On the hysteresis and the recording process in magnetic media

The information densities in magnetic recording systems have been in- creased considerably over the last few decades. Much af the gain in the in- formation density can be ascribed ta the improvement af the magnetic prop- erties af the recording media. However, recording on a magnetic medium is a camplex process which, despite extensive research in the past, is only partly understood. The purpose af the research described in this thesis is ta contribute towards a better understanding af the recording process. The approach that has been followed is a combination af experiment al study with numerical modelling. The thesis consists af four parts, two on the magnetic hysteresis and two on the recording process. 1: Experimental study of the hysteresis The technique af magnetic recording is possible only thanks ta the magnetic hysteresis. (Hysteresis is the property af magnetic media ta remain magne- tized af ter a temporary exposure ta a magnetic field. ) Two new methods for studying the hysteresis af recording media have been developed. The first methad is based upon a generalization af the Wohlfarth re- lation. With this methad, the interactions between the magnetic units in the medium can be studied. In the original methad, the effects af these interactions could be compared for only two initial states af the material, the bulk-erased and the entirely-magnetized state. The generalization en- ables a comparison ta be made between any two initial remanent states. Application af this new methad on a particulate tape sample has revealed the complexity af the interactions. The second methad cap be used ta study the intrinsic magnetic proper- ties af recording media, irrespective af their easy magnetization direction. The demagnetizing fields that may arise from components af the magne- tization perpendicular ta a sample are compensated in this methad. The methad has been applied ta a sputtered Co-Cr medium and a Metal Evap- orated Go-Ni-O medium. The results show that there is a remarkablë re- semblance between these two media in respect of the angular dependence of their coercivity. 2: Modelling the hysteresis For an accurate simulation of the magnetic hysteresis process, the well- known moving-Preisach and Stoner-Wohlfarth hysteresis models have been combined. With a simple modification, a fast numerical implementation of the combined hysteresis model has been obtained. The magnetic interac- tions, incorporated in a fashion similar to that used in the moving-Preisach model, can account for many of the details of the minor loops, including the anhysteretic susceptibility. The vector properties of the new hystere- sis model are the same as those of the Stoner- Wohlfarth model. With the combination of the two models, the hysteresis 100ps of different recording media, including the Metal Evaporated Go-Ni-O medium, can be repro- duced very accurately. Only the very fine details of the hysteresis process, involving the interactions between the magnetic units, are not simulated correctly. This has been demonstrated with the new method for studying these interactions. 3: Experimental study of the recording process Much about the recording process itself can be learned from specific record- ing experiments. The recording process on thick particulate media has been investigated by applying a new analysis method. This method is based upon a scaling argument. The length of a magnetic transition is assumed to be proportion al to the magnetic potential over the gap of the recording head. With this scaling-based analysis, the spacing 1088 can be estimated from a series of frequency responses measured at selected yalues-of the recording current. The method yields a reasonably accurate indication of the head- to-tape spacing. However, the indicated value is somewhat sensitive to the range of recording currents usesl in the experiments. Application of the method to different particulate recording media shows that the Hi8 MP tape has the best intrinsic frequency response, whereas un-oriented barium ferrite media have a relatively high optimal recording depth. Entirely different phenomena are encountered when thin Go-Gr me- dia with a perpendicular easy axis orientation are rec?rded with a ring head. Additional minima appear in the frequency response of these media. The occurrence of these minima can be described with a double-transition model, in which the transition recorded at the leading edge of the recording head is only partially erased at the trailing edge. 4: Modelling the recording process The basic principles of the recording process can be understood from simple recording models. For a more detailed simulation of the recording process, the combined hysteresis model has been implemented in a numerical record- ing model. A modified Newton iteration scheme reduces the computation time considerably. The scaling-based analysis has been evaluated with this recording model. The assumptions on which the analysis was based proved not to be justified. However, owing to a cancellation of errors, the value of the head-to-tape spacing that results from the analysis is still within 25% of the actual value. The recording model is still under development. Simulations with the current version show that the basic recording characteristics of very differ- ent tapes can be reproduced within 2 dB. However, similar results can be obtained with simpIer models. Future research will have to show that this new recording model is better suited for also reproducing more complex phenomena such as overwrite and bias recording.