360 research outputs found
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
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
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
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
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
Anomalous behavior of spin wave resonances in Ga_{1-x}Mn_{x}As thin films
We report ferromagnetic and spin wave resonance absorption measurements on
high quality epitaxially grown Ga_{1-x}Mn_{x}As thin films. We find that these
films exhibit robust ferromagnetic long-range order, based on the fact that up
to seven resonances are detected at low temperatures, and the resonance
structure survives to temperatures close to the ferromagnetic transition. On
the other hand, we observe a spin wave dispersion which is linear in mode
number, in qualitative contrast with the quadratic dispersion expected for
homogeneous samples. We perform a detailed numerical analysis of the
experimental data and provide analytical calculations to demonstrate that such
a linear dispersion is incompatible with uniform magnetic parameters. Our
theoretical analysis of the ferromagnetic resonance data, combined with the
knowledge that strain-induced anisotropy is definitely present in these films,
suggests that a spatially dependent magnetic anisotropy is the most likely
reason behind the anomalous behavior observed.Comment: 9 pages, including 6 figure
Tverberg-type theorems for intersecting by rays
In this paper we consider some results on intersection between rays and a
given family of convex, compact sets. These results are similar to the center
point theorem, and Tverberg's theorem on partitions of a point set
Quantum spin fluctuations in the dipolar Heisenberg-like rare earth pyrochlores
The magnetic pyrochlore oxide materials of general chemical formula R2Ti2O7
and R2Sn2O7 (R = rare earth) display a host of interesting physical behaviours
depending on the flavour of rare earth ion. These properties depend on the
value of the total magnetic moment, the crystal field interactions at each rare
earth site and the complex interplay between magnetic exchange and long-range
dipole-dipole interactions. This work focuses on the low temperature physics of
the dipolar isotropic frustrated antiferromagnetic pyrochlore materials.
Candidate magnetic ground states are numerically determined at zero temperature
and the role of quantum spin fluctuations around these states are studied using
a Holstein-Primakoff spin wave expansion to order 1/S. The results indicate the
strong stability of the proposed classical ground states against quantum
fluctuations. The inclusion of long range dipole interactions causes a
restoration of symmetry and a suppression of the observed anisotropy gap
leading to an increase in quantum fluctuations in the ground state when
compared to a model with truncated dipole interactions. The system retains most
of its classical character and there is little deviation from the fully ordered
moment at zero temperature.Comment: Latex2e, 18 pages, 4 figures, IOP forma
Spin texture on top of vortex avalanches in Nb/Al_2O_3/Co thin film heterostructures
We report on magneto-optical imaging, magnetization, Hall effect and
magneto-resistance experiments in Nb/Al_2O_3/Co thin film heterostructures. The
magnetic field is applied perpendicularly to the plane of the film and gives
rise to abrupt flux penetration of dendritic form. A magnetization texture is
imprinted in the Co layer in perfect coincidence with these ramifications. The
spin domains that mimic the vortex dendrites are stable upon the field removal.
Moreover, the imprinted spin structure remains visible up to room temperature.
Complementary magnetization, Hall effect and magneto-resistance experiments
were performed in a similar sample where electrical contacts were placed on the
Co layer. In the region of the field - temperature diagram where flux
instabilities are known to occur in Nb films, irregular jumps are observed in
the magnetic hysteresis and large amplitude noise is detected in the
magneto-resistance and Hall resistivity data when measured as a function of the
field.Comment: 13 pages, 8 figure
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