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.

We introduce a method for transporting colloidal particles, large molecules, cells, and other materials across surfaces and for assembling them into highly regular patterns. In this method, nonmagnetic materials are manipulated by a fluid dispersion of magnetic nanoparticles. Manipulation of materials is guided by a program of magnetic information stored in a substrate. Dynamic control over the motion of nonmagnetic particles can be achieved by reprogramming the substrate magnetization on the fly. The unexpectedly large degree of control over particle motion can be used to manipulate large ensembles of particles in parallel, potentially with local control over particle trajectory.colloidal ͉ manipulation ͉ self-assembly ͉ tweezing ͉ transport

We present a general study of the frequency and magnetic field dependence of the specific heat power produced during field-driven hysteresis cycles in magnetic nanoparticles with relevance to hyperthermia applications in biomedicine. Employing a kinetic Monte-Carlo method with natural time scales allows us to go beyond the assumptions of small driving field amplitudes and negligible inter-particle interactions, which are fundamental to the applicability of the standard approach based on linear response theory. The method captures the superparamagnetic and fully hysteretic regimes and the transition between them. Our results reveal unexpected dipolar interaction-induced enhancement or suppression of the specific heat power, dependent on the intrinsic statistical properties of particles, which cannot be accounted for by the standard theory. Although the actual heating power is difficult to predict because of the effects of interactions, optimum heating is in the transition region between the superparamagnetic and fully hysteretic regimes.

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.

Nano‐ and micromotors are fascinating objects that can navigate in complex fluidic environments. Their active motion can be triggered by external power sources or they can exhibit self‐propulsion using fuel extracted from their surroundings. The research field is rapidly evolving and has produced nano/micromotors of different geometrical designs, exploiting a variety of mechanisms of locomotion, being capable of achieving remarkable speeds in diverse environments ranging from simple aqueous solutions to complex media including cell cultures or animal tissue. This review aims to provide an overview of the recent developments with focus on predominantly experimental demonstrations of the various motor designs developed in the past 24 months. First, externally driven motors are discussed followed by considering fuel‐driven approaches. Finally, a short future perspective is provided.

Large thermal changes driven by a magnetic field have been proposed for environmentally friendly energy-efficient refrigeration 1 , but only a few materials which suffer hysteresis show these giant magnetocaloric effects 2-11 . Here we create giant and reversible extrinsic magnetocaloric effects in epitaxial films of the ferromagnetic manganite La 0.7 Ca 0.3 MnO 3 using strain-mediated feedback from BaTiO 3 substrates near a first-order structural phase transition. Our findings should inspire the discovery of giant magnetocaloric effects in a wide range of magnetic materials, and the parallel development of nanostructured bulk samples for practical applications.2 Magnetocaloric (MC) effects may be parameterized as adiabatic changes of temperature, or isothermal changes of entropy or heat, and have long been used to achieve millikelvin temperatures in the laboratory 12 . More recently, the discovery of giant MC effects near room temperature has led to suggestions for household and industrial cooling applications 1 . However, these giant MC effects arise in only a few materials [2][3][4][5][6][7][8][9][10][11] (Table 1), where strongly coupled magnetic and structural degrees of freedom produce magnetic phase transitions that are accompanied by changes in crystal symmetry 2-10 or volume 11 . It is therefore interesting to explore whether giant MC effects in magnetic materials can be created-rather than merely tuned 16 -via strain. (Table 1). By exploiting a first-order structural phase transition in BaTiO 3 (BTO) substrates, we create giant and reversible MC effects in epitaxial films of LCMO via the entropic interconversion of ferromagnetic and paramagnetic phases, whose coexistence 17,18 we reveal using photoemission electron microscopy (with magnetic contrast from x-ray magnetic circular dichroism) and ferromagnetic resonance.These extrinsic MC effects arise due to a strain-mediated feedback mechanism near the rhombohedral-orthorhombic transition in BTO at ~200 K, i.e. well away from LCMO C T at which the small intrinsic MC effects are seen. 3At temperature T, the isothermal entropy change ) (H S  of a magnetic material due to applied magnetic field H may be obtained via the Maxwell relationprovided that thermally driven changes in measured magnetization M arise due to changes in the magnitude and not the direction of the local magnetization (μ 0 is the permeability of free space, the prime indicates the dummy variable of integration). The Clausius-Clapeyron equation:represents a nominally equivalent indirect method for evaluating S  across first-order phase transitions in terms of the corresponding change in spontaneous magnetization 0 M  and the field-induced shift in transition temperature T 0 . Equations 1 and 2 follow from thermodynamics and do not depend on microscopic details. X-ray diffraction (XRD) of room-temperature LCMO//BTO reveals that the film reflections are weak and broad, and confirms the presence of 90° BTO domains (Fig. 1).The relative population of BTO twins varies between substrates, wi...

