50 research outputs found
Directed network modules
A search technique locating network modules, i.e., internally densely
connected groups of nodes in directed networks is introduced by extending the
Clique Percolation Method originally proposed for undirected networks. After
giving a suitable definition for directed modules we investigate their
percolation transition in the Erdos-Renyi graph both analytically and
numerically. We also analyse four real-world directed networks, including
Google's own webpages, an email network, a word association graph and the
transcriptional regulatory network of the yeast Saccharomyces cerevisiae. The
obtained directed modules are validated by additional information available for
the nodes. We find that directed modules of real-world graphs inherently
overlap and the investigated networks can be classified into two major groups
in terms of the overlaps between the modules. Accordingly, in the
word-association network and among Google's webpages the overlaps are likely to
contain in-hubs, whereas the modules in the email and transcriptional
regulatory networks tend to overlap via out-hubs.Comment: 21 pages, 10 figures, version 2: added two paragaph
On the homomorphism order of labeled posets
Partially ordered sets labeled with k labels (k-posets) and their
homomorphisms are examined. We give a representation of directed graphs by
k-posets; this provides a new proof of the universality of the homomorphism
order of k-posets. This universal order is a distributive lattice. We
investigate some other properties, namely the infinite distributivity, the
computation of infinite suprema and infima, and the complexity of certain
decision problems involving the homomorphism order of k-posets. Sublattices are
also examined.Comment: 14 page
Nanoscale mechanics of antiferromagnetic domain walls
Antiferromagnets offer remarkable promise for future spintronics devices,
where antiferromagnetic order is exploited to encode information. The control
and understanding of antiferromagnetic domain walls (DWs) - the interfaces
between domains with differing order parameter orientations - is a key
ingredient for advancing such antiferromagnetic spintronics technologies.
However, studies of the intrinsic mechanics of individual antiferromagnetic DWs
remain elusive since they require sufficiently pure materials and suitable
experimental approaches to address DWs on the nanoscale. Here we nucleate
isolated, 180{\deg} DWs in a single-crystal of CrO, a prototypical
collinear magnetoelectric antiferromagnet, and study their interaction with
topographic features fabricated on the sample. We demonstrate DW manipulation
through the resulting, engineered energy landscape and show that the observed
interaction is governed by the DW's elastic properties. Our results advance the
understanding of DW mechanics in antiferromagnets and suggest a novel,
topographically defined memory architecture based on antiferromagnetic DWs.Comment: 3 pages, 3 figures plus Supplementary Material. Questions and
comments are welcom
Uncovering the overlapping community structure of complex networks in nature and society
Many complex systems in nature and society can be described in terms of
networks capturing the intricate web of connections among the units they are
made of. A key question is how to interpret the global organization of such
networks as the coexistence of their structural subunits (communities)
associated with more highly interconnected parts. Identifying these a priori
unknown building blocks (such as functionally related proteins, industrial
sectors and groups of people) is crucial to the understanding of the structural
and functional properties of networks. The existing deterministic methods used
for large networks find separated communities, whereas most of the actual
networks are made of highly overlapping cohesive groups of nodes. Here we
introduce an approach to analysing the main statistical features of the
interwoven sets of overlapping communities that makes a step towards uncovering
the modular structure of complex systems. After defining a set of new
characteristic quantities for the statistics of communities, we apply an
efficient technique for exploring overlapping communities on a large scale. We
find that overlaps are significant, and the distributions we introduce reveal
universal features of networks. Our studies of collaboration, word-association
and protein interaction graphs show that the web of communities has non-trivial
correlations and specific scaling properties.Comment: The free academic research software, CFinder, used for the
publication is available at the website of the publication:
http://angel.elte.hu/clusterin
Nanomagnetism of magnetoelectric granular thin-film antiferromagnets
Antiferromagnets have recently emerged as attractive platforms for
spintronics applications, offering fundamentally new functionalities compared
to their ferromagnetic counterparts. While nanoscale thin film materials are
key to the development of future antiferromagnetic spintronics technologies,
experimental tools to explore such films on the nanoscale are still sparse.
