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Journal of Magnetism and Magnetic Materials 149 (1995) 418-424

Visualisation of magnetic domain structures through the interaction of their stray fields with magneto-optic garnet films R. Atkinson a,*, N.F. Kubrakov b, M. O'Neill a, p. Papakonstantinou a a Department of Pure and Applied Physics, The Queen's University of Belfast, Belfast BT7 INN, Northern Ireland, UK b Theoretical Department, General Physics Institute, Russian Academy of Sciences, 38 Vavilov St., Moscow 117942, Russia Received 6 February 1995; in revised form 7 March 1995

Abstract

A technique is described for the indirect visualisation of magnetic domain structures through the coupling of stray surface magnetic fields into a bismuth-substituted garnet layer. The resulting magnetisation distribution within the garnet layer is visualised optically using the polar Kerr effect. A small single-crystal platelet of barium hexaferrite is used to demonstrate the method and the complex, field-induced magnetisation patterns in the garnet are discussed qualitatively. The effect of a small bias field on the magnetisation structures is also considered and has been shown to be a means of simplifying the complex images which are produced. This indirect magneto-optical method of visualising magnetic domains is both effective and convenient, although the ultimate limits of resolution for the technique remain to be determined. I. Introduction It is well known that the observation of domain structures provides a means of gaining a deep insight into the properties of magnetic materials and devices. Consequently, it is not surprising that considerable efforts have been made in the past to develop a variety of techniques of domain observation [1]. The best known of these are the Bitter technique [2], transmission and scanning electron microscopies [3,4], magneto-optic visualisation [5,6] including scanning laser microscopy [7], X-ray topography [8] and most recently the magnetic force microscope [9]. In most instances those techniques require considerable sample preparation and more importantly de-

* Corresponding author. Fax: + 44-1232-438918.

mand expensive equipment in order to produce high quality images. In contrast, the magneto-optic technique is relatively cheap, can deal with most magnetic orientations, is non-invasive, non-contaminating and can address both thin films and bulk sampies. It also has the specific advantage of allowing dynamic studies to be made of domain behaviour in varying applied fields. The major drawback of this method, however, is the limited resolution determined by optical diffraction and the requirement that the material have significant magneto-optic sensitivity at a convenient region of the electromagnetic spectrum. For this latter reason many materials fail to have their magnetic structure revealed by magneto-optic methods and even where there are, their magnetooptic properties may be too weak to provide good photographic contrast between areas of different magnetic orientations.

0304-8853/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved

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R. Atkinson et al. /Journal of Magnetism and Magnetic Materials 149 (1995) 418-424

In this paper we will describe a method of visualising magnetic domain structures based upon a magneto-optic technique of mapping static inhomogeneous magnetic fields [10,11]. Similar techniques have been used to map magnetic flux distributions in high-temperature superconductors [12-14]. In principle, the magnetic domain structures in the sample are indirectly revealed through the interaction of the stray fields above the sample with the domain structure of a magnetically soft thin film of bismuth substituted iron garnet placed in close proximity. The magneto-optic properties of the garnet films are so large that the effects of this interaction are easily seen by conventional optical microscopic techniques [15]. An additional advantage of the method is that the application of a small bias field can be used to study the nature and magnitude of the stray fields as well as providing a means of simplifying the images produced by the method. In the discussion of the observations reported here we assume that the magnetic structure of the sample, a thin single-crystal platelet of barium ferrite, is not perturbed by the presence of the garnet film or the small applied bias fields. In addition, we assume that there is no problem in relation to the resolution of the magnetic detail which is limited only by the optics. This point will be elaborated further. The interest in this technique of domain observation has four-fold importance. First, it is cheap and quick to utilise. Second, it requires no special magneto-optic qualities for the sample itself. Third, it may eventually provide additional quantitative information through its sensitivity to applied bias fields. Finally, the details of the micromagnetic structures provide a challenge to produce a mathematical model of the new stable magnetic structures which form in thin garnet layers subjected to a variety of complex spatial distributions associated with stray magnetic fields above the domain structures.

