530 research outputs found
Busy Beaver Scores and Alphabet Size
We investigate the Busy Beaver Game introduced by Rado (1962) generalized to
non-binary alphabets. Harland (2016) conjectured that activity (number of
steps) and productivity (number of non-blank symbols) of candidate machines
grow as the alphabet size increases. We prove this conjecture for any alphabet
size under the condition that the number of states is sufficiently large. For
the measure activity we show that increasing the alphabet size from two to
three allows an increase. By a classical construction it is even possible to
obtain a two-state machine increasing activity and productivity of any machine
if we allow an alphabet size depending on the number of states of the original
machine. We also show that an increase of the alphabet by a factor of three
admits an increase of activity
Observation of thickness dependence of magnetic surface anisotropy in ultrathin amorphous films.
Copyright © 1990 The American Physical SocietyFerromagnetic resonance (FMR) and SQUID magnetometry measurements have been made on multilayers of amorphous Fe70B30/Ag. The dependence of the magnetic surface anisotropy constant Ks on the magnetic layer thickness 2L has been determined in the range 1.6 Å16.5 Å, but decreases monotonically towards zero as 2L decreases from 16.5 Å towards zero. The FMR results can be well described by a theory developed for ultrathin amorphous ferromagnetic layers
The case for interoperability in global research
”This Discussion Paper, focussing on defining
interoperability in a global research sense, is the first of two that will capture the main threads of discussion from a recent symposium hosted jointly by FEAST and The University of Queensland (UQ) on Enhancing
interoperability in the emerging global research order." - page 2Australian National Universit
Interferometric measurement of an axi-symmetric density field
We have used Fourier transform techniques and an Abel deconvolution to analyse a finite-fringe inter- ferogram produced by an axisymmetric shock wave flow, to produce a density map that can be used for the validation of a numerical model. The Abel deconvolution method enables the use of a basis that is particularly suitable for modeling phase maps produced by shock wave flows. A steady flow problem is studied, and compared with a numerical simulation. Good agreement between theoretical and experimental results are obtained
Stability of Bose Einstein condensates of hot magnons in YIG
We investigate the stability of the recently discovered room temperature
Bose-Einstein condensate (BEC) of magnons in Ytrrium Iron Garnet (YIG) films.
We show that magnon-magnon interactions depend strongly on the external field
orientation, and that the BEC in current experiments is actually metastable -
it only survives because of finite size effects, and because the BEC density is
very low. On the other hand a strong field applied perpendicular to the sample
plane leads to a repulsive magnon-magnon interaction; we predict that a
high-density magnon BEC can then be formed in this perpendicular field
geometry.Comment: Submitted to Physical Review Letter
Revealing the origin of the vertical hysteresis loop shifts in an exchange biased Co/YMnO bilayer
We have investigated exchange bias effects in bilayers composed by the
antiferromagnetic o-YMnO and ferromagnetic Co thin film by means of SQUID
magnetometry, magnetoresistance, anisotropic magnetoresistance and planar Hall
effect. The magnetization and magnetotransport properties show pronounced
asymmetries in the field and magnetization axes of the field hysteresis loops.
Both exchange bias parameters, the exchange bias field as well as
the magnetization shift , vanish around the N\'eel temperature K. We show that the magnetization shift is also measured by
a shift in the anisotropic magnetoresistance and planar Hall resistance having
those a similar temperature dependence as the one obtained from magnetization
measurements. Because the o-YMnO film is highly insulating, our results
demonstrate that the shift originates at the interface within the
ferromagnetic Co layer. To show that the main results obtained are general and
not because of some special characteristics of the o-YMO layer, similar
measurements were done in Co/CoO micro-wires. The transport and magnetization
characterization of the micro-wires supports the main conclusion that these
effects are related to the response of the ferromagnetic Co layer at the
interface.Comment: 16 Figures, in press at J. Phys.: Condensed Matter 201
Towards Autopoietic Computing
A key challenge in modern computing is to develop systems that address
complex, dynamic problems in a scalable and efficient way, because the
increasing complexity of software makes designing and maintaining efficient and
flexible systems increasingly difficult. Biological systems are thought to
possess robust, scalable processing paradigms that can automatically manage
complex, dynamic problem spaces, possessing several properties that may be
useful in computer systems. The biological properties of self-organisation,
self-replication, self-management, and scalability are addressed in an
interesting way by autopoiesis, a descriptive theory of the cell founded on the
concept of a system's circular organisation to define its boundary with its
environment. In this paper, therefore, we review the main concepts of
autopoiesis and then discuss how they could be related to fundamental concepts
and theories of computation. The paper is conceptual in nature and the emphasis
is on the review of other people's work in this area as part of a longer-term
strategy to develop a formal theory of autopoietic computing.Comment: 10 Pages, 3 figure
Effect of spacer material on the magnetic surface anisotropy in ultrathin Fe70B30 multilayer films
It has been found recently that the magnetic surface anisotropy Ks in Fe70B30/Ag multilayer films decreases monotonically with magnetic layer thickness (2L) for 2L<16.5 Å. In order to determine possible effects of the spacer material on the surface anisotropy in the aforementioned system, Ag has been replaced with Al2O3 and ferromagnetic resonance (FMR) measurements have been made on these films. These Fe70B30/Al2O3 films were fabricated by magnetron sputtering and were characterized by X-ray-diffraction and vibrating sample magnetometer (VSM) measurements in addition to FMR. In the region where Ks depends upon 2L, the data is insufficient to confirm the thickness dependence of Ks that was observed in Fe70B30/Ag, while in the region where Ks is independent of 2L, the values of Ks deduced for Fe70B30/Ag and Fe70B30/Al2O3 are in good agreement. The latter is particularly interesting in light of the enormous difference in conductivity between Ag and Al2O3
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