Finite Element Micromagnetic Simulation of High Frequency Magnetization Reversal

Future information storage technology will require a data transfer rate of 520 Mbytes/s. The corresponding switching frequencies of 250 MHz will be a challenge to both head and media design. The magnetization reversal time will approach the intrinsic relaxation time of the media. Thermally activated fast relaxation takes place by magnetization reversal over energy barriers determined by the intrinsic magnetic properties and the microstructure of the material. The theoretical description of the complex response of the magnetization to a rapidly varying applied field requires to extend the theory of micromagnetism to finite temperatures. Micromagnetic simulation using the finite element method reveals the correlation between the local arrangement of the magnetic moments and the microstructural features at a significant length scale of several nanometers. Correlated fluctuations of. the magnetization can be incorporated into micromagnetic simulations describing magnetization reversal as stochastic process with multiplicative random noise. Simulations of the influence of the microstructure on thermally activated fast relaxation will provide theoretical insight for the development of future recording media. The proposed project will be part of the international COST action on "Simulation of physical phenomena in technological applications" (COST action P3) and was approved as an official COST project at the Management Committee meeting on March 14th , 1998.

During the three-year period the research activities within the COST P3 project "Finite element micromagnetic simulation of high frequency magnetization reversal", has concentrated to the dynamics of the magnetic switching behaviour of nanoscale magnetic structures and the influence of thermal activation processes on the switching behaviour. In detail, the influence of the shape (square, rectangular, circular) and size of the nanoelements with an extension of several hundreds of nanometer on the fast switching process was studied using the micromagnetic Landau-Lifshitz Gilbert precession model. The results of NiFe elements with zero magneto-crystalline anisotropy were compared with those of granular Co- thin films and single crystalline Co-elements. In addition to the improvement of the aerial density of magnetic recording media, the data rate becomes increasingly important. For different types of magnetic memories ranging from magnetic core memory, hard discs, magneto-optical media to MRAMS the switching time is an important factor. We investigated the switching time of thin film elements under the influence of an unidirectional or rotational applied field with different sweep rate, solving the Landau-Lifshitz Gilbert equation numerically. For fields slightly larger than the smallest possible switching field, the switching time increases with increasing field. This fast switching occurs for quasi-uniform rotation of the magnetization in small particles with small damping constant and at low fields. Non-uniform demagnetizing fields of irregular particles and thermal fluctuation does not significantly change the field dependence of the switched time. Fast switching modes are possible if the rise time of the external field is shorter than the relaxation of the magnetization toward the local minimum close to the initial state. Then the external field, which has to be small enough to create a complex energy landscape, moves the magnetization far away from its initial state at nearby constant energy. The large angle between the effective field and the magnetization creates a large torque which reduces the switching time. The switching time reaches a maximum at an external field close to the Stoner-Wohlfarth coercivity. For larger external fields the expected decrease of the switching time with increasing field is obtained. The solution of the Langevin-equation shows that thermal fluctuations does not significantly influence the dependence of the switching time on the field strength. These fast switching modes may become important in perpendicular recoding. In the last years there has been a renew interest in perpendicular recording since an improvement of the areal density in longitudinal recording is getting increasingly difficult. The main advantage of perpendicular recording over longitudinal is that the aerial density can be increased without reducing the volume of the magnetic bit. An important question in perpendicular recoding is the reversal mode. Besides recording materials we have investigated the reversal process in magnetic nanoelements. Nanostructured magnetic elements may be used as storage elements, field sensors, or logic gates. The functional behavior of these devices depends on the domain configuration and the reversal mechanism of the nanomagnets. During magnetization reversal of isolated circular nanomagnets, which may be used for magnetic logic gates, inhomogeneous transient states are formed. The exchange energy of the intermediates states, which is a measure for the uniformity of the magnetization, increases with decreasing Gilbert damping constant or increasing applied field. The magnetostatic interactions lead to a further enhancement of the non-uniformity during reversal of interacting nanomagnets. Our results were published in a large number of publications (more than 30) and presented at international seminars and conferences (more than 50).