Peter A. J. de Groot

Electrochemical deposition of macroporous magnetic networks using colloidal templates

Nanostructuring of magnetic materials, on a scale comparable to the dimensions of the magnetic domain boundaries, is known to have a significant effect on the magnetic properties of a material. Here we demonstrate a simple and versatile technique for the preparation of two- or three-dimensional highly ordered macroporous cobalt, iron, nickel, and nickel iron alloy films containing close packed arrays of spherical holes of uniform size (an inverse opal structure). The films were prepared by electrochemical deposition from aqueous solution within the interstitial spaces of pre-assembled templates. The templates were assembled from colloidal polystyrene latex spheres (0.20, 0.50, 0.75 or 1.00 µm in diameter) assembled onto gold electrode surfaces from aqueous solution by slow evaporation. Following the electrochemical deposition of the metal or alloy films the polystyrene templates were removed by dissolution in toluene. Scanning electron microscopy of the resulting films show well-formed two- or three-dimensional porous structures consisting of interconnected close packed arrays of spherical voids. X-Ray diffraction analysis confirms that the nickel iron alloy in the walls of the structure is polycrystalline fcc in structure with a grain size which is significantly smaller than the thickness of the walls. The diameter of the spherical voids within the structures can be varied by changing the diameter of the polystyrene latex spheres used to form the template. Studies of the magnetic properties of the macroporous films show a large coercivity enhancement in comparison to the corresponding plain films and we find that the coercive field gradually increases as the diameter of the spherical voids decreases for films of a constant thickness.

The Electrochemical Deposition of Nanostructured Cobalt Films from Lyotropic Liquid Crystalline Media

Lyotropic liquid crystalline phases formed at high concentrations of nonionic surfactants provide a versatile templating medium that can be used to produce nanostructured materials with regular arrays of pores of nanometer dimension and extended periodicity. In this work, we have used this technique to prepare nanostructured cobalt films on gold substrates by electrochemical deposition of cobalt from cobalt acetate dissolved in the aqueous domains of the hexagonal lyotropic liquid crystalline phase of Brij 56(C16EO10). Low angle X-ray and transmission electron microscope studies show that the resulting cobalt films have a regular nanostructure consisting of a hexagonal array of cylindrical pores. The repeat distance for this nanostructure lies in the range of 7-11 nm and can be systematically varied by varying the amount of n-heptane added to the template mixture. Preliminary magnetic measurements on the nanostructured samples suggest that the coercivity (H-c) of these cobalt films is three to five times greater than that for polycrystalline cobalt and that it varies systematically with the size of the pores.

Magnetic control of a meta-molecule

Metamaterials offer the prospect of new science and applications. They have been designed by shaping or changing the material of the individual meta-molecules to achieve properties not naturally attainable. Composite meta-molecules incorporating a magnetic component offer new opportunities. In this work we report on the interaction between a non-magnetic split ring resonator (SRR) and a thin film of yttrium iron garnet (YIG). Strong hybridized resonances are observed. While the SRR is characterized by a magnetic and electric resonance, in practice, it is found that the YIG couples strongly to this symmetric (electric) mode of the SRR. It is also demonstrated that the anti-crossing region provides fertile ground for the creation of elementary excitations such as backward volume magnetostatic waves.

A new approach to (quasi) periodic boundary conditions in micromagnetics: The macrogeometry

We present a new method to simulate repetitive ferromagnetic structures. This macrogeometry approach combines treatment of short-range interactions (i.e., the exchange field) as for periodic boundary conditions with a specification of the arrangement of copies of the primary simulation cell in order to correctly include effects of the demagnetizing field. This method (i) solves a consistency problem that prevents the naive application of three-dimensional periodic boundary conditions in micromagnetism and (ii) is well suited for the efficient simulation of repetitive systems of any size.

Vortex dynamics in two-dimensional systems at high driving forces

We study numerically the dynamics of two-dimensional vortex systems at zero temperature. In addition to pinned states and turbulent plastic flow, we find motion of vortices in rough channels along the direction of the driving force. In this decoupled channel regime we demonstrate how topological defects mediate the phase slip of different channels moving with different velocities. We thus provide important confirmation of recent analytical work describing vortex dynamics at high driving forces such as the moving glass theory of Giamarchi and Le Doussal. For the largest driving forces we find that the channels couple and observe elastic motion.

