolar spins are arranged on a face-centred cubic lattice with
periodic boundary conditions. Finite-size scaling relations for the pure
dipolar ferromagnetic system are derived by a renormalisation group
calculation. These functions include logarithmic corrections to the expected
mean field behaviour since the system is in its upper critical dimension.
Scaled data confirm the validity of the finite-size scaling description and
results are compared with subsequent analysis of weakly disordered systems. A
disorder-temperature phase diagram displays the preservation of the
ferromagnetic ground state with the addition of small amounts of disorder,
suggesting the irrelevance of weak disorder in these systems.Comment: 6 pages, 4 figures; proceedings of the 3rd NEXT-Sigma-Phi Conference,
Kolymbari, Greece, August 200
Emergence of skyrmion lattices and bimerons in chiral magnetic thin films with nonmagnetic impurities
Skyrmions are topologically protected field structures with particlelike characteristics that play important roles in several areas of science. Recently, skyrmions have been directly observed in chiral magnets. Here, we investigate the effects of pointlike nonmagnetic impurities on the distinct initial states (random or helical ones) and on the formation of the skyrmion crystal in a discrete lattice. Using Monte Carlo techniques, we have found that even a small percentage of spin vacancies present in the chiral magnetic thin film considerably affects the skyrmion order. The main effects of impurities are somewhat similar to thermal effects. The presence of these spin vacancies also induces the formation of bimerons in both the helical and skyrmion states. We also investigate how adjacent impurities forming a hole affect the skyrmion crystal
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
A Circuit Model for Domain Walls in Ferromagnetic Nanowires: Application to Conductance and Spin Transfer Torques
We present a circuit model to describe the electron transport through a
domain wall in a ferromagnetic nanowire. The domain wall is treated as a
coherent 4-terminal device with incoming and outgoing channels of spin up and
down and the spin-dependent scattering in the vicinity of the wall is modelled
using classical resistances. We derive the conductance of the circuit in terms
of general conductance parameters for a domain wall. We then calculate these
conductance parameters for the case of ballistic transport through the domain
wall, and obtain a simple formula for the domain wall magnetoresistance which
gives a result consistent with recent experiments. The spin transfer torque
exerted on a domain wall by a spin-polarized current is calculated using the
circuit model and an estimate of the speed of the resulting wall motion is
made.Comment: 10 pages, 5 figures; submitted to Physical Review
Probing La(0.7)Sr(0.3)MnO3 multilayers via spin wave resonances
La(0.7)Sr(0.3)MnO3/BiFeO3 and La(0.7)Sr(0.3)MnO3/PbZr20Ti80O3 epitaxial
heterostructures have been grown on SrTiO3 substrates. Spin wave resonances are
used to study interface properties of the ferromagnetic La(0.7)Sr(0.3)MnO3. We
find that the addition of the BiFeO3 or PbZr20Ti80O3 causes out-of-plane
surface pinning of the La(0.7)Sr(0.3)MnO3. We are able to place new limits on
the exchange constant D of La(0.7)Sr(0.3)MnO3 grown on these substrates and
confirm the presence of uniaxial and biaxial anisotropies caused by the SrTiO3
substrate.Comment: 7 pages, 5 figures, 3 table
Theory for nucleation at an interface and magnetization reversal of a two-layer nanowire
Nucleation at the interface between two adjoining regions with dissimilar physical properties is investigated using a model for magnetization reversal of a two-layer ferromagnetic nanowire. Each layer of the nanowire is considered to have a different degree of magnetic anisotropy, representing a hard magnetic layer exchange-coupled to a softer layer. A magnetic field applied along the easy axis causes the softer layer to reverse, forming a domain wall close to the interface. For small applied fields this state is metastable and complete reversal of the nanowire takes place via activation over a barrier. A reversal mechanism involving nucleation at an interface is proposed, whereby a domain wall changes in width as it passes from the soft layer to the hard layer during activation. Langer’s statistical theory for the decay of a metastable state is used to derive rates of magnetization reversal, and simple formulas are found in limiting cases for the activation energy, rate of reversal, and critical field at which the metastable state becomes unstable. These formulas depend on the anisotropy difference between each layer, and the behavior of the reversal rate prefactor is interpreted in terms of activation entropy and domain-wall dynamics
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