596 research outputs found
Exchange bias with Fe substitution in LaMnO_3
The exchange bias (EB) in LaMn_{0.7}Fe_{0.3}O_3 is observed by the negative
shift and training effect of the hysteresis loops, while the sample was cooled
in external magnetic field. The analysis of cooling field dependence of EB
gives the size of the ferromagnetic (FM) cluster ~ 25 Angstrom, where the
magnetic anisotropy of FM cluster is found two order of magnitude higher than
the FM bulk manganites. We propose that the nanoscale FM clusters are embedded
in the glassy magnetic host with EB at the FM/glassy magnetic interface.Comment: 6 figure
Aggregatable Distributed Key Generation
In this paper, we introduce a distributed key generation (DKG) protocol with aggregatable and publicly-verifiable transcripts. Compared with prior publicly-verifiable approaches, our DKG reduces the size of the final transcript and the time to verify it from O(n2) to O(nlogn) , where n denotes the number of parties. As compared with prior non-publicly-verifiable approaches, our DKG leverages gossip rather than all-to-all communication to reduce verification and communication complexity. We also revisit existing DKG security definitions, which are quite strong, and propose new and natural relaxations. As a result, we can prove the security of our aggregatable DKG as well as that of several existing DKGs, including the popular Pedersen variant. We show that, under these new definitions, these existing DKGs can be used to yield secure threshold variants of popular cryptosystems such as El-Gamal encryption and BLS signatures. We also prove that our DKG can be securely combined with a new efficient verifiable unpredictable function (VUF), whose security we prove in the random oracle model. Finally, we experimentally evaluate our DKG and show that the per-party overheads scale linearly and are practical. For 64 parties, it takes 71 ms to share and 359 ms to verify the overall transcript, while for 8192 parties, it takes 8 s and 42.2 s respectively
A Fistful of Bitcoins: Characterizing Payments among Men with No Names
Bitcoin is a purely online virtual currency, unbacked by either physical commodities or sovereign obligation; instead, it relies on a combination of cryptographic protection and a peer-to-peer protocol for witnessing settlements. Consequently, Bitcoin has the unintuitive property that while the ownership of money is implicitly anonymous, its flow is globally visible. In this paper we explore this unique characteristic further, using heuristic clustering to group Bitcoin wallets based on evidence of shared authority, and then using re-identification attacks (i.e., empirical purchasing of goods and services) to classify the operators of those clusters. From this analysis, we consider the challenges for those seeking to use Bitcoin for criminal or fraudulent purposes at scale
Speech-language Pathology Services Delivered by Telehealth in a Rural Educational Setting: the School’s Perspective
Introduction: Access to speech-language pathology services for children in rural and remote communities is often limited. Telehealth is increasingly used to provide these services to schools, demonstrating high satisfaction with both service providers and recipients, but the requirements for successful program implementation are as yet unclear. We aimed to explore the implementation requirements for a telehealth speech-language pathology service for children from the perspective of a rural school. Methods: A qualitative approach, supplemented by program activity data, was used to understand the experiences and perceptions of the benefits, limitations, enablers and barriers of a telehealth speech-language pathology program delivered to a school servicing approximately 400 children in a small rural town in the state of Queensland, Australia. Thematic analysis was conducted of transcripts of individual semi-structured interviews with nine school teaching staff and field notes of informal discussions regarding 85 speech-language pathology telehealth sessions (n = 9 children) during program establishment and implementation. Results: The speech-language pathology telehealth service was acceptable to teaching staff at the rural school, who cited improved access, the suitability of the technology for child engagement, and perceived effectiveness. Implementation issues were highlighted as critical to program success and scalability, particularly staff workload, technological issues, communication processes, and sustainability. Conclusion: School-based speech-language pathology services delivered via telehealth were perceived as a suitable way of increasing access for children by rural school staff. Future implementations of telehealth speech-language pathology programs should prospectively consider workload implications and develop strategies to communicate with and involve school staff.
