A new class of magnetically actuated pumps and valves for microfluidic applications

This is the final version of the article. Available from Springer Nature via the DOI in this record.We propose a new class of magnetically actuated pumps and valves that could be incorporated into microfluidic chips with no further external connections. The idea is to repurpose ferromagnetic low Reynolds number swimmers as devices capable of generating fluid flow, by restricting the swimmers’ translational degrees of freedom. We experimentally investigate the flow structure generated by a pinned swimmer in different scenarios, such as unrestricted flow around it as well as flow generated in straight, cross-shaped, Y-shaped and circular channels. This demonstrates the feasibility of incorporating the device into a channel and its capability of acting as a pump, valve and flow splitter. Different regimes could be selected by tuning the frequency and amplitude of the external magnetic field driving the swimmer, or by changing the channel orientation with respect to the field. This versatility endows the device with varied functionality which, together with the robust remote control and reproducibility, makes it a promising candidate for several applications.This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 665440. We also acknowledge support via the EPSRC Centre for Doctoral Training in Metamaterials (Grant No. EP/L015331/1)

Hippopede curves for modelling radial spin waves in an azimuthal graded magnonic landscape

This is the final version. Available from the American Physical Society via the DOI in this record. We propose a mathematical model for describing radially propagating spin waves emitted from the core region in a magnetic patch with n vertices in a magnetic vortex state. The azimuthal anisotropic propagation of surface spin waves (SSW) into the domain, and confined spin waves (or Winter's Magnons, WM) in domain walls increases the complexity of the magnonic landscape. In order to understand the spin wave propagation in these systems, we first use an approach based on geometrical curves called 'hippopedes', however it provides no insight into the underlying physics. Analytical models rely on generalized expressions from the dispersion relation of SSW with an arbitrary angle between magnetization M and wavenumber k. The derived algebraic expression for the azimuthal dispersion is found to be equivalent to that of the 'hippopede' curves. The fitting curves from the model yield a spin wave wavelength for any given azimuthal direction, number of patch vertices and excitation frequency, showing a connection with fundamental physics of exchange dominated surface spin waves. Analytical results show good agreement with micromagnetic simulations and can be easily extrapolated to any n-corner patch geometry.Engineering and Physical Sciences Research Council (EPSRC

Graded index confined spin waves in a mixed Bloch-Néel domain wall

This is the final version. Available from the American Physical Society via the DOI in this recordWe propose a mathematical model for describing propagating confined modes in domain walls of intermediate angle α ( 0 < α < π / 2 radians ) between domains. The model is obtained from the linearized Bloch equations of motion and under reasonable assumptions that can apply to the scenario of a thick (80 nm) magnetic patch, which simplifies the calculations without a high impact on the model accuracy. The model shows that there is a clear dependence of the local wave number of the confined spin wave on the local angle of domain magnetization with respect to the wall and on the excitation magnetic field frequency. From this model, we can define a local mode index in the wall as a function of such angle and excitation frequency. Therefore, the model can be applied to 1D propagating modes, although it also has physical implications for 2D scenarios where a domain wall merges with a saturated magnetic region. Micromagnetic simulations are in good agreement with the predictions of the model. Our model can also give insight on the effects that curved edge structures may have on the propagating characteristics of spin waves bounded in domain walls.Engineering and Physical Sciences Research Council (EPSRC

