Giant magnetoimpedance in tri-layer structures of patterned magnetic amorphous ribbons

We have measured the giant magnetoimpedance (GMI) and giant magnetoresistance (GMR) of Metglas®/Cu/Metglas trilayer stacks of micropatterned amorphous magnetic Metglas 2714a ribbons and Cu foils. We obtain roomtemperature GMI ratios [Z(0)-Z(Hs)]/Z(Hs) up to 800 % and GMR ratios [R(0)- R(Hs)]/R(Hs) up to 2400 % at frequencies of 100 kHz-10 MHz, with Hs the magnetic field where impedance and resistance saturate towards their minimum values. These high values are a direct consequence of the large dynamic relative permeability of the magnetic ribbons and the chosen trilayer geometry. We analyze our results in the context of a theoretical model of L. V. Panina and K. Mohri [Sens. Actuators A 81, 71 (2000)]

Batch-Type Millimeter-Size Transformers for Miniaturized Power Applications

We have developed a novel batch-type technology for making three-dimensional (3-D) millimeter-sized transformers for ultrasmall low-power (0.1 to 1 Watt) applications. The technology uses the 3-D micropatterning of ferrite wafers by powder blasting to form the magnetic cores of the inductive devices, and combines these cores with electrical windings made by flex-foil printed circuit board technology. Microfabrication and assembly of the parts can be done in a batch process on a wafer/foil level, opening the way to further size reduction of the components. We have measured the inductive and resistive properties of our devices as a function of frequency and device geometry. The results clearly show the high potential of our technology for power applications in which small-size is important

High frequency magnetic behavior through the magnetoimpedance effect in CoFeB/(Ta, Ag, Cu) multilayered ferromagnetic thin films

We studied the dynamics of magnetization through an investigation of the magnetoimpedance effect in CoFeB/(Ta, Ag, Cu) multilayered thin films grown by magnetron sputtering. Impedance measurements were analyzed in terms of the mechanisms responsible for their variations at different frequency intervals and the magnetic and structural properties of the multilayers. Analysis of the mechanisms responsible for magnetoimpedance according to frequency and external magnetic field showed that for the CoFeB/Cu multilayer, ferromagnetic resonance (FMR) contributes significantly to the magnetoimpedance effect at frequencies close to 470 MHz. This frequency is low when compared to the results obtained for CoFeB/Ta and CoFeB/Ag multilayers and is a result of the anisotropy distribution and non-formation of regular bilayers in this sample. The MImax values occurred at different frequencies according to the used non-magnetic metal. Variations between 25% and 30% were seen for a localized frequency band, as in the case of CoFeB/Ta and CoFeB/Ag, as well as for a wide frequency range, in the case of CoFeB/Cu.Comment: 14 pages, 5 figure

The evolving SARS-CoV-2 epidemic in Africa: Insights from rapidly expanding genomic surveillance

INTRODUCTION Investment in Africa over the past year with regard to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) sequencing has led to a massive increase in the number of sequences, which, to date, exceeds 100,000 sequences generated to track the pandemic on the continent. These sequences have profoundly affected how public health officials in Africa have navigated the COVID-19 pandemic. RATIONALE We demonstrate how the first 100,000 SARS-CoV-2 sequences from Africa have helped monitor the epidemic on the continent, how genomic surveillance expanded over the course of the pandemic, and how we adapted our sequencing methods to deal with an evolving virus. Finally, we also examine how viral lineages have spread across the continent in a phylogeographic framework to gain insights into the underlying temporal and spatial transmission dynamics for several variants of concern (VOCs). RESULTS Our results indicate that the number of countries in Africa that can sequence the virus within their own borders is growing and that this is coupled with a shorter turnaround time from the time of sampling to sequence submission. Ongoing evolution necessitated the continual updating of primer sets, and, as a result, eight primer sets were designed in tandem with viral evolution and used to ensure effective sequencing of the virus. The pandemic unfolded through multiple waves of infection that were each driven by distinct genetic lineages, with B.1-like ancestral strains associated with the first pandemic wave of infections in 2020. Successive waves on the continent were fueled by different VOCs, with Alpha and Beta cocirculating in distinct spatial patterns during the second wave and Delta and Omicron affecting the whole continent during the third and fourth waves, respectively. Phylogeographic reconstruction points toward distinct differences in viral importation and exportation patterns associated with the Alpha, Beta, Delta, and Omicron variants and subvariants, when considering both Africa versus the rest of the world and viral dissemination within the continent. Our epidemiological and phylogenetic inferences therefore underscore the heterogeneous nature of the pandemic on the continent and highlight key insights and challenges, for instance, recognizing the limitations of low testing proportions. We also highlight the early warning capacity that genomic surveillance in Africa has had for the rest of the world with the detection of new lineages and variants, the most recent being the characterization of various Omicron subvariants. CONCLUSION Sustained investment for diagnostics and genomic surveillance in Africa is needed as the virus continues to evolve. This is important not only to help combat SARS-CoV-2 on the continent but also because it can be used as a platform to help address the many emerging and reemerging infectious disease threats in Africa. In particular, capacity building for local sequencing within countries or within the continent should be prioritized because this is generally associated with shorter turnaround times, providing the most benefit to local public health authorities tasked with pandemic response and mitigation and allowing for the fastest reaction to localized outbreaks. These investments are crucial for pandemic preparedness and response and will serve the health of the continent well into the 21st century

Giant Magneto-Impedance in multilayers of magnetic amorphous ribbons

We have patterned multilayers of amorphous magnetic foils of Metglas/sup /spl reg// 2714a (cobalt-based) and Cu foil into microstructures of different sizes. We have studied the GM effect in these structures, and we have characterized the behavior of the relative permeability as a function of frequency and magnetic field. We obtain GMI ratios [Z/sub max/-Z(B/sub sat/)]/Z(B/sub sat/) up to 800% and GMR ratios [R/sub max/-R(B/sub sat/)]/R(B/sub sat/) up to 2400% at frequencies of 100 kHz-10 MHz, with B/sub sat/ the magnetic field where impedance and resistance saturate (typically a few mT)

Giant magnetoimpedance of chemically thinned and polished magnetic amorphous ribbons

We have measured the giant magnetoimpedance (GMI) of microstructured Co-based Metglas (R) 2714a amorphous magnetic ribbons. Before micropatterning, the amorphous ribbons were chemically thinned or polished, to study the influence of sample thickness and surface state on the GMI effect. The magnetic domain structure of polished foils near saturation is observed using the bitter fluid technique. The well-defined geometry of the samples allows a quantitative comparison of the GMI effect with a simple skin depth model, from which we obtain the frequency-dependent permeability of the material. We have measured the GMI effect for magnetic fields parallel and transverse to the long axis of the sample. We compare the different measurement geometries and film preparation methods and, typically, find a room-temperature GMI ratio of about 35%-90% in fields of a few mT and at frequencies of 10-40 MHz