M. Kirschner

Exchange spring media for perpendicular recording

A novel type of exchange spring media is proposed for magnetic recording systems consisting of a hard/soft bilayer. Finite element micromagnetic simulations show that the reversal modes induced by the external write field are significantly different from the thermally activated switching processes. Thus, the bilayers can be optimized in order to achieve a high thermal stability without increase of coercive field. In grains with identical size and coercivity an optimized bilayer reaches an energy barrier exceeding those of optimized single phase media by more than a factor of two. Additionally the lower angular dependence of coercivity of exchange spring media will improve the signal to noise ratio.

Optimization of exchange spring perpendicular recording media

Exchange spring media are proposed for magnetic-recording systems consisting of a hard/soft bilayer. By varying the fraction of thickness for the hard and soft layer and by varying their saturation polarizations, the media properties can be optimized in order to achieve high thermal stability without increase of coercive field. In grains with identical size and coercivity, an optimized bilayer reaches an energy barrier exceeding those of optimized single-phase media by more than a factor of two. Thus, exchange spring media allow to reduce the grain diameter by more than a factor of 1//spl radic/2. Additionally, the lower angular dependence of coercivity of exchange spring media improves the signal-to-noise ratio (SNR) by a factor of 2.5.

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.

Nanostructure calculation of CoAg core-shell clusters

Detailed studies of the structure of magnetic nanoclusters are crucial for understanding their magnetic properties. We have investigated the structure of CoxAg1−x nanoparticles by means of molecular dynamics simulations utilizing the embedded atom method. Starting from a completely random distribution of Co and Ag atoms, the clusters were heated up to 1300K and subsequently cooled down. The size of the resulting particles was 2.8nm (864 atoms). A clear segregation of the Ag atoms on the surface of the Co core was obtained.

Partitioning of the perpendicular write field into head and SUL contributions

We perform multiscale finite element simulation of the write process in perpendicular media. The Landau-Lifshitz-Gilbert equation is solved simultaneously for the head, the data layer, and the soft under layer during the motion of the head. All magnetostatic interactions between head, data layer and soft underlayer are concurrently taken into account. This fully integrated recording model enables a detailed analysis of the head field as seen by the media grains.

Cell size corrections for nonzero-temperature micromagnetics

Micromagnetic calculations at nonzero temperatures depend on the computational cell size. This paper shows that the spontaneous magnetization MS of exchange-coupled moments has to be scaled by a Bloch-like law, which is similar to the well-known temperature dependence of MS. Using this scaling law, nonatomistic Metropolis Monte Carlo and stochastic Landau–Lifshitz–Gilbert simulations are performed in an external field of 0.1T. The error of the equilibrium magnetization at a temperature of T∕TC=0.38 and a cell size of 1.5nm is then 0.9% as compared with atomistic calculations. In contrast, a cell size-independent MS leads to an overestimation of the temperature of 3.2%.

Multiscale micromagnetic simulation of giant magnetoresistance read heads

The Landau-Lifshitz-Gilbert equation and quasistatic Maxwell equations were solved simultaneously to calculate the read back signal of giant magnetoresistance read heads with a hybrid finite-element/boundary element method. The finite-element simulations show the influence of the sense current on the linearity of the reader, the effect of the exchange bias field on the sensor performance, and the influence of the Gilbert damping constant on the decay time of the read back voltage. All parts of the system, the layers of the giant magnetoresistance sensor, the hard bias magnets, the shields, and the recording layer are treated micromagnetically. In addition, the influence of the sense current onto the magnetization is taken into account self-consistently. The current distribution in the giant magnetoresistance stack is calculated from local resistivity which depends on the magnetization of the free and of the pinned layer.

Micromagnetic calculation of bias field and coercivity of polycrystalline ferromagnetic/antiferromagnetic layers

Domain processes associated with exchange bias in ferromagnetic /antiferromagnetic layers are investigated using a micromagnetic approach. The model gives quantitative estimates of the bias field and the coercivity for bilayers with fully compensated interfaces. Both simulations and transmission electron microscopy studies of IrMn-NiFe systems show 360/spl deg/ wall loops and 360/spl deg/ wall segments during the reversal of the F layer. The calculated bias field is in range of /spl mu//sub 0/H/sub eb/=3 mT to /spl mu//sub 0/H/sub eb/=20 mT. The bias field shows a maximum as a function of the antiferromagnet (AF) thickness. It increases sharply with increasing AF thickness at low thicknesses and decreases moderately with increasing AF thickness at higher thicknesses.

Influence of eddy current on magnetization processes in submicrometer permalloy structures

In this paper, we determine the critical particle size and conductivity range that leads to pronounced eddy current effects on magnetization reversal of magnetic nanostructures. An extended finite-element micromagnetic solver which includes eddy currents is used to calculate the magnetization behavior of a permalloy nanocube of 27 nm side under the influence of an applied field. It is shown that for high-conductivity /spl sigma/>10/sup 6/ (/spl Omega/ m)/sup -1/ in the intermediate state of reversal it takes the magnetization longer to fully align in the direction of the applied field.

Micromagnetic calculations of bias field and coercivity of compensated ferromagnetic antiferromagnetic bilayers

Exchange bias in polycrystalline IrMn/NiFe was found at perfectly compensated interfaces. The energy associated with unidirectional anisotropy is stored in lateral domain walls in the antiferromagnet. In addition to exchange bias, this mechanism leads to a training effect. The bias field shows a maximum of μ0Hb=4 mT at an antiferromagnetic layer thickness of 22 nm. The coercivities are on the order of μ0Hc=10 mT. The coercive field increases with decreasing intergrain exchange interactions within the ferromagnet.