Magnetically controlled ferromagnetic swimmers

This is the final version of the article. Available from Springer Nature via the DOI in this record.Microscopic swimming devices hold promise for radically new applications in lab-on-a-chip and microfluidic technology, diagnostics and drug delivery etc. In this paper, we demonstrate the experimental verification of a new class of autonomous ferromagnetic swimming devices, actuated and controlled solely by an oscillating magnetic field. These devices are based on a pair of interacting ferromagnetic particles of different size and different anisotropic properties joined by an elastic link and actuated by an external time-dependent magnetic field. The net motion is generated through a combination of dipolar interparticle gradient forces, time-dependent torque and hydrodynamic coupling. We investigate the dynamic performance of a prototype (3.6 mm) of the ferromagnetic swimmer in fluids of different viscosity as a function of the external field parameters (frequency and amplitude) and demonstrate stable propulsion over a wide range of Reynolds numbers. We show that the direction of swimming has a dependence on both the frequency and amplitude of the applied external magnetic field, resulting in robust control over the speed and direction of propulsion. This paves the way to fabricating microscale devices for a variety of technological applications requiring reliable actuation and high degree of control.This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 665440. We also acknowledge support via the EPSRC Centre for Doctoral Training in Metamaterials (Grant No. EP/L015331/1)

Microfluidic devices powered by integrated elasto-magnetic pumps (article)

This is the final version. Available on open access from the Royal Society of Chemistry via the DOI in this recordThe dataset associated with this article is located in ORE at: https://doi.org/10.24378/exe.2863We show how an asymmetric elasto-magnetic system provides a novel integrated pumping solution for lab-on-a-chip and point of care devices. This monolithic pumping solution, inspired by Purcell's 3-link swimmer, is integrated within a simple microfluidic device, bypassing the requirement of external connections. We experimentally prove that this system can provide tuneable fluid flow with a flow rate of up to 600 μL h-1. This fluid flow is achieved by actuating the pump using a weak, uniform, uniaxial, oscillating magnetic field, with field amplitudes in the range of 3-6 mT. Crucially, the fluid flow can be reversed by adjusting the driving frequency. We experimentally prove that this device can successfully operate on fluids with a range of viscosities, where pumping at higher viscosity correlates with a decreasing optimal driving frequency. The fluid flow produced by this device is understood here by examining the non-reciprocal motion of the elasto-magnetic component. This device has the capability to replace external pumping systems with a simple, integrated, lab-on-a-chip component.Engineering and Physical Sciences Research Council (EPSRC)European Union Horizon 2020Medical Research Council (MRC)Royal SocietyWellcome Trus

Asymmetry of spin wave dispersions in a hexagonal magnonic crystal

PublishedJournal ArticleWe report a study of the dispersion of spin waves in a hexagonal array of interacting ferromagnetic nanodisks for two orthogonal orientations of the in-plane applied magnetic field, i.e., either parallel or perpendicular to the direction of first neighbour disks. The experimental data were modelled using the dynamical matrix method, and the results were interpreted in terms of the effective wave vector model. We have found that spin waves propagating in the two orthogonal directions exhibit marked asymmetry concerning the existence of maxima/minima in their dispersion curves and the sign of their group velocities. © 2013 AIP Publishing LLC.This work was supported by the European Community's Seventh Framework Programme (FP7/2007-2013) under Grant Agreement Nos. 228673 (MAGNONICS) and 233552 (DYNAMAG) and by MIUR-PRIN 2010-11 Project 2010ECA8P3 “DyNanoMag.” V.V.K. also acknowledges funding received from EPSRC of the UK under project EP/E055087/1

Time-domain imaging of curling modes in a confined magnetic vortex and a micromagnetic study exploring the role of spiral spin waves emitted by the core

