17,881 research outputs found
Micromagnetic Simulation of Nanoscale Films with Perpendicular Anisotropy
A model is studied for the theoretical description of nanoscale magnetic
films with high perpendicular anisotropy. In the model the magnetic film is
described in terms of single domain magnetic grains with Ising-like behavior,
interacting via exchange as well as via dipolar forces. Additionally, the model
contains an energy barrier and a coupling to an external magnetic field.
Disorder is taken into account in order to describe realistic domain and domain
wall structures. The influence of a finite temperature as well as the dynamics
can be modeled by a Monte Carlo simulation.
Many of the experimental findings can be investigated and at least partly
understood by the model introduced above. For thin films the magnetisation
reversal is driven by domain wall motion. The results for the field and
temperature dependence of the domain wall velocity suggest that for thin films
hysteresis can be described as a depinning transition of the domain walls
rounded by thermal activation for finite temperatures.Comment: Revtex, Postscript Figures, to be published in J. Appl.Phy
Thick-Film and LTCC Passive Components for High-Temperature Electronics
At this very moment an increasing interest in the field of high-temperature electronics is observed. This is a result of development in the area of wide-band semiconductors’ engineering but this also generates needs for passives with appropriate characteristics. This paper presents fabrication as well as electrical and stability properties of passive components (resistors, capacitors, inductors) made in thick-film or Low-Temperature Co-fired Ceramics (LTCC) technologies fulfilling demands of high-temperature electronics. Passives with standard dimensions usually are prepared by screen-printing whereas combination of standard screen-printing with photolithography or laser shaping are recommenced for fabrication of micropassives. Attainment of proper characteristics versus temperature as well as satisfactory long-term high-temperature stability of micropassives is more difficult than for structures with typical dimensions for thick-film and LTCC technologies because of increase of interfacial processes’ importance. However it is shown that proper selection of thick-film inks together with proper deposition method permit to prepare thick-film micropassives (microresistors, air-cored microinductors and interdigital microcapacitors) suitable for the temperature range between 150°C and 400°C
Multiscale likelihood analysis and complexity penalized estimation
We describe here a framework for a certain class of multiscale likelihood
factorizations wherein, in analogy to a wavelet decomposition of an L^2
function, a given likelihood function has an alternative representation as a
product of conditional densities reflecting information in both the data and
the parameter vector localized in position and scale. The framework is
developed as a set of sufficient conditions for the existence of such
factorizations, formulated in analogy to those underlying a standard
multiresolution analysis for wavelets, and hence can be viewed as a
multiresolution analysis for likelihoods. We then consider the use of these
factorizations in the task of nonparametric, complexity penalized likelihood
estimation. We study the risk properties of certain thresholding and
partitioning estimators, and demonstrate their adaptivity and near-optimality,
in a minimax sense over a broad range of function spaces, based on squared
Hellinger distance as a loss function. In particular, our results provide an
illustration of how properties of classical wavelet-based estimators can be
obtained in a single, unified framework that includes models for continuous,
count and categorical data types
Domain State Model for Exchange Bias
Monte Carlo simulations of a system consisting of a ferromagnetic layer
exchange coupled to a diluted antiferromagnetic layer described by a classical
spin model show a strong dependence of the exchange bias on the degree of
dilution in agreement with recent experimental observations on Co/CoO bilayers.
These simulations reveal that diluting the antiferromagnet leads to the
formation of domains in the volume of the antiferromagnet carrying a remanent
surplus magnetization which causes and controls exchange bias. To further
support this domain state model for exchange bias we study in the present paper
the dependence of the bias field on the thickness of the antiferromagnetic
layer. It is shown that the bias field strongly increases with increasing film
thickness and eventually goes over a maximum before it levels out for large
thicknesses. These findings are in full agreement with experiments.Comment: 8 pages latex, 3 postscript figure
Uniform susceptibility of classical antiferromagnets in one and two dimensions in a magnetic field
We simulated the field-dependent magnetization m(H,T) and the uniform
susceptibility \chi(H,T) of classical Heisenberg antiferromagnets in the chain
and square-lattice geometry using Monte Carlo methods. The results confirm the
singular behavior of \chi(H,T) at small T,H: \lim_{T \to 0}\lim_{H \to 0}
\chi(H,T)=1/(2J_0)(1-1/D) and \lim_{H \to 0}\lim_{T \to 0} \chi(H,T)=1/(2J_0),
where D=3 is the number of spin components, J_0=zJ, and z is the number of
nearest neighbors. A good agreement is achieved in a wide range of temperatures
T and magnetic fields H with the first-order 1/D expansion results [D. A.
Garanin, J. Stat. Phys. 83, 907 (1996)]Comment: 4 PR pages, 4 figures, submitted to PR
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