Magnetic skyrmions have the potential to provide solutions for low-power, high-density data storage and processing. One of the major challenges in developing skyrmion-based devices is the skyrmions’ magnetic stability in confined helimagnetic nanostructures. Through a systematic study of equilibrium states, using a full three-dimensional micromagnetic model including demagnetisation effects, we demonstrate that skyrmionic textures are the lowest energy states in helimagnetic thin film nanostructures at zero external magnetic field and in absence of magnetocrystalline anisotropy. We also report the regions of metastability for non-ground state equilibrium configurations. We show that bistable skyrmionic textures undergo hysteretic behaviour between two energetically equivalent skyrmionic states with different core orientation, even in absence of both magnetocrystalline and demagnetisation-based shape anisotropies, suggesting the existence of Dzyaloshinskii-Moriya-based shape anisotropy. Finally, we show that the skyrmionic texture core reversal dynamics is facilitated by the Bloch point occurrence and propagation.

Introduction: Sudden impact disasters often result in the displacement of large numbers of people. These movements can occur prior to events, due to early warning messages, or take place post-event due to damages to shelters and livelihoods as well as a result of long-term reconstruction efforts. Displaced populations are especially vulnerable and often in need of support. However, timely and accurate data on the numbers and destinations of displaced populations are extremely challenging to collect across temporal and spatial scales, especially in the aftermath of disasters. Mobile phone call detail records were shown to be a valid data source for estimates of population movements after the 2010 Haiti earthquake, but their potential to provide near real-time ongoing measurements of population displacements immediately after a natural disaster has not been demonstrated.Methods: A computational architecture and analytical capacity were rapidly deployed within nine days of the Nepal earthquake of 25th April 2015, to provide spatiotemporally detailed estimates of population displacements from call detail records based on movements of 12 million de-identified mobile phones users.Results: Analysis shows the evolution of population mobility patterns after the earthquake and the patterns of return to affected areas, at a high level of detail. Particularly notable is the movement of an estimated 390,000 people above normal from the Kathmandu valley after the earthquake, with most people moving to surrounding areas and the highly-populated areas in the central southern area of Nepal.Discussion: This analysis provides an unprecedented level of information about human movement after a natural disaster, provided within a very short timeframe after the earthquake occurred. The patterns revealed using this method are almost impossible to find through other methods, and are of great interest to humanitarian agencies.

We study domain-wall (DW) motion induced by spin waves (magnons) in the presence of the Dzyaloshinskii-Moriya interaction (DMI). The DMI exerts a torque on the DW when spin waves pass through the DW, and this torque represents a linear momentum exchange between the spin wave and the DW. Unlike angular momentum exchange between the DW and spin waves, linear momentum exchange leads to a rotation of the DW plane rather than a linear motion. In the presence of an effective easy plane anisotropy, this DMI induced linear momentum transfer mechanism is significantly more efficient than angular momentum transfer in moving the DW.

In confined helimagnetic nanostructures, skyrmionic states in the form of incomplete and isolated skyrmion states can emerge as the ground state in absence of both external magnetic field and magnetocrystalline anisotropy. In this work, we study the dynamic properties (resonance frequencies and corresponding eigenmodes) of skyrmionic states in thin film FeGe disk samples. We employ two different methods in finite-element based micromagnetic simulation: eigenvalue and ringdown method. The eigenvalue method allows us to identify all resonance frequencies and corresponding eigenmodes that can exist in the simulated system. However, using a particular experimentally feasible excitation can excite only a limited set of eigenmodes. Because of that, we perform ringdown simulations that resemble the experimental setup using both in-plane and out-of-plane excitations. In addition, we report the nonlinear dependence of resonance frequencies on the external magnetic bias field and disk sample diameter and discuss the possible reversal mode of skyrmionic states. We compare the power spectral densities of incomplete skyrmion and isolated skyrmion states and observe several key differences that can contribute to the experimental identification of the state present in the sample. We measure the FeGe Gilbert damping, and using its value we determine what eigenmodes can be expected to be observed in experiments. Finally, we show that neglecting the demagnetisation energy contribution or ignoring the magnetisation variation in the out-of-film direction -although not changing the eigenmode's magnetisation dynamics significantly -changes their resonance frequencies substantially. Apart from contributing to the understanding of skyrmionic states physics, this systematic work can be used as a guide for the experimental identification of skyrmionic states in confined helimagnetic nanostructures.

