1,721,112 research outputs found
Thin-film magnetoresistive absolute position detector
No full textThe subject of this thesis is the investigation of a digital absolute posi- tion-detection system, which is based on a position-information carrier (i.e. a magnetic tape) with one single code track on the one hand, and an array of magnetoresistive sensors for the detection of the information on the other. The sensor array consists of a permalloy (Ni0.81Fe0.19) strip which is electrically sub-divided into adjacent sensor elements by means of cross- contact leads (Mo-Al). The sensor response on a magnetic field in the plane of the strip is a result of the anisotropic magnetoresistance effect in thin ferromagnetic films. The permalloy strip has a thickness of about 50 nm and a width in the range of 10 to 30 um. The central positions of the sensor elements are located at mutual distances of 50 to 200 um, which is dependent on the bit period in the magnetic tape. The plane of the sensor strip is positioned perpendicular to the magnetic-tape plane. The strip axis and direction of movement of the permalloy strip are lengthways along the magnetic tape. This tape is recorded with a so-called pseudo-random bit pattern, having the property that each arbitrary selected set of n adjacent bits is unique. Depending on the actual position of the sensor head, a special bit combination is detected by a sensor array con- sisting of n elements. This digital word can be decoded to a position coordi- nate. The thesis reports on the feasibility of the position-detection system on the basis of experiments with specially developed measurement heads. In introduc- tory observations the backgrounds, the theory and the design choices come up for discussion. Attention is paid to the modelling of the magnetization distribution in the permalloy strip, which is excited by homogeneous or inhomogeneous magnetic fields. Next the conversion of the magnetization distribution into magnetoresistance and so-called planar-Hall voltages is treated, resulting in a clear choice for the application of the former in a position-detector array. The potentialities of a silicon Hall-sensor array as an alternative for permalloy sensors is considered. Next the basic set-up of a complete position-detection system is discussed, including the generation of the applied position code, as well as methods to cope with problems like the sensor arrays being non-ideally positioned with respect to the bit centres and the potentialities of the application of interpolation schemes. For practical reasons commercially available, longitudinally magnetized tapes are used as a position-data carrier in an experimental system. In view of the computations of the sensor behaviour in the stray field of such a tape, the foundations of a computer simulation model are discussed, resulting in both a practical "simple" model and a more-difficult sophisticated model. Next the details of the realized sensor-chip design are outlined. The realization of the sensor head has been a very demanding activity of this research project; two subjects are discussed in detail. First the realization of the required low interface-resistivity value between permalloy and conduc- tor film is discussed. Second the positioning of the sensor strip at the very edge of the substrate (distance in the order of micrometers) is described. The measurements chapter describes the applied measurement equipment, the various properties and dimensions of the prototype sensor structures and the set-up of the actually performed measurements, i.e. the registration of the sensor-element responses in the stray fields of magnetic tapes. The results are evaluated for sensor elements and tapes with various properties, in the light of the so-called detection distances which are determined with the help of "eye patterns" composed on the basis of worst-case responses. The lower limit of the bit period is found to be in the order of a few hundred micro- meters. Conclusions are first addressed to the performance of the computer model. It is found that the simple simulation model results in reasonably accurate approximations of the sensor responses and detection distances, if the computations are carried out for relatively small values (t ~ 10 um) of the separation between the sensor strip and the magnetic-tape surface. A careful extension of the range of the experimental parameters in the computer model does not result in essentially better computed figures for the minimum bit period (i.e. resolution) of the position-detection system: Prospects of improved performances could possibly arise by the introduction of an all- embracing second design round. It is noted that the final resolution of the position-detection system, being in the order of magnitude of 100 um, can be improved by a factor of 10 to 100 with the application of interpolation techniques. In this case the increase of the accuracy which, in the first instance, is in the order of magnitude of the bit period, will be dependent on the actual quality of these techniques. Finally, the performance of a position detection system based on the alternative silicon-Hall technology is estimated to be of the same order as the permalloy-based version. In an appendix attention is paid to measurement techniques which, based on magnetoresistance sensors, at the time were applied for analysis of the form and magnitude of the compact head field of recording write heads. The utilization of the discussed absolute position-detection system should be based on the possibilities to economically realize a (mass)product with relatively good performance with respect to the established (mostly optical) systems. Having this aim in view, a special-purpose signal-processing chip should be developed, among other things, and integrated with the thin-film sensor structure. However, extensive and multidisciplinar activities of this kind are found to form a certain barrier for the utilization of the design
Theoretical and experimental investigations on short-wavelength recording
