The non-random walk of chiral magnetic charge carriers in artificial spin ice.

The flow of magnetic charge carriers (dubbed magnetic monopoles) through frustrated spin ice lattices, governed simply by Coulombic forces, represents a new direction in electromagnetism. Artificial spin ice nanoarrays realise this effect at room temperature, where the magnetic charge is carried by domain walls. Control of domain wall path is one important element of utilizing this new medium. By imaging the transit of domain walls across different connected 2D honeycomb structures we contribute an important aspect which will enable that control to be realized. Although apparently equivalent paths are presented to a domain wall as it approaches a Y-shaped vertex from a bar parallel to the field, we observe a stark non-random path distribution, which we attribute to the chirality of the magnetic charges. These observations are supported by detailed statistical modelling and micromagnetic simulations. The identification of chiral control to magnetic charge path selectivity invites analogy with spintronics.

Perk-dependent repression of miR-106b-25 cluster is required for ER stress-induced apoptosis.

Activation of the unfolded protein response sensor PKR-like endoplasmic reticulum kinase (Perk) attenuates endoplasmic reticulum (ER) stress levels. Conversantly, if the damage is too severe and ER function cannot be restored, this signaling branch triggers apoptosis. Bcl-2 homology 3-only family member Bim is essential for ER stress-induced apoptosis. However, the regulatory mechanisms controlling Bim activation under ER stress conditions are not well understood. Here, we show that downregulation of the miR-106b-25 cluster contributes to ER stress-induced apoptosis and the upregulation of Bim. Hypericin-mediated photo-oxidative ER damage induced Perk-dependent cell death and led to a significant decrease in the levels of miRNAs belonging to miR-106b-25 cluster in wild-type (WT) but not in Perk⁻/⁻ MEFs. Further, we show that expression of miR-106b-25 and Mcm-7 (host gene of miR-106b-25) is co-regulated through the transcription factors Atf4 (activating transcription factor 4) and Nrf2 (nuclear factor-erythroid-2-related factor 2). ER stress increased the activity of WT Bim 3'UTR (untranslated region) construct but not the miR-106b-25 recognition site-mutated Bim 3'UTR construct. Overexpression of miR-106b-25 cluster inhibits ER stress-induced cell death in WT but did not confer any further protection in Bim-knockdown cells. Further, we show downregulation in the levels of miR-106b-25 cluster in the symptomatic SOD1(G86R) transgenic mice. Our results suggest a molecular mechanism whereby repression of miR-106b-25 cluster has an important role in ER stress-mediated increase in Bim and apoptosis.

Dynamic oscillations of coupled domain walls.

In domain wall (DW) excitation experiments, nonlinearity (NL) intrinsic to the DW dynamics is often hard to distinguish from perturbation due to the confining potential or DW distortion. Here we numerically investigate the dynamic oscillations of magnetostatically coupled DWs: a system well understood in the quasistatic limit. NL is observed, even for a harmonic potential, due to the intrinsic DW motion. This behavior is principally dependent on terms normally associated with the DW canonical momentum and is in contrast with a NL restoring potential. This NL is not observable in quasistatic measurements, relatively insensitive to the confining potential, and may be tuned by the nanowire parameters. The shown NLs are present in any DW restoring potential and must be accounted for when probing DW potential landscapes.

Emerging chirality in artificial spin ice.

Artificial spin ice, made up of planar nanostructured arrays of simple ferromagnetic bars, is a playground for rich physics associated with the spin alignment of the bars and spin texture associated with the magnetic frustration at the bar vertices. The phase diagram is exotic, showing magnetic monopole-like defects and liquid and solid phases of spins arranged in loop states with predicted chiral order. We show that magnetotransport measurements in connected honeycomb structures yield the onset of an anomalous Hall signal at 50 kelvin. The temperature scale can be attributed to the long-range dipolar ice phase. The topological Hall signal arises because chiral loops form at the sample edges, indicating a generic route to exotic states via nanoarray edge structure.

Dynamic propagation and nucleation in domain wall nanowire devices.

The dynamic injection and propagation of domain walls (DWs) in technologically relevant geometries have been investigated. On short (~10 ns) timescales nucleation of a DW by a localized Oersted field is found to be well described using a Néel-Brown reversal mode. Using locally injected DWs, we test the propagation of DWs over long distances (~100 µm) in close proximity nanowires and beyond the Walker breakdown limit. In nanowires that act as true conduits to a DW, data can be successfully propagated without loss or inter-wire cross-talk. This is in contrast to poorly characterized systems where the DW is found to propagate asynchronously above the critical breakdown field.

