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.

Magnetic skyrmions are hailed as a potential technology for data storage and other data processing devices. However, their stability against thermal fluctuations is an open question that must be answered before skyrmion-based devices can be designed. In this work, we study paths in the energy landscape via which the transition between the skyrmion and the uniform state can occur in interfacial Dzyaloshinskii-Moriya finite-sized systems. We find three mechanisms the system can take in the process of skyrmion nucleation or destruction and identify that the transition facilitated by the boundary has a significantly lower energy barrier than the other energy paths. This clearly demonstrates the lack of the skyrmion topological protection in finite-sized magnetic systems. Overall, the energy barriers of the system under investigation are too small for storage applications at room temperature, but research into device materials, geometry and design may be able to address this.

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 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...

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.

Using rapid thermal processing (RTP) we recently demonstrated the production of high quality well ordered barium ferrite films in times much shorter than those required by a conventional annealing process. Influence over the magnetic and structural properties developed in annealed samples was also achieved by variation of the RTP heating profile (R. Carey, P. A. Gago-Sandoval, D. M. Newman, and B. W. J. Thomas, presented at Intermag-93, Stockholm, April 13–16, 1993). It is known that the magneto-optic properties of barium ferrite can be enhanced by selective substitution of some of the Fe by Co2+ and Ti4+ albeit at the expense of reducing the magnetic anisotropy. A multitarget scanning cosputtering process has been used in conjunction with rapid thermal processing to produce a series of barium ferrite films in which Co, Cr, Mn, Ni are selectively introduced to substitute for between 5 and 20 at. % of the Fe. A corresponding percentage of Ti is also added to maintain charge compensation. The magnetic and magneto-optic properties of these films are presented and discussed with reference to their composition and treatment respect to the properties of barium ferrite.

The potential for application of magnetic skyrmions in high density storage devices provides a strong drive to investigate and exploit their stability and manipulability. Through a three-dimensional micromagnetic hysteresis study, we investigate the question of existence of skyrmions in cylindrical nanostructures of variable thickness. We quantify the applied field and thickness dependence of skyrmion states, and show that these states can be accessed through relevant practical hysteresis loop measurement protocols. As skyrmionic states have yet to be observed experimentally in confined helimagnetic geometries, our work opens prospects for developing viable hysteresis process-based methodologies to access and observe skyrmionic states.There is a continuous demand for developing magnetic data storage devices with higher recording density and improved reliability and robustness. Recent research demonstrates that magnetic skyrmions show great potential to meet such demands, 1-3 which drives vigorous research activity to understand the fundamental aspects of their emergence, stability, and manipulability.Magnetic skyrmions are topologically stable quasiparticles, which have been found to exist 4 with diameters as small as 1 nm. They arise in magnetic systems that lack inversion symmetry in the crystal lattice, which gives rise to the chiral Dzyaloshinskii-Moriya interaction (DMI),5,6 such as in bulk helimagnetic materials with a non-centrosymmetric crystal lattice, 5,6 or at the interface between two dissimilar materials.7,8 Along with skyrmions, DMI may give rise to different types of magnetic spin configurations, including helical and conical structures, all of which have been observed through theory, 9-12 simulation, 1,13 and experiment. 4,14-17Sustaining stable skyrmion states in continuous magnetic films requires the application of a significant external field, 15,16 which has been seen as a disadvantage for developing applications in information storage. As shown recently, however, zero-field isolated skyrmions can be sustained in magnetic nanostructures with confined geometries, 13,18 and created through several standard techniques including spin-polarized current injection.19,20 The research so far has focused predominantly on two dimensional confined nanostructures with negligible thickness − the influence of which is not yet understood. Indeed, the recent evidence suggests that modulations of magnetization through the thickness of a nanostructure is an important factor determining the stability of skyrmions. 13,21,22To explore such thickness dependent magnetization modulations in confined geometries, in this paper we study the hysteresis behavior of nanocylinder structures through full three dimensional micromagnetic simulaa) Electronic mail: h.fangohr@soton.ac.uk tions. The external field is varied starting from a well defined saturated state to produce a hysteresis loop, and the magnetization patterns recorded along the loop are analysed and classified into accessible states. Such hysteresis loop measure...

With the continued advance in the design and production of small solid state lasers and associated optics, future systems designed to achieve very-high-density storage of digital information are likely to become increasingly dependent on sophisticated optical technology. Non-erasable storage systems that have been accepted for many years (WORM, CD-ROM and so on) show a continuous growth of applications and sales. The first erasable storage systems, produced and marketed to an agreed world standard, are firmly based on magneto-optic recording technology. This review is intended to summarize the systems and materials that have made this technology possible.