Macrospin limit and configurational anisotropy in nanoscale Permalloy triangles

In Permalloy submicron triangles, configurational anisotropy - a higher-order form of shape anisotropy - yields three equivalent easy axes, imposed by the structures' symmetry order. Supported by micromagnetic simulations, an experimental method was devised to evaluate the nanostructure dimensions for which a Stoner-Wohlfarth type of reversal could be used to describe this particular magnetic anisotropy. In this regime, a straightforward procedure using an in-plane rotating field allowed us to quantify experimentally the six-fold anisotropy fields for triangles of different thicknesses and sizes

Controllable switching of vortex chirality in magnetic nanodisks by a field pulse

We propose a way of fast switching the chirality in a magnetic nanodisk by applying a field pulse. To break the symmetry with respect to clockwise or counterclockwise chirality a mask is added by which an inhomogeneous field influences the vortex state of a nanodisk. Using numerical spin--lattice simulations we demonstrate that chirality can be controllably switched by a field pulse, whose intensity is above some critical value. A mathematical definition for the chirality of an arbitrary shaped particle is proposed.Comment: REVTeX, 4 pages, 3 figure

Coupling and induced depinning of magnetic domain walls in adjacent spin valve nanotracks

The magnetostatic interaction between magnetic domain walls (DWs) in adjacent nanotracks has been shown to produce strong inter-DW coupling and mutual pinning. In this paper, we have used electrical measurements of adjacent spin-valve nanotracks to follow the positions of interacting DWs. We show that the magnetostatic interaction between DWs causes not only mutual pinning, as observed till now, but that a travelling DW can also induce the depinning of DWs in near-by tracks. These effects may have great implications for some proposed high density magnetic devices (e.g. racetrack memory, DW logic circuits, or DW-based MRAM).Comment: The following article has been accepted by the Journal of Applied Physic

Two-dimensional control of field-driven magnetic bubble movement using Dzyaloshinskii-Moriya interactions

The field-induced asymmetric growth of magnetic bubble domains in Pt/Co/Pt out-of-plane magnetized films with Dzyaloshinskii–Moriya interactions (DMI) is used to control the lateral displacement of bubbles. We demonstrate experimentally that we can laterally translate bubbles away from their nucleation site by applying a series of alternating 3-dimensional field pulses with a controlled relative sign between the out-of-plane and in-plane components. Using magneto optical Kerr effect imaging, the domain wall velocity as a function of applied field strength was measured from which the magnitude of the DMI field was estimated.This work was supported by the European Community under the Seventh Framework Programme '3SPIN' (ERC contract 247368) and by EMRP JRP EXL04 SpinCal. The EMRP is jointly funded by the EMRP participating countries within EURAMET and the EU.This is the accepted manuscript. The final version is available from AIP at http://scitation.aip.org/content/aip/journal/apl/106/2/10.1063/1.4905600

Vortex motion in a finite-size easy-plane ferromagnet and application to a nanodot

We study the motion of a non-planar vortex in a circular easy-plane ferromagnet, which imitates a magnetic nanodot. Analysis was done using numerical simulations and a new collective variable theory which includes the coupling of Goldstone-like mode with the vortex center. Without magnetic field the vortex follows a spiral orbit which we calculate. When a rotating in-plane magnetic field is included, the vortex tends to a stable limit cycle which exists in a significant range of field amplitude B and frequency $\omega$ for a given system size L. For a fixed $\omega$, the radius R of the orbital motion is proportional to L while the orbital frequency $\Omega$ varies as 1/L and is significantly smaller than $\omega$. Since the limit cycle is caused by the interplay between the magnetization and the vortex motion, the internal mode is essential in the collective variable theory which then gives the correct estimate and dependency for the orbit radius $R\sim B L/\omega$. Using this simple theory we indicate how an ac magnetic field can be used to control vortices observed in real magnetic nanodots.Comment: 15 pages (RevTeX), 14 figures (eps

Near-field interaction between domain walls in adjacent Permalloy nanowires

The magnetostatic interaction between two oppositely charged transverse domain walls (DWs)in adjacent Permalloy nanowires is experimentally demonstrated. The dependence of the pinning strength on wire separation is investigated for distances between 13 and 125 nm, and depinning fields up to 93 Oe are measured. The results can be described fully by considering the interaction between the full complex distribution of magnetic charge within rigid, isolated DWs. This suggests the DW internal structure is not appreciably disturbed by the pinning potential, and that they remain rigid although the pinning strength is significant. This work demonstrates the possibility of non-contact DW trapping without DW perturbation and full continuous flexibility of the pinning potential type and strength. The consequence of the interaction on DW based data storage schemes is evaluated.Comment: 4 pages, 4 figures, 1 page supplimentary material (supporting.ps

Interacting circular nanomagnets

Regular 2D rectangular lattices of permalloy nanoparticles (40 nm in diameter) were prepared by the method of the electron lithography. The magnetization curves were studied by Hall magnetometry with the compensation technique for different external field orientations at 4.2K and 77K. The shape of hysteresis curves indicates that there is magnetostatic interaction between the particles. The main peculiarity is the existence of remanent magnetization perpendicular to easy plain. By numerical simulation it is shown, that the character of the magnetization reversal is a result of the interplay of the interparticle interaction and the magnetization distribution within the particles (vortex or uniform).Comment: 16 pages, 8 figure

Tailoring the magnetic properties of Fe asymmetric nanodots

Asymmetric dots as a function of their geometry have been investigated using three-dimensional (3D) object oriented micromagnetic framework (OOMMF) code. The effect of shape asymmetry of the disk on coercivity and remanence is studied. Angular dependence of the remanence and coercivity is also addressed. Asymmetric dots are found to reverse their magnetization by nucleation and propagation of a vortex, when the field is applied parallel to the direction of asymmetry. However, complex reversal modes appear when the angle at which the external field is applied is varied, leading to a non monotonic behavior of the coercivity and remanence.Comment: 5 pages, 7 figure

Confidentiality and public protection: ethical dilemmas in qualitative research with adult male sex offenders

This paper considers the ethical tensions present when engaging in in-depth interviews with convicted sex offenders. Many of the issues described below are similar to those found in other sensitive areas of research. However, confidentiality and public protection are matters that require detailed consideration when the desire to know more about men who have committed serious and harmful offences is set against the possibility of a researcher not disclosing previously unknown sensitive information that relates to the risk of someone being harmed.</p

String Method for the Study of Rare Events

We present a new and efficient method for computing the transition pathways, free energy barriers, and transition rates in complex systems with relatively smooth energy landscapes. The method proceeds by evolving strings, i.e. smooth curves with intrinsic parametrization whose dynamics takes them to the most probable transition path between two metastable regions in the configuration space. Free energy barriers and transition rates can then be determined by standard umbrella sampling technique around the string. Applications to Lennard-Jones cluster rearrangement and thermally induced switching of a magnetic film are presented.Comment: 4 pages, 4 figure