This is the final version. Available from the American Physical Society via the DOI in this recordThe curling spin wave modes of a ferromagnetic vortex confined to a microscale disk have been directly imaged in response to a microwave field excitation using time-resolved scanning Kerr microscopy. Micromagnetic simulations have been used to explore the interaction of gyrotropic vortex core dynamics with the curling modes observed in the region of circulating in-plane magnetization. Hybridization of the fundamental gyrotropic mode with the degenerate, lowest frequency, azimuthal modes has previously been reported to lead to their splitting and counterpropagating motion, as we observe in our spectra and measured images. The curling nature of the modes can be ascribed to asymmetry in the static and dynamic magnetization across the disk thickness, but here we also present evidence that spiral spin waves emitted by the core can influence the spatial character of higher frequency curling modes for which hybridization is permitted only with gyrotropic modes of the same sense of azimuthal motion. While it is challenging to identify if such modes are truly hybridized from the mode dispersion in a confined disk, our simulations reveal that spiral spin waves from the core may act as mediators of the interaction between the core dynamics and azimuthal modes, enhancing the spiral nature of the curling mode. At higher frequency, modes with radial character only do not exhibit marked curling, but instead show evidence of interaction with spin waves generated at the edge of the disk. The measured spatiotemporal character of the observed curling modes is accurately reproduced by our simulations, which reveal the emission of propagating short-wavelength spiral spin waves from both core and edge regions of the disk. Our simulations suggest that the propagating modes are not inconsequential, but may play a role in the dynamic overlap required for hybridization of modes of the core and in-plane magnetized regions. These results are of importance to the fields of magnonics and spintronics that aim to utilize spin wave emission from highly localized, nanoscale regions of nonuniform magnetization, and their subsequent interaction with modes that may be supported nearby.Engineering and Physical Sciences Research Council (EPSRC

Dynamics of spiral spin waves in magnetic nanopatches: influence of thickness and shape

This is the final version. Available from American Physical Society via the DOI in this record. We explore the dynamics of spiral spin waves in permalloy nanoelements with variable aspect ratio of geometric dimensions, and their potential use as improved spin wave emitters with no or little biasing field required. Numerical results show that above a certain thickness, propagating spiral waves can be obtained in circular and square shaped elements in a flux closure state. VNA-FMR experiments on 20-nm (thin) and 80-nm (thick) samples confirm two type of spectra corresponding to different dispersions for thinner and thicker elements. We show that, for the thicker films, the vortex core region acts as a source of large amplitude spiral spin waves, which dominate over other modes. In case of the thinner elements, these modes are critically damped. For different shapes of the patch, we show that a rich collection of confined propagating modes can also be excited, modifying the final wave front and enriching the potential of the nanodot as a spin wave emitter. We give an explanation for the intense spiral modes from the perspective of a balance of dipolar and exchange energies in the sample.EPSRCSpanish MINECOComunidad de Madri

Exploring carbon nanotubes / BaTiO3 / Fe3O4 Nanocomposites as microwave absorbers

This is the final version of the article. Available from EM Academy via the link in this record.Open access journalWe report the modelling and characterization of microwave absorbing materials specially designed for 26–37 GHz frequency range (Ka-band). Composite materials based on carbon nanotubes/BaTiO3/Fe3O4 in a phosphate ceramic matrix were produced, and their electromagnetic response was investigated. Both theoretical and experimental results demonstrate that this material can absorb up to 100% of the power of an incident plane wave at a normal incidence angle. The physics underlying such absorption level is discussed in terms of refractive index of the material.This work was supported in part by FP7-PEOPLE-2013-IRSES-610875 NAmiceMC, FP7 Twinning Grant Inconet EaP 004. P. Kuzhir is thankful for support by Tomsk State University Competitiveness Improvement Program. Lab-STICC is UMR CNRS 6285

Complex permittivity and permeability of composite materials based on carbonyl iron powder over ultrawide frequency band

This is the final version. Available from the American Physical Society via the DOI in this record The complex electrical permittivity and magnetic permeability of composite materials made of a polymer binder filled with micron-scale carbonyl iron powder (CIP) are measured between 0.1 and 39 GHz. Permeability is measured in overlapping frequency subbands using two different techniques: a free-space method from 3 to 39 GHz and a coaxial impedance cell from 0.1 to 5 GHz. The dependence on filler concentration is studied for composites based on phosphated CIP R-100F-2. It is found that the static permittivity and permeability of the composites increase logarithmically with increasing percentage of CIP volume loading; this corresponds to Lichtenecker's law for a mixture of two components. It is demonstrated that by using the R-100F-2 type CIP it is possible to produce single-layer radar-absorbing materials with a relatively small thickness (less than 1.5 mm) and a deep and broad normal-incidence reflectivity minimum (less than -20 dB) from 10 to 30 GHz