2. Sample preparation The barium hexaferrite (BaFe12 O 19) sample whose domain structures have been visualised was a bulk single crystal grown by a slow cooling flux method [16]. From this crystal a platelet, with surface area 30 mm 2, was cut perpendicular to the hexagonal (0001)

419

T $

P

E

A

~E

Fig. 1. Experimental arrangement for indirect visualisation of domain structures. S: optical source, B: beam splitter, P: polariser, A: analyser, T: sample, G: garnet layer, O: microscope objective, E: eyepiece, C: solenoid.

axis and thinned down to a thickness of 25 ixm by mechanical polishing. The important magnetic parameters such as saturation magnetisation (M s = 352 kA/m), coercivity ( H c < 398 A / m ) and anisotropy field ( H k = 1.3 X 103 k A / m ) were determined by VSM. The domain patterns of barium ferrite in this form have been studied extensively in the past mainly by the Bitter technique or by SEM [17]. The general pattern of the sample in the form described above is the usual labyrinthine stripe pattern.

3. Experimental arrangement The experimental arrangement shown in Fig. 1 is quite conventional, consisting of a high-quality polarising metallurgical microscope operating in reflection at normal incidence. The sample and visualising film were mounted on the objective stage, together and within a solenoid capable of providing small homogeneous bias fields perpendicular to the film surface. The visualising film was a monocrystalline Bi-substituted iron garnet layer on a GGG substrate with film thickness 4.5 Ixm, saturation magnetisation 6.37 k A / m , coercivity < 80 A / m , and anisotropy field 2 x 10 4 k A / m perpendicular to the plane of the film. The magnetic structures in the garnet film were observed in reflection using the polar Kerr effect. To improve the sensitivity of the method the film was backed by an opaque aluminium reflector. Besides

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R. Atkin~on ct al. /.hmrnal of Magnetism and Magnetie Materials 149 (1995) 418-424

increasing the observed Kerr effect, this layer also eliminated any deterioration in image quality through multiple reflections of the radiation with the actual sample itself. Because the stray fields decrease rapidly with distance from the sample surface it is important that the sample and garnet film be placed in close proximity to each other. It is usual in magneto-optical observation of domains using the polar Kerr effect to be dealing with two anti-parallel states of magnetisation. The resulting images consisting of light and dark domains. In this case however, the effects of the stray fields, is to produce four principal magnetised microscopic regions in the garnet layer. These are two anti-parallel states perpendicular to the film plane and two antiparallel in-plane (or near in-plane) components. At normal incidence it is not easy to distinguish between the two in-plane states though it is possible to provide contrast between the two perpendicular states and those that are in-plane. Because of the rapid variations of the stray fields other magnetisations states also exist forming smooth transitions between the various regions. The extreme example of course being a domain wall. However, as we shall see, the three important states are + M ± , - M l and _MII, where the signs refer to the magnetisation direction with respect to some Cartesian coordinate system and the subscripts refer to directions relative to the plane of the film surface. By suitable adjustment of the orientation of the analyser we may distinguish between these three orientations. In the optical micrographs shown in Fig. 2 the analyser was adjusted so that + M . regions appear white, - M ± appear grey and ___MII always appear dark.

(a)

I

I 100 ~m

(b)

I

I 100 p,m

Fig. 2. Magnetic structures visualised in the garnet layer under the influence of stray fields from the domain structure of the sample. (a) n 0 = 0, (b) n 0 = 1.3 k A / m .

4. Observations and qualitative explanation The magnetic structures observed are shown for increasing bias field H 0. In Figs. 2(a) and (b) there are two major regions. One corresponds to the natural domain structure of the garnet layer itself, having a coarse maze domain pattern of domain stripe width ~ 14 Ixm and contrast consists mainly of white and grey regions separated by narrow (black) domain walls. The major areas of the micrographs show the

finer domain structures resulting from the stray-field interactions with the garnet. Here the domain widths are much smaller ( ~ 3.5 p~m) and there exist regions, of comparable widths, showing white, black and grey scales. The boundary between these two major regions coincides with the sample edge. To establish the integrity of the stray-field-induced patterns the position of the garnet layer was changed, switching it back and forth and even re-