Full text linkIn this thesis CoCr and CoCr/NiFe double layers are studied by ferromagnetic resonance. The coercivity and the initial susceptibility of these layers are measured. An approximation of the Kooy and Enz model which is suitable for calculating the initial suceptibility is presented [3.36]. A theoretical model [3.34] for magnetization curves which includes hysteresis is given. Neutron depolarization [4.5] is used to check a model [4.1] which describes the interaction between the two layers of the double layer. Chapter 1 gives a brief introduction on perpendicular recording and describes some of the properties of CoCr. The preparation of our samples is explained. A short account of the apparatus used to measure the magnetic parameters is included. Chapter 2 is divided into three parts. Part I gives a general introduction on FMR. The general theory, which is elaborated further on in this chapter for specific application for CoCr and NiFe, is given. Part II describes FMR measurements carried out on single NiFe layers and NiFe layers which form part of a double layer configuration of CoCr/NiFe. An X-band spectrometer is used. Both the resonant field Hr and the linewidth AH are measured for a range of values of ~, the angle between the applied field and the normal to the film. Theoretical curves for ~(~) are calculated. A new low field, low frequency, theoretical model [2.18] for AHC~) is presented. These theoretical curves also enable us to study the effect of each individual magnetic parameter on Hr(~) , AHC~) and AHCHr). The magnetic parameter ~ can be obtained from experimental values of Hr(~). Using this model the damping parameter « can be obtained from experimental values of AH. In part III HrC~) of both CoCr and NiFe in the double layer and of single CoCr layers is measured for ~ = 00- 90°. A Q band spectrometer is used. In order to obtain information on the origin of the different CoCr peaks these measurements are also carried out on the single CoCr layers before and af ter various etching steps. Theoretical curves for H(~) are calculated and a new high field model [2.32] is presented enabling us to determine the magnetic parameters of the different layers of microstratified CoCr. FMR measurements were also carried out on single CoCr layers using an X band spectrometer [2.33]. A peak due to the uniform precession mode and a resonance peak in the zone where the film is still unsaturated was measured. In chapter 3 the hysteresis loop and in particular the perpendicular coercivity H c and the initial susceptibility or the initial slope T of single CoCr layers and CoCr/NiFe double layers are studied. This provides information on the possible interaction between the two layers, on the origin of coercivity and on the domain structure. H c of both magnetron sputtered CoCr layers and CoCr/NiFe double layers, as well as the coercivity of the CoCr layer of the double layer is measured. Hysteresis loops are calculated using a modification [3.34] of the Kooy and Enz model [3.5] to include hysteresis, using the friction force which opposes domain wall movement as a source of hysteresis. The initial susceptibility T of single and double layers is also measured. A low field approximation [3.36] of the Kooy and Enz model gives that T-l is proportional to t-l/2 where t is the thickness of the fïlm. When applying this model to RF sputtered CoCr films there is a difference in behaviour depending on the thickness of the film. Chapter 4 gives the response [4.1] of the NiFe layer to the stray field of the domains of CoCr i.e. the magnetostatic interaction in a double layer consisting of a medium with horizontal anisotropy and one with vertical anisotropy. In order to study the interaction experimentally, neutron depolarizat1on experiments [4.5] were carried out on double layers and the corresponding single layers. Neutron depolarizat1on enabled us to study the interaction between the NiFe and the domains of CoCr. Furthermore the coercivity of the NiFe of the double layer ls compared with that of the single layer
On the hysteresis and the recording process in magnetic media
No full textThe 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
Simulation of the perpendicular magnetic recording process
No full textThis thesis describes a simulation modelof the complete perpendicular magnetic recording process, from write current to readback voltage. Gontradictory to most models published in literature, it is neither analytical nor iterative. In the model of this thesis, the actual physical process is piecewisely linearized during every stage of recording. Extensive attention is paid to the influence of the so-called "image-charge effect", caused by the presence of a head and a keeper layer in the vicinity of a magnetized recording layer. Part I gives an introduction to the problem and to the topic of image charges. The assumptions determining the validity of the model are described and accounted for. Also, a theoretical derivation concerning the "switching criterion", the level in the recording layer at which self-consistency is assumed, is given. In part II, the actual model is set up. First, the recording of a single magnetization reversal in a DG pre-magnetized recording layer with no head-to-medium motion is analyzed and modeled. Then, the recording layer is shifted along the activated head, which causes a relaxation of the original magnetization distribution. This part of the process, so far predominantly neglected in literature, is described in detail. Also, the influence of a varying head field is investigated to complete the modeling of the write process. Finally, the readback process is modeled using the well-known theorem of reciprocity. The effects of variations of the model's parameters are described in part III. The image-charge effect is treated in a quantitative manner and the influence of the obliquity of the recording layer's intrinsic hysteresis loop is investigated. Also, the influences of the bit density and the head field to coercive field ratio are described. Finally, the thicknesses of the recording layer and the air gap are varied to find that, as expected, the air gap should be chosen as small as possible. Some mathematical derivations concerning demagnetizing fields, image-charge fields and a conformal mapping have been put into appendices completing the thesis
A slider motion monitoring system, measuring contact and friction forces in rigid disk storage devices
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