Magnetic hyperthermia (MH) based on magnetic nanoparticles (MNPs) is a promising adjuvant therapy for cancer treatment. Particle clustering leading to complex magnetic interactions affects the heat generated by MNPs during MH. The heat efficiencies, theoretically predicted, are still poorly understood because of a lack of control of the fabrication of such clusters with defined geometries and thus their functionality. This study aims to correlate the heating efficiency under MH of individually coated iron oxide nanocubes (IONCs) versus soft colloidal nanoclusters made of small groupings of nanocubes arranged in different geometries. The controlled clustering of alkyl-stabilized IONCs is achieved here during the water transfer procedure by tuning the fraction of the amphiphilic copolymer, poly(styrene-co-maleic anhydride) cumene-terminated, to the nanoparticle surface. It is found that increasing the polymer-to-nanoparticle surface ratio leads to the formation of increasingly large nanoclusters with defined geometries. When compared to the individual nanocubes, we show here that controlled grouping of nanoparticles—so-called “dimers” and “trimers” composed of two and three nanocubes, respectively—increases specific absorption rate (SAR) values, while conversely, forming centrosymmetric clusters having more than four nanocubes leads to lower SAR values. Magnetization measurements and Monte Carlo-based simulations support the observed SAR trend and reveal the importance of the dipolar interaction effect and its dependence on the details of the particle arrangements within the different clusters.

There has been much interest recently in the discovery of thermally induced magnetisation switching using femtosecond laser excitation, where a ferrimagnetic system can be switched deterministically without an applied magnetic field. Experimental results suggest that the reversal occurs due to intrinsic material properties, but so far the microscopic mechanism responsible for reversal has not been identified. Using computational and analytic methods we show that the switching is caused by the excitation of two-magnon bound states, the properties of which are dependent on material factors. This discovery allows us to accurately predict the onset of switching and the identification of this mechanism will allow new classes of materials to be identified or designed for memory devices in the THz regime.T hermally induced magnetisation switching (TIMS) occurs when an applied sub-picosecond heat pulse causes the magnetic state of a system to deterministically switch without any external or implicit magnetic field to determine the final state 1 . This unexpected switching was first observed in the amorphous rare earthtransition metal ferrimagnet GdFeCo and later, only in the similar materials TbCo 2 and TbFe 3 , but always for a limited range of rare-earth concentration. As yet there is no explanation why this should be the case, nor what properties of the material allow TIMS to occur at all. Here we show that the microscopic origin of TIMS is the thermal excitation of two magnon bound states whose properties are dependent on the composition of the material and the exchange interactions present. Our results explain the plethora of somewhat contradictory and paradoxical experimental observations of all-optical switching (AOS), where each postulated material requirement for reversal has been countered by an opposing example in the literature 1,2,4-8 . This study focuses on the prototypical material GdFeCo, but our theory can be generalised, allowing the occurrence of TIMS to be predicted from a knowledge of the magnon band structure of a material. Our work gives the insight required to select materials or design heterostructures which can exploit TIMS for applications, such as all-optical magnetic storage, where the replacement of rare-earth materials is needed due to the issues surrounding the sourcing these elements.In the area of all-optical switching, the explanation for switching magnetisation by laser light has shifted as more experiments are performed. In early papers it was thought that the inverse Faraday effect produced a large intrinsic magnetic field within GdFeCo which drove the magnetisation reversal 4,5 . More recent work by Khorsand et al. shows that the helicity dependence of AOS is due to magnetic circular dichroism causing a difference in the absorption of energy into the magnetic system 8 , supporting the results of Ref. 1. They show that there is a general threshold energy for AOS which is independent of the helicity. This result suggests that helicity dependent AOS is a subset of TIMS, where the magn...