268 research outputs found
Surface Aided Polarization Reversal In Small Ferroelectric Particles.
Polarization reversal in ferroelectric particles driven by a pulsed electric field is examined theoretically using Landau-Devonshire-Khalatnikov theory. A significant reduction in reversal times is shown to be possible if certain surface properties and size criteria are met. The surface properties are also shown to control the magnitude of the applied field needed for irreversible switching. An interesting signature of surface effects is found in the switching current. The theory predicts that the switching current for small ferroelectric particles can exhibit double peaks as a function of time. The size and relative times of the peaks provide specific information on the
magnitude and rate of surface reversal dynamics
Competing anisotropies in exchange biased nano-structured thin films
The magnetic anisotropies of a patterned, exchange biased Fe50 Mn50 /Ni80 Fe20 system are studied using ferromagnetic resonance, supplemented by Brillouin light scattering experiments and Kerr magnetometry. The exchange biased bilayer is partially etched into an antidot geometry so that the system approximates a Ni80 Fe20 layer in contact with antidot structured Fe50 Mn50 . Brillouin light scattering measurements of the spin wave frequency dependence on the wave vector reveal a magnonic band gap as expected for a periodic modulation of the
magnetic properties. Analysis of the ferromagnetic resonance spectra reveals eightfold and fourfold contributions to the magnetic anisotropy. Additionally, the antidot patterning decreases the magnitude of the exchange bias and modifies strongly its angular dependence. Softening of all resonance modes is most pronounced for the applied magnetic field aligned within 10◦ of the antidot axis, in the direction of the bias. Given the degree to which one can tailor the ground state, the resulting asymmetry at low frequencies could make this an interesting candidate for applications such as selective/directional microwave filtering and multistate magnetic logic
On thermalization of magnetic nano-arrays at fabrication
We propose a model to predict and control the statistical ensemble of
magnetic degrees of freedom in Artificial Spin Ice (ASI) during thermalized
adiabatic growth. We predict that as-grown arrays are controlled by the
temperature at fabrication and by their lattice constant, and that they can be
described by an effective temperature. If the geometry is conducive to a phase
transition, then the lowest temperature phase is accessed in arrays of lattice
constant smaller than a critical value, which depends on the temperature at
deposition. Alternatively, for arrays of equal lattice constant, there is a
temperature threshold at deposition and the lowest temperature phase is
accessed for fabrication temperatures {\it larger rather than smaller} than
this temperature threshold. Finally we show how to define and control the
effective temperature of the as-grown array and how to measure critical
exponents directly. We discuss the role of kinetics at the critical point, and
applications to experiments, in particular to as-grown thermalized square ASI,
and to magnetic monopole crystallization in as-grown honeycomb ASI.Comment: 14 pages, 2 figures. A theoretical approach to experimental results
reported in: Morgan J P, Stein A, Langridge S and Marrows C (2010) Nature
Physics 7 7
Spatial and temporal dynamics at an actively silicifying hydrothermal system
Steep Cone Geyser is a unique geothermal feature in Yellowstone National Park (YNP), Wyoming, actively gushing silicon-rich fluids along outflow channels possessing living and actively silicifying microbial biomats. To assess the geomicrobial dynamics occurring temporally and spatially at Steep Cone, samples were collected at discrete locations along one of Steep Cone’s outflow channels for both microbial community composition and aqueous geochemistry analysis during field campaigns in 2010, 2018, 2019, and 2020. Geochemical analysis characterized Steep Cone as an oligotrophic, surface boiling, silicious, alkaline-chloride thermal feature with consistent dissolved inorganic carbon and total sulfur concentrations down the outflow channel ranging from 4.59 ± 0.11 to 4.26 ± 0.07 mM and 189.7 ± 7.2 to 204.7 ± 3.55 μM, respectively. Furthermore, geochemistry remained relatively stable temporally with consistently detectable analytes displaying a relative standard deviation <32%. A thermal gradient decrease of ~55°C was observed from the sampled hydrothermal source to the end of the sampled outflow transect (90.34°C ± 3.38 to 35.06°C ± 7.24). The thermal gradient led to temperature-driven divergence and stratification of the microbial community along the outflow channel. The hyperthermophile Thermocrinis dominates the hydrothermal source biofilm community, and the thermophiles Meiothermus and Leptococcus dominate along the outflow before finally giving way to more diverse and even microbial communities at the end of the transect. Beyond the hydrothermal source, phototrophic taxa such as Leptococcus, Chloroflexus, and Chloracidobacterium act as primary producers for the system, supporting heterotrophic growth of taxa such as Raineya, Tepidimonas, and Meiothermus. Community dynamics illustrate large changes yearly driven by abundance shifts of the dominant taxa in the system. Results indicate Steep Cone possesses dynamic outflow microbial communities despite stable geochemistry. These findings improve our understanding of thermal geomicrobiological dynamics and inform how we can interpret the silicified rock record