 
Exchange Field Induced Magnetoresistance in Colossal Magnetoresistance Manganites
The effect of an exchange field on electrical transport in thin films of
metallic ferromagnetic manganites has been investigated. The exchange field was
induced both by direct exchange coupling in a ferromagnet/antiferromagnet
multilayer and by indirect exchange interaction in a ferromagnet/paramagnet
superlattice. The electrical resistance of the manganite layers was found to be
determined by the absolute value of the vector sum of the effective exchange
field and the external magnetic field.Comment: 5 pages, 4 figure
Magnetization Vorticity and Exchange Bias Phenomena in Arrays of Small Asymmetric Magnetic Rings
Arrays of nanoscopic magnetic asymmetric rings, 150 nm in outer diameter, are
fabricated using the techniques of electron-beam lithography, angular
deposition and ion-beam etching. Magnetic measurements for cobalt asymmetric
rings at room temperature verifies previous reports of vortex magnetic state
formation of a desired circulation direction for the application of external
magnetic field along the asymmetry axis of the rings. However, the main theme
of this article is the observation of exchange bias phenomena when the ring
samples are cooled down to low temperature in the presence of a positive
magnetic field. Very interestingly, the observed exchange bias effect is
negative for along and perpendicular orientations of ring's asymmetry axis with
respect to the in-plane external magnetic field. This is in good quantitative
agreement with the random interface model proposed by Malozemoff et al. For the
application of inplane external magnetic field at 45 degree with respect to the
asymmetry axis, the exchange bias effect is positive. Unlike the exchange bias
effects in thin films, this is a very unusual observation indicating that
exchange bias phenomena of opposite natures can be manipulated by appropriate
combination of geometrical constraint and external magnetic field direction, in
addition to the interfacial interactions between ferromagnetic (FM) and
antiferromagnetic (AFM) layer.Comment: Asymmetric magnetic rings arrays; Exchange bias phenomen
Metastable Random Field Ising model with exchange enhancement: a simple model for Exchange Bias
We present a simple model that allows hysteresis loops with exchange bias to
be reproduced. The model is a modification of the T=0 random field Ising model
driven by an external field and with synchronous local relaxation dynamics. The
main novelty of the model is that a certain fraction f of the exchange
constants between neighbouring spins is enhanced to a very large value J_E. The
model allows the dependence of the exchange bias and other properties of the
hysteresis loops to be analyzed as a function of the parameters of the model:
the fraction f of enhanced bonds, the amount of the enhancement J_E and the
amount of disorder which is controlled by the width sigma of the Gaussian
distribution of the random fields.Comment: 8 pages, 11 figure
Exchange Bias and Vertical Shift in CoFe2O4 nanoparticles
Magnetic properties of core-shell cobalt ferrite nanoparticles 15 to 48nm
prepared by a sol-gel route have been studied. It is shown that the coercivity
follows non-monotonic size dependence varying as 1/d above the maximum (d is
the particle size). Field cooled magnetization exhibited both horizontal
(exchange bias) and vertical shifts. The exchange bias is understood as
originating at the interface between a surface region with structural and spin
disorder and a core ferrimagnetic region. The dependence of the exchange bias
and vertical shifts on the particle sizes and cooling fields are found to have
significant differences and the differences are explained in the light of
recent results which suggest that both weakly and strongly pinned spins are
present at the interface. It is suggested that the exchange bias is dominated
by the weakly pinned spins while the vertical shift is affected by the strongly
pinned ones.Comment: 2
Ferromagnetic Domain Distribution in Thin Films During Magnetization Reversal
We have shown that polarized neutron reflectometry can determine in a
model-free way not only the mean magnetization of a ferromagnetic thin film at
any point of a hysteresis cycle, but also the mean square dispersion of the
magnetization vectors of its lateral domains. This technique is applied to
elucidate the mechanism of the magnetization reversal of an exchange-biased
Co/CoO bilayer. The reversal process above the blocking temperature is governed
by uniaxial domain switching, while below the blocking temperature the reversal
of magnetization for the trained sample takes place with substantial domain
rotation
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