Electron Beam Tomography of Recording Head Fields

The quantitative evaluation of inductive recording head fields has been achieved by electron beam tomography. The differential phase contrast (DPC) mode of Lorentz microscopy implemented on a 200 kV scanning transmission instrument provides a novel technique for recording head field investigations and in particular the acquisition of the experimental data sets required for field reconstruction. The absolute determination of the recording head field has been obtained by calibration of the DPC image contrast.

This thesis starts with a brief discussion of the basics of ferromagnetism and the application of magnetic materials in magnetic recording technology. Development trends and some recent advances in magnetic recording head design are also discussed.

The second chapter gives a review of two dimensional techniques developed previously for magnetic stray field measurement and particular attention is given to the DPC Lorentz microscopy, since it is the experimental basis of 3D field characterisation by means of electron beam tomography.

The fundamental principles and the realisation of electron beam tomography for recording head field study are discussed in chapter 3. The two reconstruction algorithms of the RTM and the ART are introduced. The emphasis of this chapter is put upon the derivation of the magnetic field vector ART algorithm and the experimental implementation of the electron beam tomography using DPC Lorentz microscopy based on the modified JEOL 2000FX (S)TEM. The acquisition of the experimental data sets for tomographic reconstruction is also described in this chapter.

The ART tomography program described in chapter 3 is tested and compared with the RTM in chapter 4. The performance of the tomography programs are evaluated by simulation of DPC data sets for a model thin film head using different reconstruction parameters. It is confirmed by these simulations that the ART and the RTM can produce satisfactory reconstruction of recording head fields. Reconstructions using fewer projections by the ART and using truncated input data sets by the ART and the RTM can still provide reasonable information on the major field distributions; this situation is encountered in practice. The computer simulations also provide information on the suitable reconstruction parameters which may be adopted in the experimental reconstruction of recording head fields.

In chapter 5 the electron beam tomography method is applied to study the stray field from inductive thin film heads. A novel method of mounting the thin film head for data collection makes it possible to reconstruct the stray field on a plane ~0.25 m m from the head gap. By etching part of the alumina present in the vicinity of the poletips, it has proved possible to identify magnetic flux leakage from regions of the poles, other than the polegap. The saturation behaviour of the writing field can also be obtained by studying the integrated stray field in the head gap direction for different dc driving currents.

Chapter 6 presents the experimental results from the study of the stray field from tape heads. The specimens used in this chapter are a pair of Metal-in-Gap write/ferrite read heads and laminated alloy film tape heads. Electron beam tomography and the DPC experiments can provide quantitative information on the stray field gradient and the half height of the field amplitude. The results obtained also show that DPC Lorentz microscopy is the most powerful tool to observe stray field defects, such as the secondary gap effect from the MIG head and the remanence effect from both the MIG and the laminated alloy film heads.

A method to calibrate the relative value of the DPC signal acquired from a quadrant DPC detector is described in chapter 7. The actual value of the electron beam deflection at certain point(s) on the DPC image is measured in-situ as part of the DPC experiment. From the calibration data set obtained, which is consistent with the theoretical analysis of the detector response, the absolute determination of the 3D stray field is achieved.

Conclusions and suggestions for further work are given in chapter 8.