ANALYSIS OF MAGNETIC DOMAIN STRUCTURES IN CoCr SPUTTERED FILMS USING DIFFERENTIAL PHASE CONTRAST ELECTRON MICROSCOPY

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ANALYSIS OF MAGNETIC DOMAIN STRUCTURES

IN CoCr SPUTTERED FILMS USING

DIFFERENTIAL PHASE CONTRAST ELECTRON

MICROSCOPY

J. Chapman, D. Rogers, J. Bernards

To cite this version:

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JOURNAL DE PHYSIQUE

Colloque C8, SupplBment au no 12, Tome 49, dkembre 1988

ANALYSIS OF MAGNETIC DOMAIN STRUCTURES IN CoCr SPUTTERED FILMS

USING

DIFFERENTIAL PHASE

CONTRAST

ELECTRON MICROSCOPY

J. N. Chapman (I), D. J. Rogers ( I ) and J. P. C. Bernards (2)

(I) Department of Physics and Astronomy, University of Glasgow, Glasgow G l 2 8QQ, G.B. (2) Philips Research Laboratories, PO Boz 80000, 5600JA Eindhoven, The Netherlands

Abstract. - The microstructural and micromagnetic properties of GoCr films have been studied in the scanning trans- mission electron microscope. For a sample deposited on a polyester substrate at 20 OC domains were found to extend through the thickness of the film and have a mean width approximately four times the columnar diameter.

1. Introduction

The magnetic properties of CoCr film suitable for perpendicular recording depend on the physical microstructure of the film. This, in turn, is strongly in- fluenced by the conditions under which it is grown. We have used a vibrating sample magnetometer and a torque magnetometer t o determine the macroscopic magnetic constants of the material, and a range of electron microscopy techniques to study both its micromagnetic and microstructural properties. In this paper we describe the electron microscopy techniques which have proved most useful and present results from a film of composition CorgCr21 deposited onto a polyester substrate at 20 OC. In the final section we combine the results from the macroscopic and microscopic measure- ments to obtain a value for the domain wall energy.

2. Experimental details

400 n m thick films of CoCr were grown by RF sput- tering onto polyester (PET) substrates as described

Whilst poor by general TEM standards this represents

a very high resolution for imaging magnetic structures. The detector in DPC microscopy is split into quadrants and images are produced by taking difference signals from opposite segments. In principle, such images al- low both the magnetisation distribution within and the stray field distribution beyond the film surfaces to be determined. Figure 1 shows a relatively low magnifi- cation DPC image mapping induction components in a vertical direction. Stray fields, denoted by regions of high and low intensity, are clearly visible beyond the upper and lower surfaces of the cross-section. Further- more, their distribution above and below the section

previously [I]. Such specimens are too thick for direct

examination in the transmission electron microscope ,'

m

"

so cross-sections through the films were prepared by

argon ion beam thinning. These were examined using

.

C

an extended VG HB5 scanning transmission electron , I I

52C'nm

microscope (STEM) equipped with a Link Analytical AN10/95S system suitable for acquisition and processing of both images and spectra. Bright field imaging and convergent beam diffraction techniques confirmed that the material had a textured columnar structure which extended throughout the entire thickness of the film. In the case of the sample deposited on a substrate at 20 OC the mean column diameter was determined to be 40 nm.

The magnetic microstructure of the material was in- vestigated using the differential phase contrast (DPC) mode of Lorentz microscopy [2, 31. For domain stud- ies, as undertaken here, the specimen must be in magnetic field free space and under these conditions the minimum probe size (and hence resolution) is 3 nm.

Fig. 1. - DPC image of CoCr cross-section showing com- ponents of stray field in a vertical direction.

is very similar, providing strong evidence that vertical domains run through the entire thickness of the CoCr film. In the thin cross-sectional specimen in figure 1

the domain width varies between 35 and 480 nm, the mean value being 160 nm.

Whilst stray fields can readily be seen in figure 1

no useful information can be obtained from within the cross-section itself. Here there is strong crystallo- graphic contrast from the columns which conceals the

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C8 - 1966 JOURNAL DE PHYSIQUE

magnetic signals of interest. Furthermore, the high amplifier gains required to reveal the small difference signals from the stray fields result in saturation effects within the sample so that even the columnar structure is barely distinguishable. Thus a single DPC image fails to provide direct information on the relation between the columns and the domain structure. Figure 2

shows how this difficulty can be overcome. The same signals from the four sectors of the quadrant detector can be combined in different ways so that as well as forming DPC images from highly amplified difference signals it is possible to sum the signals from the four segments and so form a normal bright field image. The sum signal is passed through a separate amplifier and so an image revealing the columnar structure, which is in perfect registration with the DPC images, is produced. In figure 2 we have used the image processing system t o form a composite comprising a DPC image beyond the edges of the section and a sum image within the section itself. Thus, both the columnar structure and the micromagnetic structure (as inferred from the stray fields) are revealed together. Examination of figure 2 suggests that whilst some domain boundaries appear t o coincide with column boundaries others do not. Further images are being collected and analysed t o try to determine whether or not the positions of the two are related.

3. Domain wall energy

From a knowledge of the saturation magnetiza- tion, the uniaxial anisotropy constant, the width of the thinned CoCr cross-section and the mean domain width, it is possible to estimate the domain wall energy following Kooy and Enz [4]. Using values for the four quantities of 460 k ~ . m - l , 125 k ~ . m - ~ , 300 nm and 160 nm respectively a wall energy of 2.6 m ~ . m - ~ is 'obtained. It should be noted that in the case of a thin cross-section in zero field the expression of Kooy and Enz reduces to the Kittel formula [5] but for a p*

factor. Neglect of the p* effect yields a value for the wall energy of 3.1 m ~ . m - ~ .

4. Conclusions

A combination of imaging techniques on a STEM provides a powerful and direct method of revealing mi-

cromagnetic and microstructural information. From the images shown in figures 1 and 2 it is clear that there is a modest variation in domain width but that almost all the domains are rather bigger than a single column diameter. The mean domain width is approximately equal to four column diameters. These observations suggest that there is significant exchange coupling at column boundaries. By combining the microscopic information with experimentally determined macroscopic magnetic constants a value of 3 m ~ . m - ~ is obtained for the wall energy.

Acknowledgements

We would like to thank Mr. S. McVitie for his help with the STEM imaging and the SERC for provision of equipment.

[I] Bernards, J. P. C., Schrauwen, C. P. G., Luitjens, S. B., Zieren, V., De Bie, R. W., IEEE Trans.

Mag. MAG-23 (1987) 125.

[2] Chapman, J. N., Morrison, G. R., J. Magn. Magn. Muter. 35 (1983) 254.

[3] Chapman, J. N., McVitie, S., McFadyen, .I. R., Scanning Microscopy Suppl.. 1, Ed. 0. Johari

Fig. 2. - Composite image showing components of stray (AMf 0%lare, chicago), i987, p. 221.

field in a horizontal direction outside the cross-section and

[4i

K O O ~ ,

'.,

U., l5 7.