Dennis Nissen

Micromagnetic simulation of exchange coupled ferri-/ferromagnetic composite in bit patterned media

Ferri-/ferromagnetic exchange coupled composites are promising candidates for bit patterned media because of the ability to control the magnetic properties of the ferrimagnet by its composition. A micromagnetic model for the bilayer system is presented where we also incorporate the microstructural features of both layers. Micromagnetic finite element simulations are performed to investigate the magnetization reversal behaviour of such media. By adding the exchange coupled ferrimagnet to the ferromagnet, the switching field could be reduced by up to 40% and also the switching field distribution is narrowed. To reach these significant improvements, an interface exchange coupling strength of 2 mJ/m2 is required.

Controlling the magnetic structure of Co/Pd thin films by direct laser interference patterning

Pulsed two beam direct laser interference patterning (DLIP) is used to generate two-dimensional temperature patterns on a magnetic sample. In contrast to other methods like electron beam lithography, DLIP offers the possibility to pattern large areas on a timescale of a few nanoseconds in a one-step process. Usually DLIP is used to pattern surfaces [1,2], but here we focus on local periodic heating on the nanoscale.

Single vortex core recording in a magnetic vortex lattice

We investigated the reversal characteristics of magnetic vortex cores in a two dimensional assembly of magnetic vortices. The vortex lattice was created by film deposition of 30-nm-thick permalloy onto large arrays of self-assembled spherical SiO2-particles with a diameter of 330 nm. The vortex core reversal was investigated by employing a write/read tester. This device uses a state-of-the-art magnetic recording head of a hard disc drive, which allows imaging as well as applying a local magnetic field pulse to individual vortices. The successful writing and reading of individual vortex cores is demonstrated, including a switching map, which indicates the switching behavior dependent on the relative position of the field pulse with respect to the vortex core.

Sub-micron period lattice structures of magnetic microtraps for ultracold atoms on an atom chip

We report on the design, fabrication and characterization of magnetic nanostructures to create a lattice of magnetic traps with sub--micron period for trapping ultracold atoms. These magnetic nanostructures were fabricated by patterning a Co/Pd multilayered magnetic film grown on a silicon substrate using high precision e-beam lithography and reactive ion etching. The Co/Pd film was chosen for its small grain size and high remanent magnetization and coercivity. The fabricated structures are designed to magnetically trap

Magnetic coupling of vortices in a two-dimensional lattice

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Magnetic vortices in magnetic nano-caps formed on densely packed particle array

Rb atoms above the surface of the magnetic film with 1D and 2D (triangular and square) lattice geometries and sub-micron period. Such magnetic lattices can be used for quantum tunneling and quantum simulation experiments, including using geometries and periods that may be inaccessible with optical lattice.

Control of the magnetic structure of Co/Pd thin films by direct laser interference patterning

We investigated the magnetization reversal of magnetic vortex structures in a two-dimensional lattice. The structures were formed by permalloy (Py) film deposition onto large arrays of self assembled spherical SiO(2)-particles with a diameter of 330 nm. We present the dependence of the nucleation and annihilation field of the vortex structures as a function of the Py layer thickness(aspect ratio) and temperature. By increasing the Py thickness up to 90 nm or alternatively by lowering the temperature the vortex structure becomes more stable as expected. However, the increase of the Py thickness results in the onset of strong exchange coupling between neighboring Py caps due to the emergence of Py bridges connecting them. In particular, we studied the influence of magnetic coupling locally by in-field scanning magne to-resistive microscopy and full-field magnetic soft x-ray microscopy, revealing a domain-like nucleation process of vortex states, which arises via domain wall propagation due to exchange coupling of the closely packed structures. By analyzing the rotation sense of the reversed areas, large connected domains are present with the same circulation sense. Furthermore, the lateral core displacements when an in-plane field is applied were investigated, revealing spatially enlarged vortex cores and a broader distribution with increasing Py layer thickness. In addition, the presence of some mixed states, vortices and c-states, is indicated for the array with the thickest Py layer.

Bistable self-assembly in homogeneous colloidal systems for flexible modular architectures

When the size of a magnetic structure becomes comparable to a critical length scale, such as exchange length, the multidomain spin configuration becomes energetically unfavorable and either a single domain or an inhomogeneous magnetization configuration is formed. One example is a magnetic vortex state [1], which is of considerable technological interest due to its unique physical properties, such as low magnetic stray field and high integration density. A magnetic vortex is characterized by the in-plane magnetic flux closure, which can rotate clockwise or counterclockwise and the direction of the out-of-plane vortex core polarization, which can point up or down [2]. The reversal mechanism of a magnetic vortex is to a large extending controlled by the contributions of exchange and magnetostatic energy. Thus, additional exchange coupling induced by an antifer-romagnetic layer [3,4] can be used to alter these properties.

Trapping ultracold atoms in a sub-micron-period triangular magnetic lattice

Pulsed two beam direct laser interference patterning (DLIP) is used to generate two-dimensional temperature patterns on a magnetic sample. In contrast to other methods like electron beam lithography, DLIP offers the possibility to pattern large areas on a timescale of a few nanoseconds in a one-step process. Usually DLIP is used to pattern surfaces [1,2], but here we focus on local periodic heating on the nanoscale.