Giant magnetic modulation of a planar, hybrid metamolecule resonance

Coupling magnetic elements to metamaterial structures creates hybrid metamolecules with new opportunities. Here we report on the magnetic control of a metamolecule resonance, by utilizing the interaction between a single split ring resonator (SRR) and a magnetic thin film of permalloy. To suppress eddy current shielding, the permalloy films are patterned into arrays of 30–500 μm diameter discs. Strong hybridized resonances were observed at the anticrossing between the split ring resonance and the ferromagnetic resonance (FMR) of the permalloy. In particular, it is possible to achieve 40 dB modulation of the electric (symmetric) mode of the SRR on sweeping the applied magnetic field through the SRR/FMR anticrossing. The results open the way to the design of planar metamaterials, with potential applications in nonlinear metamaterials, tunable metamaterials and spintronics. S Online supplementary data available from stacks.iop.org/NJP/16/063002/ mmedia

Micromagnetic simulation studies of ferromagnetic part spheres

Self-assembly techniques can be used to produce periodic arrays of magnetic nanostructures. We have developed a double-template technique using electrochemical deposition. This method produces arrays of dots which are of spherical shape, as opposed to those prepared by standard lithographic techniques, which are usually cylindrical. By varying the amount of material that is deposited electrochemically, spheres of diameter d can be grown up to varying heights h<d. Thus different spherical shapes can be created ranging from shallow dots to almost complete spheres. Using micromagnetic modeling, we calculate numerically the magnetization reversal of the soft part spherical particles. The observed reversal mechanisms range from single domain reversal at small radii to vortex movement in shallow systems at larger radii and vortex core reversal, as observed in spheres at larger heights. We present a phase diagram of the reversal behavior as a function of radius and growth height. Additionally, we compare simulation results of hybrid finite element/boundary element and finite difference calculations for the same systems.

Orientation and symmetry control of inverse sphere magnetic nanoarrays by guided self-assembly

Inverse sphere shaped Ni arrays were fabricated by electrodeposition on Si through the guided self-assembly of polystyrene latex spheres in Si/SiO2 patterns. It is shown that the size commensurability of the etched tracks is critical for the long range ordering of the spheres. Moreover, noncommensurate guiding results in the reproducible periodic triangular distortion of the close packed self-assembly. Magnetoresistance measurements on the Ni arrays were performed showing room temperature anisotropic magnetoresistance of 0.85%. These results are promising for self-assembled patterned storage media and magnetoresistance devices.

Micromagnetic simulation of ferromagnetic part-spherical particles

The paramagnetic size limit for current magnetic storage media, particularly in sputtered grain storage, is being approached rapidly. To further increase media storage density, patterned media can be used which only need a single grain to store one bit of data. Chemical self-assembly techniques offer cost-effective methods to create templates, from which periodic arrays of magnetic structures can be formed. In contrast to systems of dots prepared by standard lithography, which have a cylindrical shape, dots prepared by chemical self-assembly template techniques are often spherical or part spherical in shape. In this article, we investigate the properties of such magnetic shapes using micromagnetic simulations. To represent accurately the geometry produced through chemical self-assembly methods, we attach a partial sphere (lower part) to a small ellipsoidal dome. We compute the hysteresis loops for various dot sizes and compare them with experimental results. In those below a critical diameter (140 nm in nickel), the hysteresis loop is square-like, resembling the uniform rotation of magnetization once the critical field is exceeded. For larger sizes, the hysteresis loop reverses reversibly around zero applied field but shows minor loops, placed symmetrically at the onset of magnetization reversal. These correspond to vortices penetrating and exiting the structure. In summary, we find that the coercive field of the droplets becomes zero above a critical diameter where the magnetization reversal behavior changes from single domain-like to vortex-like. Our results agree with experimental measurements performed on such structures.

Micromagnetic simulation of the magnetic exchange spring system DyFe2∕YFe2

Magnetic measurements of [110] [50ADyFe2∕200AYFe2] reveal a rich switching behavior: the formation of exchange springs in this system of alternating hard and soft layers can be observed for low temperatures (LTs). For high temperatures (HTs), the appearance of the hysteresis loop changes significantly, implying a more complicated reversal process. In this article, we reproduce hysteresis loops for net and compound-specific magnetizations by means of micromagnetic simulations and assess the quality by a direct comparison to recent x-ray magnetic circular dichroism measurements. The HT switching characteristics, showing a magnetization reversal of the hard magnetic layer before the soft magnetic layer, are investigated and understood on the basis of detailed magnetic configuration plots. The crossover of LT to HT switching patterns is explained by energy considerations, and the dependence on different parameters is outlined.