K. Ramstöck

Corners and nucleation in Micromagnetics

The divergence of the stray field in corners and its consequence for micromagnetic calculations was studied numerically in two dimensions for high-anisotropy materials. The results show that no atomistic theory has to be invoked because the singularity is smoothed out already within micromagnetics. The magnetic configurations and its deduced critical quantity, the coercivity, are determined correctly if the configurations are well approximated on the exchange length SQRT(A/Kd). The singularity in the stray field remains only visible in the torque balance where it is compensated by an exchange torque.

Micromagnetics of a soft underlayer

The micromagnetic properties of a soft underlayer in perpendicular media are studied. It is shown that stray fields from a recording layer can substantially modify magnetic properties of soft underlayer material. The degree of modification is determined by the characteristic bit size in the recording layer and by the soft underlayer material magnetic characteristics. It is found that there exists a thin layer within a soft underlayer with reduced permeability. The effects of the presence of such a reduced permeability region within a soft underlayer on both reading and writing processes are discussed.

Micromagnetic simulation of a flux guide for a read head with sub-100 nm resolution

Large scale 3D micromagnetic simulations are performed on single pole read heads with pole tip cross sections of 45345 and 1003200 nm. It is shown that tips with these dimensions have a much higher output than predicted by an analytical theory using the reciprocity theorem. The larger of these tips still behaves in a relatively soft manner and has an output waveform shape very close to the one predicted by the analytical model. The response of the small tip however is hysteretic.

Formation And Annihilation Of Edge Walls In Thin-film Permalloy Strips

Comparison between magneto-resistance measurements and Kerr observations confirm that the formation and annihilation of edge walls in thin Permalloy strips, formed during rotation of an external field, is responsible for hysteresis in angular positioning sensors. The annihilation field required to avoid this unwanted behavior strongly depends on the strip cross section. Micromagnetic calculations assuming rectangular cross section yield higher absolute field values than those observed, but help to clarify the sensor behavior as field and material parameters are varied.

Numerical micromagnetics in low-anisotropy materials

The algorithms of finite element micromagnetics are investigated systematically. Improvements are presented in the discretization and in the energy calculation, as well as in the energy minimization method. The program is tested on two classical two-dimensional examples. >

Micromagnetic simulations of the domain structure and the magnetization reversal of Co50Ni50/Pt multilayer dots

The domain structure and the switching field of Co50 Ni50 /Pt multilayer dots,prepared by laser interference lithography,were micromagnetically simulated.The simulations were carried out with a three-dimensional simulation package,optimized for large-scale problems. The single-domain state is the lowest energy state for dots with a diameter below 75 nm.The switching field was computed by using suitable minimization techniques,and was used to analyze the effect of size,dot shape and edge defects.

A read and write element for magnetic probe recording

We present our results on the development of magnetic sensors for application in magnetic probe recording. Successful writing experiments on a magnetic medium with perpendicular anisotropy show that magnetic domains of 130 nm can be reversed in a heat-assisted process. For reading purposes we propose a magnetoresistive sensor. The optimization of the shape of the sensor was performed using micromagnetic simulations with the requirement that the sensor has to be capable of both read and write operations. At this stage, the experimental realization of the sensor was carried out at a wafer-base level. The fabrication technique consists of a combination of optical lithography and focused ion beam etching.

Techniques for the computation of embedded micromagnetic structures

The techniques to compute a micromagnetic object within an analytically known environment were refined. A novel multigrid method was developed which allows the computation of structures with widely differing length scales. The techniques are applied and tested on the problem of Bloch lines in bulk uniaxial materials. Energy and structure of such lines are calculated for the first time as a function of their orientation. In these calculations both the lattice density and the embedding range are extrapolated to infinity.

Simulation Of Magnetooptical Domain Boundary Images

Simulated magnetooptical images of domain walls are compared quantitatively with direct observations. The simulation is based on a micromagnetic model of V-lines in (110) oriented iron, a linear theory of magnetooptical diffraction and image formation based on Fourier optics.

Analytic and numerical two-dimensional micromagnetic calculations of thin films with a laterally oscillating interface coupling

The effects of a laterally oscillating interface energy competing with the exchange energy and uniaxial volume anisotropy were studied both analytically and numerically. The analytic prediction for the magnetization and the total energy in the ground state is expressed as a power series expansion in the interface coupling strength. The first three terms of this expansion are constructed explicitly. The results compare well with results of micromagnetic simulations within the expected range of applicability of the series expansion. The analytic solution can serve as a valuable benchmark for micromagnetic simulations.