R. Atkinson et al. /Journal of Magnetism and Magnetic Materials 149 (1995) 418-424

moving it totally from the field of view. Whilst this had a profound effect on the natural domain pattern of the garnet, the stray-field-induced pattern remained constant. This simple test is of considerable importance since in this case it removes any serious doubt concerning any ambiguities regarding the reliable transfer of the stray-field pattern to the garnet. We emphasise that it remains to be determined what are the limitations of resolution of this transfer mechanism, particularly in the case of very small domains. It is fairly clear what will happen in the case of large domains and it is likely that such situations may be readily interpreted. Having established the reliability and consistency of the stray-field-induced pattern it remains to interpret its significance. In this respect it is helpful to realise that the domain structure of the sample is typical of thin-film media having large perpendicular anisotropy and is of a labyrinth-type of structure as

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revealed by alternative techniques [17]. In this respect the natural domain patterns of the sample and visualising garnet are quite similar, though not in size. For simplicity and for the sake of subsequent discussions it is convenient to regard the intrinsic domain pattern of the sample as a set of stripe domains as shown in Fig. 3(a). Though they are intuitively obvious, we calculate the stray-field distribution above the stripes in terms of both the perpendicular and parallel field components. These calculations were carried out on the assumption that the domain walls were negligibly thin compared to the average domain size and that the stripes are periodic and of infinite extent. The magnetisation within each domain was assumed to be homogeneous. Areas can be found in the micrographs of Figs. 2(a,b) which approximate to these simplifications. The stray fields are illustrated in Fig. 3(b) for

Domain Walls.~

isualising Film Mirror 3ample

X

(a)

Hy/Ms

z = 0.1 ixm

(b) Fig. 3. (a) Symmetrical periodic stripe domain structure in the sample and the possible magnetic structures in the visualising layer. Magnetisation in the black regions of the grey-black-grey and white-black-white transitions is shown qualitatively.(b) Normal and in-plane componentsof the stray field abovethe sample normalisedby the saturationmagnetisation Ms. Samplethickness 25 p,m, period of the structure p = 7 p.m.

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R. Atkin.son et al. /.I,mrnal of Magnetism and Magnetic Materials 149 (1995) 418-424

distances of 0.1 and 1 p.m from the sample surface. Clearly, there is a large in-plane component above domain walls with significant perpendicular component directly above the centres of each of the stripe domains. The stray-field distributions are, of course, quite symmetrical in relation to the underlying domain pattern. As will be shown later, the large in-plane components of the stray fields produce magnetostatic traps into which domain walls, within the garnet material, may fall. Such traps have a significant effect on the subsequent domain structure of the garnet. The interaction of the stray fields above a stripe domain pattern with the garnet appears to lead to two equally likely stable magnetic configurations, resulting in two observable intensity distributions in the optical field. These can be seen in Fig. 2(a). One of the stable magnetisation distributions in the garnet is + M~ regions, above equivalent regions in the sample, separated by in-plane (or slightly negatively canted) regions above the - M x regions of the sample. The mirror image of this configuration is a - M I region above its equivalent in the sample, separated by in-plane (or positively canted) regions above + M j_ regions of the sample. These two equilibrium states are both shown schematically in Fig. 3(a) and it should be fairly obvious that, if it is energetically favourable for a domain wall to exist in the garnet, these two types of regions will coexist, separated by the 180° wall. The simple result of this is that there exist white, grey and dark regions in the field of view each of which coincide with an underlying domain. In addition, the existence and position of a domain wall in the garnet indicates the position and existence of a domain wall in the sample. This then is the basic configuration of the system though in practice there are subtle variations which manifest themselves through the complex domain structure of the sample. For example, one may observe junctions of three black stripes (Mll) within a grey sea ( - M j ) where, at the intersection, there may exist a reduced in-plane component with the consequent appearance of a 'blurred' white triangular region. This is illustrated schematically in Fig. 4(a). The application of a small bias field H 0 can enhance or eliminate these triangular regions very easily as can be seen in Fig. 2(b). Prior to their complete disappearance, these take on the form of a

(a)

Ha=O (b) HaO Fig. 4. Schematic diagram of visualised structure in the garnet layer for zero, and small bias fields.

