IEEE TRANSACTIONS ON MAGNETICS, VOL. 26, NO. 5, SEPTEMBER 1990 1527

W A L L TRANSITIONS IN PE-Y DOUBLE FILMS

E-NIEDOBA, H.O.GUPTA, L . J . H E Y D E m , I.TOW', I.B.PUCH?iLSKA Laboratoire de Magnetisme et d'0ptique des Solides C.N.R.S.- 1 p1.Aristide-Briand - 92195 MEUDON France * on leave from Institute of Physics, Czechoslovak Academy of Sciences, Na Slovance 2, Cs 18040 PRAHA 8'- Czechoslovakia

Abstract - The transition of twin walls into superimposed walls, and vice-versa, in two layer laminated Permalloy films have been observed. The pictures of walls and quasi- walls have been taken using high resolution Kerr effect. The transition occurs at reproductible values of magnetic field applied in the hard direction of the sample and the experiment indicates that it takes place by the motion of Bloch lines along the walls.

INTRODUCTION

In double magnetic films separated by a nonmagnetic layer, the flux closure between domain walls in the two magnetic films gives rise to the presence of Nee1 wall/ Nee1 quasi- wall configurations (e.g. ref.(!)). Using the recent development of the high resolution Kerr effect, both walls and quasi-walls could be observed in double Permalloy (Niso Fezo) layers with superimposed domainsc2'. The purpose of the present paper is to report transitions in double films between the two possible wall configurations, namely the superimposed NBel walls of opposite chirality and twin walls of the same chirality accompanied by quasi-walls (Fig.1).

0 I I , I Q

@ IC1 Q

( c )

Fig.1 - Possible arrangements of Nee1 walls in double layered films : a) superimposed walls with opposite polarity; b) shifted (twin) walls with the same polarity together with the induced quasi-walls (open arrows); c) top view of wall and quasi-wall of Fig.b.

The transition takes place under the action of an external field, Hh, applied in the hard direction, perpendicular to the domain walls.

Thus Hh is parallel and/or antiparallel to the direction of magnetization in the centre of the walls. As this direction plays a substantial role in the investigated transitions, the word "polarity" rather than "chirality" will be used in order to emphasize its importance. As shown in Fig.lb, c, the polarity of quasi-walls (open arrows) is opposite to that of the walls by which they were induced. If Hh is zero, the energy of the superimposed Nee1 walls (Fig.la) is approximately half that of the twin walls accompanied by the quasi-walls. The energy of superimposed walls increases with increasing H, while that of twin walls decreases, and the transition from the former to the latter configuration is expected.

EXPERIMENT& TECHNIQUE

The double Permalloy films, separated by a carbon layer, were prepared in a standard electron beam evaporator (Edwards 306A) in a vacuum of 1 x lo-= Torr, as in Ref.(2). A uniform magnetic field was applied during the deposition to induce uniaxial anisotropy. The thickness of the symmetric Permalloy layers, d,, ranges from 200 A to 1200 A. The thickness of carbon used as the nonmagnetic spacer was kept constant, d,=30 A . The characteristics of investigated samples are given in Table I.

Table I. H, - coercive field, H, - uniaxial anisotropy field

Sample d, ( A ) H, (Oe) Hk(Oe)

1 200 0.5 7.3 2 300 0.4 6 . 6 3 400 0 . 5 7.1 4 700 0.5 6 . 6 5 900 0 . 5 8.1 6 1200 0 . 6 7.3

The observations of NBel walls and quasi- walls were carried out using the high resolution longitudinal Kerr effect with digital contrast enhancement. In this technique, the contrast originates from the top film, since the penetration depth of the light is about 80 A . To detect wall contrast, the optical plane of incidence is set nearly normal to the wall. Depending on the wall polarity, a white or black contrast is observed. Fig.2a shows a wall with two segments of white and black contrast corresponding to a N6el wall with two segments of opposite polarity. This picture is a typical one for the superimposed wall configuration. In Fig.2b a wall with continuous black contrast is seen together with a white quasi-wall parallel to it.

OO18-9464/90/0900-1527$01 .OO 0 1990 IEEE

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I 10

( b ) Fig.2 - Walls and quasi-walls observed by

Kerr effect : a) single Nee1 wall; b) wall and quasi-wall (sample 1).

To study the wall transitions, a single twin domain wall is created before every experimental run by decreasing the field H, from the saturated state. During this slow decrease, a small field gradient along the easy axis is simultaneously applied. This easy axis field (H,) generated by two identical coils with currents I1 and I, running in opposite directions determines the position of the domain wall. Initially I1 = I2 and the wall is created in the middle of the sample. Then, if necessary, a relative chacge of the values I, and I, provides the displacement of the wall.