Tunable remote pinning of domain walls in magnetic nanowires.

Domain wall (DW) pinning in ferromagnetic nanowires is in general a complex process. Distortions of the DW shape make quantitative agreement between modeling and experiment difficult. Here we demonstrate pinning using nanometer scale localized stray fields. This type of interaction gives well-characterized, tailorable potential landscapes that do not appreciably distort the DW. Our experimental results are in excellent quantitative agreement with an Arrhenius-Néel model of depinning--a result only possible when the modeled potential profile agrees fully with that experienced by the DW.

Fast domain wall motion in magnetic comb structures.

Modern fabrication technology has enabled the study of submicron ferromagnetic strips with a particularly simple domain structure, allowing single, well-defined domain walls to be isolated and characterized. However, these domain walls have complex field-driven dynamics. The wall velocity initially increases with field, but above a certain threshold the domain wall abruptly slows down, accompanied by periodic transformations of the domain wall structure. This behaviour is potentially detrimental to the speed and proper functioning of proposed domain-wall-based devices, and although methods for suppression of the breakdown have been demonstrated in simulations, a convincing experimental demonstration is lacking. Here, we show experimentally that a series of cross-shaped traps acts to prevent transformations of the domain wall structure and increase the domain wall velocity by a factor of four compared to the maximum velocity on a plain strip. Our results suggest a route to faster and more reliable domain wall devices for memory, logic and sensing.

Near-field interaction between domain walls in adjacent Permalloy nanowires.

The magnetostatic interaction between two oppositely charged transverse domain walls (TDWs) in adjacent Permalloy nanowires is experimentally demonstrated. The dependence of the pinning strength on wire separation is investigated for distances between 13 and 125 nm. The results can be described fully by considering the distribution of magnetic charge within rigid, isolated TDWs. Alternative DW internal structure cannot reproduce this observed dependence. Modeling suggests the TDW internal structure is not appreciably disturbed, and remains rigid although the pinning strength is significant.

Magnetization reversal in individual cobalt micro- and nanowires grown by focused-electron-beam-induced-deposition.

We systematically study individual micro- and nanometric polycrystalline cobalt wires grown by focused-electron-beam-induced-deposition. The deposits were grown in a range of aspect ratios varying from 1 up to 26. The minimum lateral dimension of the nanowires was 150 nm, for a thickness of 40 nm. Atomic force microscopy images show beam-current-dependent profiles, associated with different regimes of deposition. The magnetization reversal of individual nanowires is studied by means of the spatially resolved magneto-optical Kerr effect. Abrupt switching is observed, with a systematic dependence on the wire's dimensions. This dependence of the coercive field is understood in magnetostatic terms, and agrees well with previous results on cobalt wires grown with different techniques. The influence of compositional gradients along the structural profile on the magnetic reversal is studied by using micromagnetic simulations. This work demonstrates the feasibility of using this technique to fabricate highly pure magnetic nanostructures, and highlights the advantages and disadvantages of the technique with respect to more conventional ones.

Measuring domain wall fidelity lengths using a chirality filter.

The motion of transverse domain walls (DWs) in thin Permalloy nanowires has been studied by locally detecting the chirality of the moving DW, using a cross-shaped trap acting as a chirality filter. We find that structural changes of the DW occur over a characteristic minimum distance: the "DW fidelity length." The measured field dependence of the fidelity length is in good qualitative agreement with a 1D analytical model and with published results of numerical simulations and experiments. We also demonstrate extension of the fidelity length to meter length scales using a series of filters.

Age-related changes in performance on a visual-closure task.

A sample of 645 volunteer subjects, 50-79 years of age, took part for two consecutive years in a neuropsychological study of perception and memory. The sample was divided into three age groups (50-59, 60-69, and 70-79) in order to examine any age-related changes in performance. On a visual-closure task, similar to that of Gollin (1960), all age groups showed significant improvements in performance (savings) with repeated exposure to the drawings, both after a 15-min, and again after a 1-year, time interval. There were significant differences, however, between the different age groups on all aspects of the task, including number of errors made during initial perceptual performance, percentage of savings over both short- and long-time intervals, and number of items recalled. These findings are contrasted with those from studies of implicit and explicit aspects of verbal memory in normal young and old people, which have found no age-related differences in implicit memory abilities. Implications of these findings, as they relate to the neuropsychological assessment of memory in older people, are discussed briefly.