Topology by Design in Magnetic nano-Materials: Artificial Spin Ice
Artificial Spin Ices are two dimensional arrays of magnetic, interacting
nano-structures whose geometry can be chosen at will, and whose elementary
degrees of freedom can be characterized directly. They were introduced at first
to study frustration in a controllable setting, to mimic the behavior of spin
ice rare earth pyrochlores, but at more useful temperature and field ranges and
with direct characterization, and to provide practical implementation to
celebrated, exactly solvable models of statistical mechanics previously devised
to gain an understanding of degenerate ensembles with residual entropy. With
the evolution of nano--fabrication and of experimental protocols it is now
possible to characterize the material in real-time, real-space, and to realize
virtually any geometry, for direct control over the collective dynamics. This
has recently opened a path toward the deliberate design of novel, exotic
states, not found in natural materials, and often characterized by topological
properties. Without any pretense of exhaustiveness, we will provide an
introduction to the material, the early works, and then, by reporting on more
recent results, we will proceed to describe the new direction, which includes
the design of desired topological states and their implications to kinetics.Comment: 29 pages, 13 figures, 116 references, Book Chapte
Exchange bias effect in alloys and compounds
The phenomenology of exchange bias effects observed in structurally
single-phase alloys and compounds but composed of a variety of coexisting
magnetic phases such as ferromagnetic, antiferromagnetic, ferrimagnetic,
spin-glass, cluster-glass and disordered magnetic states are reviewed. The
investigations on exchange bias effects are discussed in diverse types of
alloys and compounds where qualitative and quantitative aspects of magnetism
are focused based on macroscopic experimental tools such as magnetization and
magnetoresistance measurements. Here, we focus on improvement of fundamental
issues of the exchange bias effects rather than on their technological
importance
Spatial Fluctuations of Loose Spin Coupling in CuMn/Co Multilayers
A detailed investigation of magnetic impurity-mediated interlayer exchange coupling observed in Cu 0.94 Mn 0.06 / Co multilayers using polarized neutron reflectometry and magnetic x-ray techniques is reported. Excellent descriptions of temperature and magnetic field dependent biquadratic coupling are obtained using a variant of the loose spin model that takes into account the distribution of the impurity Mn ions in three dimensions. Positional disorder of the magnetic impurities is shown to enhance biquadratic coupling via a new contribution J fluct 2 , leading to a temperature dependent canting of magnetic domains in the multilayer. These results provide measurable effects on RKKY coupling associated with the distribution of impurities within planes parallel to the interfaces
Simultaneous CXCL12 and ESR1 CpG island hypermethylation correlates with poor prognosis in sporadic breast cancer
<p>Abstract</p> <p>Background</p> <p>CXCL12 is a chemokine that is constitutively expressed in many organs and tissues. <it>CXCL12 </it>promoter hypermethylation has been detected in primary breast tumours and contributes to their metastatic potential. It has been shown that the oestrogen receptor α (<it>ESR1</it>) gene can also be silenced by DNA methylation. In this study, we used methylation-specific PCR (MSP) to analyse the methylation status in two regions of the <it>CXCL12 </it>promoter and <it>ESR1 </it>in tumour cell lines and in primary breast tumour samples, and correlated our results with clinicopathological data.</p> <p>Methods</p> <p>First, we analysed <it>CXCL12 </it>expression in breast tumour cell lines by RT-PCR. We also used 5-aza-2'-deoxycytidine (5-aza-CdR) treatment and DNA bisulphite sequencing to study the promoter methylation for a specific region of <it>CXCL12 </it>in breast tumour cell lines. We evaluated <it>CXCL12 </it>and <it>ESR1 </it>methylation in primary tumour samples by methylation-specific PCR (MSP). Finally, promoter hypermethylation of these genes was analysed using Fisher's exact test and correlated with clinicopathological data using the Chi square test, Kaplan-Meier survival analysis and Cox regression analysis.</p> <p>Results</p> <p><it>CXCL12 </it>promoter hypermethylation in the first region (island 2) and second region (island 4) was correlated with lack of expression of the gene in tumour cell lines. In the primary tumours, island 2 was hypermethylated in 14.5% of the samples and island 4 was hypermethylated in 54% of the samples. The <it>ESR1 </it>promoter was hypermethylated in 41% of breast tumour samples. In addition, the levels of ERα protein expression diminished with increased frequency of <it>ESR1 </it>methylation (p < 0.0001). This study also demonstrated that <it>CXCL12 </it>island 4 and <it>ESR1 </it>methylation occur simultaneously at a high frequency (p = 0.0220).</p> <p>Conclusions</p> <p>This is the first study showing a simultaneous involvement of epigenetic regulation for both <it>CXCL12 </it>and <it>ESR1 </it>genes in Brazilian women. The methylation status of both genes was significantly correlated with histologically advanced disease, the presence of metastases and death. Therefore, the methylation pattern of these genes could be used as a molecular marker for the prediction of breast cancer outcome.</p
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