Dieter Suess

Transition from single-domain to vortex state in soft magnetic cylindrical nanodots

We have investigated the magnetic properties of submicron soft magnetic cylindrical nanodots using an analytical model as well as three dimensional numerical finite element simulations. A detailed comparison of the magnetic vortex state shows the differences between these two models. It appears that the magnetic surface charges play a crucial role in the equilibrium magnetization distribution especially for shifted vortices. Finally, the magnetic phase diagram for soft magnetic particles with varying aspect ratio is presented.

Perpendicular FePt-based exchange-coupled composite media

Exchange-coupled composite media were realized by combining perpendicular hard magnetic FePtCu alloy films with perpendicular Co/Pt multilayers which are magnetically softer. We demonstrate that the switching field of the hard layer can be efficiently altered by modifying the material properties of the soft layer by varying the number of Co/Pt bilayers. Moreover, the possibility of effectively tuning the interlayer exchange coupling using rapid thermal annealing was shown. These studies were supported by theoretical modeling revealing the relevant factors to reduce the switching field of the hard layer which are important for future media design.

Recording simulations on graded media for area densities of up to 1Tbit∕in.2

We report recording simulations on graded media with area densities of 1Tbit∕in.2. The media are composed of a nucleation layer exchange coupled to a hard magnetic storage layer. The nucleation layer has an anisotropy K(z) that gradually varies in order to adjust the domain wall propagation field to the write field. Bits were written with a bit length of 12nm and a track width of 53nm on graded media with total thickness of 21nm and maximum anisotropy of 1MJ∕m3. The computed values for transition jitter are around 0.65nm, depending on the intergrain exchange.

Magnetic multilayers on porous anodized alumina for percolated perpendicular media

Co∕Pt multilayers deposited on anodized alumina substrates are introduced as percolated perpendicular media. The pores act as pinning sites due to variations of thickness and anisotropy direction around the perimeters. Coercivity, domain size, and switching field can be engineered by controlling pore density. The media exhibit strong perpendicular anisotropy and the switching field remains unchanged at its minimum up to an angular deviation of 50° from the easy axis. A better tolerance of switching-field distributions can thus be achieved, which may help to achieve a high signal-to-noise ratio. The thermal stability of the proposed media is investigated by micromagnetic simulations.

Domain wall motion in nanowires using moving grids (invited)

The magnetization reversal process of Co nanowires was investigated using a moving mesh technique. The nucleation and expansion of reversed domains is calculated by solving the Gilbert equation of motion for different damping constants. The adaptive finite element method reduces the total CPU time by more than a factor of 4 as compared to a uniform mesh. Two different domain wall types are observed. For a wire diameter of d=10 nm transverse walls occur and gyromagnetic precession limits the domain wall velocity. The domain wall velocity increases from 50 to 520 m/s as the Gilbert damping constant increases from α=0.05 to α=1 at an applied field of 500 kA/m. For a diameter greater than 20 nm vortex walls are formed. The vortex mobility increases with decreasing damping constant. Thus velocities up to 2000 m/s are reached for a wire diameter of 40 nm, α=0.05, and an applied field of 250 kA/m.

Thermal stability of graded exchange spring media under the influence of external fields

The thermal stability of graded exchange spring media for perpendicular recording is investigated. It is found that the energy barrier linearly decreases with the external field strength. This leads to a high thermal stability in the low frequency recording limit, where large demagnetizing fields act on the media. The attempt frequency of graded media is about a factor of 5 larger than the attempt frequency of comparable single phase media.

Micromagnetic study of pinning behavior in percolated media

A micromagnetic model has been developed describing the influence of the domain wall pinning in a magnetic thin film consisting of a granular microstructure with perpendicular magnetocrystalline anisotropy and a film thickness in the range of 5–16nm. The diameter of the prismatic domain wall pinning centers has been varied between 2 and 10nm. The stability of a single written bit is investigated. Thermal stability (energy barrier=80kBT300) can be guaranteed if the diameters of the pinning sites are equal or larger than the domain wall width and the film thickness is about 16nm. For defects on the order of 6nm magnetically very hard materials with values of Ku of 4×106J∕m3 are required.

Microwave-assisted three-dimensional multilayer magnetic recording

Layer-selective writing of two layer bit patterned media is demonstrated by performing micromagnetic finite element simulations. Selectivity is achieved by controlling the frequency of an oscillating magnetic field in the gigahertz range, applied in addition to the head field. Generation of the microwave field by means of a wire next to the tip of a single pole head is proposed. The Oersted field from the alternating current induces magnetic oscillations in the pole tip which create a high frequency field that is superimposed to the perpendicular write field. The amplitude of the ac field component is in the order of 0.1 T.

3D print of polymer bonded rare-earth magnets, and 3D magnetic field scanning with an end-user 3D printer

3D print is a recently developed technique, for single-unit production, and for structures that have been impossible to build previously. The current work presents a method to 3D print polymer bonded isotropic hard magnets with a low-cost, end-user 3D printer. Commercially available isotropic NdFeB powder inside a PA11 matrix is characterized, and prepared for the printing process. An example of a printed magnet with a complex shape that was designed to generate a specific stray field is presented, and compared with finite element simulation solving the macroscopic Maxwell equations. For magnetic characterization, and comparing 3D printed structures with injection molded parts, hysteresis measurements are performed. To measure the stray field outside the magnet, the printer is upgraded to a 3D magnetic flux density measurement system. To skip an elaborate adjusting of the sensor, a simulation is used to calibrate the angles, sensitivity, and the offset of the sensor. With this setup, a measurement resolut...

Three-dimensional micromagnetic finite element simulations including eddy currents

We developed a micromagnetic eddy current method that allows arbitrary geometries, requires no mesh outside the ferromagnet, and uses a stable integration scheme. We simultaneously solve the Landau–Lifshitz–Gilbert equation and the quasistatic Maxwell equations using a hybrid finite element/boundary element method (FEM/BEM). The eddy current field is directly calculated from the space time behavior of the magnetization rate of change. The boundary conditions of the eddy current field at infinity are taken into account using a FEM/BEM scheme. The resulting system of differential algebraic equations is solved using a backward differentiation method.