magnetic 'bubble' though the magnetisation in the vicinity of these is unlike that of a normal cylindrical bubble domain (see Fig. 4b). It is clear that the strong in-plane stray fields above domain walls in the sample create magnetostatic traps for domain walls associated with the visualising film. In some cases such walls may cross into regions where the trapping energy is much reduced. We call these regions propagator fronts and are shown schematically in Fig. 4(c). On application of an appropriately directed field that part of the wall, not in a deep magnetostatic trap, can move along quite easily converting, for example, a white ( + M ± ) region into a black (+MII) region or vice versa. The same argument applies to grey ( - M ± ) and black (+MII) regions and the two processes are illustrated in Fig. 5. Through the movement of propagator fronts under the influence of a strong bias field (to the point which would normally saturate the intrinsic domain structure of the garnet) the overall stray-field-induced pattern is simplified by converting it totally into one of either of the two basic equilibrium configurations. The resulting situation is a clear representation of the domain configuration of the sample. It should be noted that such a pattern would indicate + M ~ (or - M ~) and MII magnetisation regions in the garnet which of course lie directly over + M 1 regions in the sample. Clearly, under

R. Atkinson et al. /Journal of Magnetism and Magnetic Materials 149 (1995) 418-424

HaO Fig. 5. Schematic diagram of visualised structure in the garnet layer indicating the movement of propagator fronts.

such circumstances there are no 180 ° domain walls in the garnet since these have been swept away by the applied bias field.

5. Discussion A simple technique has been described for indirectly visualising magnetic domain structures through the coupling of their surface stray fields into a bismuth-substituted garnet layer. The resulting magnetisation distribution within the garnet layer may be visualised using the polar Kerr effect to reveal contrast between areas of differing magnetisation. In the case presented, the technique has been applied to reveal the labyrinth-type of structures in thin single-crystal platelets of barium hexaferrite. The stray fields above such domains induce several types of fundamental magnetisation distributions within the garnet which are in magnetostatic equilibrium. The discovery of these equilibrium structures should provide an interesting challenge to confirm, through micromagnetic calculations, their detailed nature in unaxial media. Small bias fields applied to the garnet have been shown to provide additional information on the mi-

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cromagnetic structures induced in the garnet and on the nature of magnetostatic traps for domain walls and the dynamics of domain propagation. It has also been shown that the application of such bias fields can be used to simplify the general appearance of the complex magnetisation patterns. With further study of this technique it may be anticipated that the sensitivity of the stray-field images to bias fields may eventually lead to quantitative information concerning the direction and magnitude of the stray fields and ultimately to the magnitude and directions of the magnetisation distribution of the sample itself. The indirect magneto-optic visualisation technique has been shown to be an effective, quick and convenient means of revealing domains in media whose intrinsic magneto-optical properties are too weak to utilise conventional methods. The experimental equipment remains unchanged, requiring the same polarising microscope that would be used for ordinary magneto-optic methods. Finally, we make the point that the ultimate spatial resolution limits will depend upon the optical diffraction limit and the details of the magnetic properties of the visualising medium. In this respect further work is required to optimise the sensitivity of the method, to determine its physical limitations and to establish what, if any, quantitative information can be obtained.

Acknowledgements This work was supported by the NATO Linkage scheme, ref. no. 930126.

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[9] A. Kikukawa, S. Hosaka, Y. Honda and S. Tanaka, Appl. Phys. Lett. 61 (1992) 2607. [10] N.F. Kubrakov, Proc. Gen. Phys. Inst., Moscow 35 (1992) 136. [11] N.F. Kubrakov, SPIE 1126 (19891 82. [12] M.V. lndenbom, V.I. Nikitenko, A.A. Polanskii and V.K. Vlaskovlasov, Cryogenics 30 (1990) 747. [13] M.V. Indenbom, N.N. Kolesnikov, M.P. Kulakov, I.G. Naumenko, V.I. Nikitenko, A.A. Polanskii, N.F. Vershinin and V.K. Vlaskovlasov, Physica C 166 (1990) 486.

[14] A.A. Polanskii, M.V. Indenbom, V.I. Nikitenko, Y.A. Osipian and V.K. Vlaskovlasov, IEEE Trans. Magn. 26 (1990) 1445. [15] P. Hansen and J.P. Krumme, Thin Solid Films 117 (1987) 69. [16] Z. Simsa, P. Gornet, A.S. Pointon and R. Gerber, IEEE Trans. Magn. 26 (1990) 2789. [17] C. Kooy and U. Enz, Philips Res. Rep. 15 (1960) 7.