Magnetization reversal in a novel gradient nanomaterial

The deposition of Co/Pd multilayer films onto self-assembled particle arrays with particle sizes down to 50 nm leads to pronounced curvature-induced physical properties. Unlike in classical nanosystems, the so-formed single caps on top of the spherical particles exhibit a radial symmetric anisotropy orientation across their surface. Its impact on the magnetization reversal process was analyzed experimentally for different particle sizes and compared to micromagnetic simulations, offering new opportunities in the functionalization of magnetic nanostructures.     

Head and bit patterned media optimization at areal densities of 2.5 Tbit/in and beyond

Global optimization of writing head is performed using micromagnetics and surrogate optimization. The shape of the pole tip is optimized for bit patterned, exchange spring recording media. The media characteristics define the effective write field and the threshold values for the head field that acts at islands in the adjacent track. Once the required head field characteristics are defined, the pole tip geometry is optimized in order to achieve a high gradient of the effective write field while keeping the write field at the adjacent track below a given value. We computed the write error rate and the adjacent track erasure for different maximum anisotropy in the multilayer, graded media. The results show a linear trade off between the error rate and the number of passes before erasure. For optimal head media combinations we found a bit error rate of 10⁻⁶ with 10⁸ pass lines before erasure at 2.5 Tbit/in².     

Transverse and vortex domain wall structure in magnetic nanowires with uniaxial in-plane anisotropy

Micromagnetic and analytical models are used to investigate how in-plane uniaxial anisotropy affects transverse and vortex domain walls in nanowires where shape anisotropy dominates. The effect of the uniaxial anisotropy can be interpreted as a modification of the effective wire dimensions. When the anisotropy axis is aligned with the wire axis (θ(a) = 0), the wall width is narrower than when no anisotropy is present. Conversely, the wall width increases when the anisotropy axis is perpendicular to the wire axis (θ(a) = π/2). The anisotropy also affects the nanowire dimensions at which transverse walls become unstable. This phase boundary shifts to larger widths or thicknesses when θ(a) = 0, but smaller widths or thicknesses when θ(a) = π/2.     

Pulsed inductive microwave magnetometer response calculated for IrMn/FeNi bilayers

Micromagnetic simulations of a pulsed inductive microwave magnetometer (PIMM) experiment are performed using a well established model for exchange bias. The model (Interacting Grain Model) consists of ferromagnetic grains and antiferromagnetic grains with randomly distributed easy axes. A perfectly compensated interface between the ferromagnet and the antiferromagnet is assumed which leads to spin flop coupling. The antiferromagnetic layer is modelled as two totally antiparallel sublattices with a small intergrain exchange between each antiferromagnetic sublattice. Simulations of an experimental PIMM setup provide a shift of the minimum of the resonance frequency which is also observered experimentally.     

Self-organizing magnetic beads for biomedical applications

In the field of biomedicine magnetic beads are used for drug delivery and to treat hyperthermia. Here we propose to use self-organized bead structures to isolate circulating tumor cells using lab-on-chip technologies. Typically blood flows past microposts functionalized with antibodies for circulating tumor cells. Creating these microposts with interacting magnetic beads makes it possible to tune the geometry in size, position and shape. We developed a simulation tool that combines micromagnetics and discrete particle dynamics, in order to design micropost arrays made of interacting beads. The simulation takes into account the viscous drag of the blood flow, magnetostatic interactions between the magnetic beads and gradient forces from external aligned magnets. We developed a particle-particle particle-mesh method for effective computation of the magnetic force and torque acting on the particles.     

Exchange-coupled perpendicular media

The potential of exchange spring bilayers and graded media is reviewed. An analytical model for the optimization of graded media gives an optimal value of the magnetic polarization of J_s=0.8 T. The optimum design allows for thermally stable grains with grain diameters in the order of 3.3 nm, which supports ultra-high density up to 5-10 Tbit/in². The switching field distribution is significantly reduced in bilayer media and graded media compared to single-phase media. For the graded media the switching field distribution is reduced by about a factor of two. For bilayer media the minimum switching field distribution is obtained for soft-layer anisotropies that are about one fifth of the hard-layer anisotropy. The influence of precessional switching on the reversal time and the reversal field is investigated in detail for magnetic bilayers. Exchange spring bilayers can be reversed with field pulses of 20 ps.     

Enhanced nucleation fields due to dipolar interactions in nanocomposite magnets

One approach to construct powerful permanent magnets while using less rare-earth elements is to combine a hard magnetic material having a high coercive field with a soft magnetic material having a high saturation magnetization at the nanometer scale and create so-called nanocomposite magnets. If both materials are strongly coupled, exchange forces will form a stable magnet. We use finite element micromagnetics simulations to investigate the changing hysteresis properties for varying arrays of soft magnetic spherical inclusions in a hard magnetic body. We show that the anisotropy arising from dipolar interactions between soft magnetic particles in a hard magnetic matrix can enhance the nucleation field by more than 10% and strongly depends on the arrangement of the inclusions.     

Fast stray field computation on tensor grids

A direct integration algorithm is described to compute the magnetostatic field and energy for given magnetization distributions on not necessarily uniform tensor grids. We use an analytically-based tensor approximation approach for function-related tensors, which reduces calculations to multilinear algebra operations. The algorithm scales with N^4/3 for N computational cells used and with N^2/3 (sublinear) when magnetization is given in canonical tensor format. In the final section we confirm our theoretical results concerning computing times and accuracy by means of numerical examples.