143 research outputs found
Switchable Adhesion of Soft Composites Induced by a Magnetic Field
Switchable adhesives have the potential to improve the manufacturing and
recycling of parts and to enable new modes of motility for soft robots. Here,
we demonstrate magnetically-switchable adhesion of a two-phase composite to
non-magnetic objects. The composite's continuous phase is a silicone elastomer,
and the dispersed phase is a magneto-rheological fluid. The composite is simple
to prepare, and to mould to different shapes. When a magnetic field is applied,
the magneto-rheological fluid develops a yield stress, which dramatically
enhances the composite's adhesive properties. We demonstrate up to a nine-fold
increase of the pull-off force of non-magnetic objects in the presence of a 250
mT field
Tomographic reconstruction of a three-dimensional magnetization vector field
Using x-ray magnetic nanotomography the internal magnetization structure within extended samples can be determined with high spatial resolution and element specificity, without the need for assumptions or prior knowledge of the magnetic properties of a sample. Here we present the details of a new algorithm for the reconstruction of a three-dimensional magnetization vector field, discussing both the mathematical description of the problem, and details of the gradient-based iterative reconstruction routine. To test the accuracy of the algorithm the method is demonstrated for a complex simulated magnetization configuration obtained from micromagnetic simulations. The reconstruction of the complex three-dimensional magnetic nanostructure, including the surroundings of magnetic singularities (or Bloch points), exhibits an excellent qualitative and quantitative agreement with the simulated magnetic structure. This method provides a robust route for the reconstruction of internal three-dimensional magnetization structures obtained from x-ray magnetic tomographic datasets, which can be acquired with either hard or soft x-rays, and can be applied to a wide variety of three-dimensional magnetic systems
Three-dimensional magnetization structures revealed with X-ray vector nanotomography
In soft ferromagnetic materials, the smoothly varying magnetization leads to the formation of fundamental patterns such as domains, vortices and domain walls<sup>1</sup>. These have been studied extensively in thin films of thicknesses up to around 200 nanometres, in which the magnetization is accessible with current transmission imaging methods that make use of electrons or soft X-rays. In thicker samples, however, in which the magnetization structure varies throughout the thickness and is intrinsically three dimensional, determining the complex magnetic structure directly still represents a challenge<sup>1, 3</sup>. We have developed hard-X-ray vector nanotomography with which to determine the three-dimensional magnetic configuration at the nanoscale within micrometre-sized samples. We imaged the structure of the magnetization within a soft magnetic pillar of diameter 5 micrometres with a spatial resolution of 100 nanometres and, within the bulk, observed a complex magnetic configuration that consists of vortices and antivortices that form cross-tie walls and vortex walls along intersecting planes. At the intersections of these structures, magnetic singularities—Bloch points—occur. These were predicted more than fifty years ago<sup>4</sup> but have so far not been directly observed. Here we image the three-dimensional magnetic structure in the vicinity of the Bloch points, which until now has been accessible only through micromagnetic simulations, and identify two possible magnetization configurations: a circulating magnetization structure<sup>5</sup> and a twisted state that appears to correspond to an ‘anti-Bloch point’. Our imaging method enables the nanoscale study of topological magnetic structures<sup>6</sup> in systems with sizes of the order of tens of micrometres. Knowledge of internal nanomagnetic textures is critical for understanding macroscopic magnetic properties and for designing bulk magnets for technological applications<sup>7</sup>
Long-range order in arrays of composite and monolithic magneto-toroidal moments
Magneto-toroidal order, also called ferrotoroidicity, is the most recently
established type of ferroic state. It is based on a spontaneous and uniform
alignment of unit-cell-sized magnetic whirls, called magneto-toroidal moments,
associated with a macroscopic toroidization. Because of its intrinsic
magnetoelectric coupling, this new ferroic state could be useful in the
development of spintronic devices. We exploit two-dimensional periodic arrays
of magnetostatically coupled nanomagnets as model systems for the investigation
of long-range magneto-toroidal order. We present two pathways promoting this
order, namely (i), structures comprising a ring of uniformly magnetized
sub-micrometer-sized bar magnets and (ii), structures in which each magnetic
building block itself hosts a magnetic vortex. For both cases calculations of
the magnetic-dipole interaction and micromagnetic simulations reveal the
conditions for the formation of spontaneous magneto-toroidal order. We confirm
this order and the formation of magneto-toroidal domains in our arrays by
magnetic force microscopy. We identify the presence of two types of domain-wall
states emerging from the competition of two intrinsic microscopic couplings.
Our work not only identifies the microscopic conditions promoting spontaneous
magneto-toroidal order but also highlights the possibility tailor mesoscale
magnetic arrays towards elusive types of ferroic order.Comment: 20 pages, 7 figure
Thermodynamic phase transitions in a frustrated magnetic metamaterial
The research leading to these results has received funding from the European Community’s Seventh Framework Programme (FP7/2007-2013) under grant agreement no. 290605 (COFUND: PSI-FELLOW) and from the EPSRC (grant EP/J01060X).Materials with interacting magnetic degrees of freedom display a rich variety of magnetic behaviour that can lead to novel collective equilibrium and out-of-equilibrium phenomena. In equilibrium, thermodynamic phases appear with the associated phase transitions providing a characteristic signature of the underlying collective behaviour. Here we create a thermally active artificial kagome spin ice that is made up of a large array of dipolar interacting nanomagnets and undergoes phase transitions predicted by microscopic theory. We use low energy muon spectroscopy to probe the dynamic behaviour of the interacting nanomagnets and observe peaks in the muon relaxation rate that can be identified with the critical temperatures of the predicted phase transitions. This provides experimental evidence that a frustrated magnetic metamaterial can be engineered to admit thermodynamic phases.Publisher PDFPeer reviewe
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