EXPERIMENTAL RESULTS

The wall transition was investigated by two methods. In the first method, the wall was stationary and the transitions were detected during a slow sweep of the hard axis field (H,). In the second method, the wall was moved over a large distance by varying an easy axis field, H,, while the hard axis field, H,, was kept constant. When there is no wall displacement, the wall transitions have a large hysteresis, similar to the transitions from the cross-tie to the N6el walls described by Middelhoek"' for double films with a thicker nonmagnetic layer. The width of the hysteresis may be reduced by application of a low ac field in the easy axis which produce small oscillations of the walls around their equilibrium positions. We observed, however, that large wall displacements were necessary to suppress the hysteresis of the transitions. Therefore the second method was used to determine the values of the transition fields. The results are given in Fig.3.

t

200 400 600 800 1000 1200

DJK)

Fig.3 - Field of wall transition versus thickness of the films.

i) Transition from twin to superimposed walls

Starting with the sample saturated in the hard direction, the field H,, was slowly decreased and a twin wall was nucleated with its polarity determined by direction of H,. Then, for a given value of H,, the wall was moved over large distances and its final configuration was detected. Fig. 4 shows both initial and final states of the wall for two different values of H,. Fig. 4a represents a situation typical for a high field value where a twin wall divided into segments persists after it has been moved. The wall contrast in the final position ( f ) is reversed as compared to the initial one (i) due to the use of a numerical image processingc4) (the initial image is taken as a reference from which the final image is substracted; the contrast of both images is enhanced and the difference is displayed on a TV screen). Fig.4~ shows the transition from twin (i) to superimposed wall (f) which occurs for field values lower than H, (Fig.3). The wall was moved several times in both directions over large distances before this transition occured. For all investigated specimens the transition does not take place at a well defined value of the H,, but there is a small range of the field (R between H, and H, in Fig. 3) in which both twin and superimposed walls are observed. A mixed type of walls (with superimposed and twin segments) appears also in this range resulting sometimes from repeated wall displacements (see Fig.4b).

ii) Transition from superimposed to twin wall

We have described above the way in which the superimposed walls were obtained. To induce the transition they were moved over large distances at a given value of H,, and their final state was detected using image processing. Fig. 5a shows a superimposed wall before (i) and after (f) the displacement for the field value lower than H, (see Fig.3). Fig.5b is characteristic of the transition range where, after several displacements, several short segments of twin wall areosserved. For fields higher than H, transitions from superimposed to twin walls occur (Fig.5~).

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Fig.4 - Transition from twin to superimposed wall : a) before transition; b) in the transition range and c) after transition (sample 2).

DISCUSSION

As expected from energy considerations, the superimposed walls are stable for low values of the applied field H,, while the twin walls are always created after saturation in the hard axis, and stay stable for higher field values. What is the micromagnetic mechanism of the wall transitions? It seems likely that these transitions take place by motion of Bloch lines. Bloch lines are present in the superimposed walls between the segments of the opposite polarity (Fig.2a). The theoretical approach to the description of the observed transition is based on comparison of the total energy density of the superimposed walls and that of the twin walls. The two wall energy densities were given by numerical solutions of the variational problem of each case. The computed values of the transition field are in good general agreement with the measured ones. However the computed dependence of the transition field on the film thickness is different from that observed experimentally. Refinement of the computation of the magnetostatic energy contribution and better knowledge of the coupling between the magnetization in the two ferromagnetic films is necessary.

( c ) Fig.5 - Transition from superimposed to twin

wall : a) before transition (sample 3); b) in the transition range and c) after transition (sample 5).

CONCLUSION

Field transitions between superimposed Nee1 walls and NBel twin walls accompanied by quasi-walls were directly observed and investigated. These transitions take place in fields of several tenths of H,. The suppression of wall transition hysteresis by large wall displacements allows us to conclude that the transition takes place by the displacement of Bloch lines.

Acknowledgements : The authors would like to thank A. HUBERT for helpful discussions and critical reading of the manuscript.

The work was supported by the European Economic Community (Contract No.STZ P-0382).

REFERENCES : 1. YELON, A., Physics of Thin Films, Academic

2. NIEDOBA, H., HUBERT, A., MIRECKI, B., and PUCHALSKA, I.B., J. Mag. Mag. Mat. 80, 379 1989.

3 . MIDDELHOEK, S . , J. Applied Physics 37, 1276, 1966.

4. LABRUNE, M., and DANIEL, T., in Proceedings of the Conference on Image Detection and Quality, Paris, 1986.

Press, 205-299, 1971.