Here, we offer a solution to this technological bottleneck, by addressing the
ubiquitous surface magnetisation of magnetoelectic antiferromagnets in a
granular thin film sample on the nanoscale using single-spin magnetometry in
combination with spin-sensitive transport experiments. Specifically, we
quantitatively image the evolution of individual nanoscale antiferromagnetic
domains in 200-nm thin-films of CrO in real space and across the
paramagnet-to-antiferromagnet phase transition. These experiments allow us to
discern key properties of the CrO thin film, including the mechanism of
domain formation and the strength of exchange coupling between individual
grains comprising the film. Our work offers novel insights into CrO's
magnetic ordering mechanism and establishes single spin magnetometry as a
novel, widely applicable tool for nanoscale addressing of antiferromagnetic
thin films.Comment: 22 pages, 7 figure
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Nanomagnetism of Magnetoelectric Granular Thin-Film Antiferromagnets
Antiferromagnets have recently emerged as attractive platforms for spintronics applications, offering fundamentally new functionalities compared with their ferromagnetic counterparts. Whereas nanoscale thin-film materials are key to the development of future antiferromagnetic spintronic technologies, existing experimental tools tend to suffer from low resolution or expensive and complex equipment requirements. We offer a simple, high-resolution alternative by addressing the ubiquitous surface magnetization of magnetoelectric antiferromagnets in a granular thin-film sample on the nanoscale using single-spin magnetometry in combination with spin-sensitive transport experiments. Specifically, we quantitatively image the evolution of individual nanoscale antiferromagnetic domains in 200 nm thin films of Cr 2 O 3 in real space and across the paramagnet-to-antiferromagnet phase transition, finding an average domain size of 230 nm, several times larger than the average grain size in the film. These experiments allow us to discern key properties of the Cr 2 O 3 thin film, including the boundary magnetic moment density, the variation of critical temperature throughout the film, the mechanism of domain formation, and the strength of exchange coupling between individual grains comprising the film. Our work offers novel insights into the magnetic ordering mechanism of Cr 2 O 3 and firmly establishes single-spin magnetometry as a versatile and widely applicable tool for addressing antiferromagnetic thin films on the nanoscale. © 2019 American Chemical Society
Flexomagnetism and vertically graded NĂ©el temperature of antiferromagnetic Cr2O3 thin films
Antiferromagnetic insulators are a prospective materials platform for magnonics, spin superfluidity, THz spintronics, and non-volatile data storage. A magnetomechanical coupling in antiferromagnets offers vast advantages in the control and manipulation of the primary order parameter yet remains largely unexplored. Here, we discover a new member in the family of flexoeffects in thin films of Cr2O3. We demonstrate that a gradient of mechanical strain can impact the magnetic phase transition resulting in the distribution of the NĂ©el temperature along the thickness of a 50-nm-thick film. The inhomogeneous reduction of the antiferromagnetic order parameter induces a flexomagnetic coefficient of about 15 ÎŒB nmâ2. The antiferromagnetic ordering in the inhomogeneously strained films can persist up to 100 °C, rendering Cr2O3 relevant for industrial electronics applications. Strain gradient in Cr2O3 thin films enables fundamental research on magnetomechanics and thermodynamics of antiferromagnetic solitons, spin waves and artificial spin ice systems in magnetic materials with continuously graded parameters
Spin transport and spin torque in antiferromagnetic devices
Ferromagnets are key materials for sensing and memory applications. In contrast, antiferromagnets which represent the more common form of magnetically ordered materials, have found less practical application beyond their use for establishing reference magnetic orientations via exchange bias. This might change in the future due to the recent progress in materials research and discoveries of antiferromagnetic spintronic phenomena suitable for device applications. Experimental demonstration of the electrical switching and detection of the NĂ©el order open a route towards memory devices based on antiferromagnets. Apart from the radiation and magnetic-field hardness, memory cells fabricated from antiferromagnets can be inherently multilevel, which could be used for neuromorphic computing. Switching speeds attainable in antiferromagnets far exceed those of ferromagnetic and semiconductor memory technologies. Here we review the recent progress in electronic spin-transport and spin-torque phenomena in antiferromagnets that are dominantly of the relativistic quantum mechanical origin. We discuss their utility in pure antiferromagnetic or hybrid ferromagnetic/antiferromagnetic memory devices