Accurate Micromagnetic Simulation Of Nano-sized Magnetic Devices

As a basic theory in applied magnetism, the micromagnetic simulation has beenplaying an essential role in the development of hard disk drives. With the increment ofareal density of magnetic storage, there is an increasing requirement to the accuracy ofmicromagnetics. In the single pole type (SPT) write head, the sawtooth inclined edgesexisting in the finite difference method (FDM) model has been a serious problem, andthis inaccuracy will be more serious in smaller sized heads. In addition, the magneticproperties of FeCo soft magnetic thin films used in writers have not been wellunderstood. Therefore, to improve the accuracy of the micromagnetic simulation, wehave introduced prism edge cells to the SPT head to form smooth slope edges; andsimulated the microstructure to explore the mechanism of good soft magnetic propertiesof FeCo film obtained in experiments.To simulate a ferromagnetic thin film nano-sized device with arbitrary shape,prism edge cells are introduced in the model. We have analytically calculated thedemagnetizing matrices of a rectangle and triangle, respectively, and accordingly thedemagnetizing matrices of prism cells can be acquired by summing up the matrices ofall their facets. In the basic studies of micromagnetics, we also check our programs viastudying two standard problems and a square-shape thin film device with three differentsorts of discretization ways. It is found that the cell size can be larger than the Neelexchange length, which will enable higher simulation speed.Consequently, we have established an accurate model of the SPT head. In ourmodel, the effects of sawtooth edges in FDM model of the SPT head are distinct whenthe pole thickness is20nm; i.e., the models with sawtooth edges are inaccurate in thiscase. Two sorts of magnetic motive forces are compared: one is driven by magnetic poleat the top surface of the main pole tip, the other is driven by current in coils. Althoughthe fields generated by current coils (10mA) are much smaller than those by magneticpoles, the SPT head can be fully reversed for both cases. Also there is an optimizedthickness (60nm) for the soft underlayer when the track is discrete in the recordingmedia, while the head field is similar for60nm-thick and100nm-thick SUL when the medium is continuous. For a hard magnetic nanodot, the anisotropy distribution affectsthe reversal time evidently.To study the magnetic property of FeCo soft magnetic thin films used inSPT heads, we have introduced the microstructure into our micromagneticmodel, including crystalline grain distribution, the amendments of magneticparameters for cells located at the grain boundaries, and the stress-inducedanisotropy. The simulation results of hysteresis loops agree well withexperimental results.Based on this model, we further discover that the switching and initialpermeabilities at high frequencies are relevant to the easy axis of the pole tip inthe SPT head. When the easy axis is perpendicular to the media, the write fieldis larger with a little smaller cut-off frequency. The initial permeabilities of aSPT pole tip with the microstructures included have similar high frequencyresponse to that in a model with slightly larger cells and only the stress-inducedanisotropy included.