We demonstrate the use of the magnetic-field-dependence of highly spatially confined, GHz-frequency ferromagnetic resonances in a ferromagnetic nanostructure for the detection of adsorbed magnetic nanoparticles. This is achieved in a large area magnonic crystal consisting of a thin ferromagnetic film containing a periodic array of closely spaced, nano-scale anti-dots. Stray fields from nanoparticles within the anti-dots modify resonant dynamic magnetisation modes in the surrounding magnonic crystal, generating easily measurable resonance peak shifts. The shifts are comparable to the resonance linewidths for high anti-dot filling fractions with their signs and magnitudes dependent upon the modes' localisations (in agreement with micromagnetic simulation results). This is a highly encouraging result for the development of frequencybased nanoparticle detectors for high speed nano-scale biosensing.Magnetic biosensors, in which biological analytes are tagged with magnetic nanoparticles (MNPs), have excellent potential for solid-state point-of-care medical diagnostics 1-3 . The technique is intrinsically matrix-insesntive 1 , can compete with industry-standard immunoassays 4 and can be combined with magnetic separation methods 5 . The central element of a magnetic biosensor is a detector for the stray or 'fringing' magnetic fields generated by magnetised MNPs which are used to label, typically in-vitro, analytes of interest within a biological sample. Previously used sensors include SQuIDs 6 , Hall sensors 7 , ferromagnetic rings 8,9 and magneto-impedance devices 10 . However one of the most widely used methods is that employing magnetoresistive (MR) magnetic field sensors [1][2][3][4][5][11][12][13][14] which are typically fabricated with at least one

Recent studies have demonstrated that skyrmionic states can be the ground state in thin-film FeGe disk nanostructures in the absence of a stabilising applied magnetic field. In this work, we advance this understanding by investigating to what extent this stabilisation of skyrmionic structures through confinement exists in geometries that do not match the cylindrical symmetry of the skyrmion -such as as squares and triangles. Using simulation, we show that skyrmionic states can form the ground state for a range of system sizes in both triangular and square-shaped FeGe nanostructures of 10 nm thickness in the absence of an applied field. We further provide data to assist in the experimental verification of our prediction; to imitate an experiment where the system is saturated with a strong applied field before the field is removed, we compute the time evolution and show the final equilibrium configuration of magnetization fields, starting from a uniform alignment.

We demonstrate that chiral skyrmionic magnetization configurations can be found as the minimum energy state in B20 thin film materials with easy-plane magnetocrystalline anisotropy with an applied magnetic field perpendicular to the film plane. Our observations contradict results from prior analytical work, but are compatible with recent experimental investigations. The size of the observed skyrmions increases with the easy-plane magnetocrystalline anisotropy. We use a full micromagnetic model including demagnetization and a three-dimensional geometry to find local energy minimum (metastable) magnetization configurations using numerical damped time integration. We explore the phase space of the system and start simulations from a variety of initial magnetization configurations to present a systematic overview of anisotropy and magnetic field parameters for which skyrmions are metastable and global energy minimum (stable) states. Skyrmions are topological defects1 that can be observed in the magnetization configuration of materials that lack inversion symmetry, 2 either due to a noncentrosymmetric crystal lattice, 3,4 or at interfaces between different materials.5 This lack of inversion symmetry results in a chiral interaction known as the Dzyaloshinskii-Moriya (DM) interaction.3,4 The DM interaction results in a rich variety of chiral magnetization configurations, including helical, conical, and skyrmionic magnetization configurations. Skyrmionic configurations were predicted 6 and later observed in helimagnetic materials, 7-10 and materials with an interfacial DM interaction. 11-15Skyrmions demonstrate potential for applications in data storage and processing devices. Skyrmions have been observed with diameters of the order of atom spacings in mono-atomic Fe layers, 16 which is significantly smaller than the magnetic domains proposed for the racetrack memory design.17 This results in a greater storage density. The movement of skyrmions has also been demonstrated 18,19 using spin-polarized current densities of the order 10 6 Am −2 , which is orders of magnitude less than what is required to move magnetic domain walls.17,20 These observations demonstrate potential for skyrmion-based racetrack memory technology 21 and other data storage and processing devices. 22Certain material restrictions need to be overcome before skyrmions can be used in such technologies. While skyrmions can be stabilized, they are only stable in a limited region of the parameter space defined by an applied magnetic field and the temperature. This region is narrow in bulk materials, 7 larger in thin film materials, 9and further stabilized in laterally confined geometries 23 and materials with pinning defects. 24 Analytical analysis of helimagnetic thin film material models find that skyrmion lattice states are ground states in helimagnetic thin films with an applied magnetic field only in systems with easy-axis magnetocrystalline anisotropy, 2,25where the easy axis and the applied field are perpendicular to the plane of the film. However, si...

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