W.A.P. Nicholson

Coherent magnetic imaging by TEM

A novel transmission electron microscope optimised for the study of magnetic material is described. Using this instrument, a new method for revealing magnetic structures, coherent Foucault imaging, can be realised. Images appear directly in the form of magnetic interferograms and provide immediate access to a quantitative description of the induction distribution across the specimen. A simple analytical approach is given to the underlying theory and the main features are confirmed by computer modelling. Experimental images of small regular permalloy elements illustrate the power of the technique for studying domain structures. >

Near-simultaneous dual energy range EELS spectrum imaging.

A system that allows the collection of the low loss spectrum and the core loss spectrum, covering different energy regions, at each pixel in a spectrum image is described. It makes use of a fast electrostatic shutter with control signals provided by the spectrum imaging software and synchronisation provided by the CCD camera controller. The system also allows simultaneous collection of the X-ray spectrum and the signals from the imaging detectors while allowing the use of the existing features of the spectrum imaging software including drift correction and sub-pixel scanning. The system allows acquisition of high-quality spectra from both the core and the low loss regions, allowing full processing of the EELS data. Examples are given to show the benefits, including deconvolution, absolute thickness mapping and determination of numbers of atoms per unit area and per unit volume. Possible further developments are considered.

In-situ magnetising experiments using coherent magnetic imaging in TEM

Using a novel electron microscope which has been optimised for the study of magnetic materials a new technique for investigating themicromagnetic structure of thin films has been developed. Images formed using this technique display very clearly the induction distribution within a specimen and examples are given from micron sized elements of permalloy. Initial results are presented from in-situ magnetising experiments which utilise the objective lens field of the instrument.

Characterisation Of MFM Tip Fields By Electron Tomography

Lorentz electron microscopy has been applied to the study of the magnetic field distribution from MFM tips. Data acquired by this technique has been used to reconstruct the field distribution from the MFM tip by tomography. Initial results have been obtained from commercial tips which have been magnetised along the tip axis. The reconstructed field is consistent with the predicted form.

A new design of specimen stage for in situ magnetising experiments in the transmission electron microscope.

A new stage for carrying out in situ magnetising experiments in the transmission electron microscope has been designed, constructed and tested. The principal advantages of the stage are that it delivers horizontal fields with negligible perturbation to the illumination and is suitable for operation in pulsed or continuous field mode. Details of its performance, including field calibration, are given. The paper concludes with a description of where the stage is likely to be of most use.

Magnetisation distributions in thin film recording heads by Type II contrast

The magnetic domain structure of the polepieces of thin-film magnetic recording heads and its variation with drive current and frequency are of interest in understanding the read/write properties of the head. The authors describe the setting up of the synchronous detection mode of type II backscattered electron contrast in a 200-kV electron microscope and its application to the study of domain wall movement in the P2 polepieces of heads with and without the 15-20- mu m-thick alumina overlayer in place. Extension of the method to allow evaluation of the changes in magnetization directions under AC drive conditions is described. The authors also demonstrate a novel technique which gives information on the remanent state of the domain structure in the P2 polepiece and its changes under DC drive conditions. It is concluded that these improvements in technique, taken together with the capability of the synchronous detection method to provide information on changes of magnetic structure with depth, mean that this experimental tool will be invaluable in its own right as well as complementary to the Kerr imaging methods. >

Focused ion beam irradiation of ferromagnetic thin films in the presence of an applied field

A stage has been constructed to supply a variable strength in situ magnetizing field for use in a focused ion beam system. Ion irradiation experiments were performed in the presence of the magnetic field in order to investigate its effect on the localized exchange bias field in thin CoFe/IrMn bilayers. Squares with dimensions 10 ? 10??m2 were directly irradiated by Ga+ ions of energy 30?keV at doses between 1 ? 1013 and 1 ? 1015?ions?cm?2. TEM studies of the magnetization reversal behaviour of the squares showed that bias field reversal could be achieved with the maximum achievable bias field strength being > 50% of that along the original direction.

TEM studies of the switching characteristics of small permalloy elements as a function of field orientation

We have studied how the switching field of permalloy elements with sub-micron widths varies as a function of the orientation of the applied field. Switching fields for individual elements were determined using Lorentz microscopy, the studies being facilitated greatly by the use of a newly designed magnetizing stage. Brief details of the stage are given. For elements with widths ~400 nm, switching fields increased modestly as the angle the applied field made with the long axis of the element increased. In contrast, as the element width was reduced to 160 nm, little variation with field orientation was observed. The results suggest that the residual domain structures close to the ends of the elements played a major role in determining their switching characteristics, even for the smallest elements investigated here.

In-situ Lorentz microscopy of magnetic nanostructures

In this paper, we describe results from a newly developed stage which comprises two wires situated close to the sample which give a field parallel to the thin film plane and which in principle should give no net deflection of the electron beam. This stage can be used for steady or pulsed fields and is specifically designed for use with TEM silicon nitride membrane substrates.