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

NEW DOMAIN CONFIGURATION IN THIN-FILM RECORDING HEADS

ALAN B. SMITH and WARREN W. GOLLER Magnetic Recording Head Development, Digital Equipment Corp.,

Shrewsbury, MA 01545

Abstract-We have used K e r r microscopy to observe t h e domain s t ruc tu re of permalloy films t h a t have t h e same shape as t h e pole pieces commonly employed in thin-film recording heads. These pole pieces were de- posited o n th in glass slides, making it possible to ob- serve the domains f rom both the top a n d bottom of the film. We found that the domain configuration can be qui te different on the two surfaces when strain-induced anisotropy is present.

INTRODUCTION

The poles of thin-film heads are normally made of plated permalloy about 3 pm thick with a small uniaxial magnetic anisotropy in the plane. The easy axis is usually transverse to the head axis (i.e. parallel to the recording media used with the head) since this is the orientation that results in the most efficient conduction of high-frequency flux changes[ 1-31. The anisotropy normally consists of two parts. The first is a uni- form anisotropy (Hk E 3. Oe) induced by the application of a magnetic field during plating. The second contribution is strain-induced and is proportional to the net strain and the magnetostriction constant (41. This strain-induced contribution to the anisotropy can be substantial and can significantly affect the domain configuration. As we shall show in this paper by means of Kerr microscopy, the strain-induced anisotropy (and particularly the non-uniform part of it) causes interesting effects that have not been previously reported.

EXPERIMENTAL PROCEDURE

To obtain the Kerr micrographs presented in this paper, we haye used previously-described video enhancement techniques [5-71. To produce each micrograph, these techniques always re- quire the subtraction of two video images, one with the domain configuration present and another with the sample magnetically saturated. However, in this paper, we wish to compare the do- mains at the top and the bottom of the sample. Thus we must obtain Kerr micrographs of the top and then of the bottom without demagnetizing the sample in between. This situation is easily achieved by acquiring the saturated video image first when obtaining the top micrograph but last when obtaining the bottom one.

If a Kerr micrograph is taken of a region containing only 180" domain walls, it is easily interpreted since the two magne- tization orientations produce different shades of gray. However, the domain configuration is usually more complicated; and the magnetization is not confined to just two opposite directions. In such a case, the Kerr contrast (i.e. the shade of gray) indicates the component of the magnetization aligned with some refer- ence direction that is determined by the way the microscope is adjusted. In such cases it is extremely useful to obtain an- other Kerr micrograph with the apparatus adjusted to display the magnetization components with respect to a reference direc-

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tion orthogonal to that in the first micrograph. We will present such pairs of micrographs in this paper. As discussed above, it is possible to take both these micrographs without disturbing the domain configuration if the saturated reference images are taken in the proper order. In this paper we utilize the longitudi- nal Kerr effect to display magnetization components along the vertical axis of the micrograph. The transverse Kerr effect [8) is used to show components along the horizontal axis. By employ- ing both the longitudinal and transverse effects, it is possible to obtain both micrographs without having to rotate the sample.

RESULTS

When the magnetic anisotropy is relatively uniform and the easy axis is transverse to the head axis, one sees the domain structure of Fig. 1. (As explained above, these two Kerr mi- crographs represent the same domain state; but each displays different components of the magnetization. The contrast cor- responding to different magnetization directions is indicated by arrows next to each micrograph.) Figure 1 shows stripes of al- ternating magnetization transverse to the pole, with triangular closure domains magnetized along the pole axis. This domain configuration is consistent with those previously reported [1,9] for similar heads. (Figure Ib also reveals the presence of Bloch lines [lo] in some of the walls.)

The domain configuration is quite different from Fig. 1 in films with appreciable strain-induced anisotropy. Let us con- sider a film that deviates from the zero-magnetostriction com- position only enough to produce a magnetostriction constant A which is approximately -1. x lo-'. In such a film we observe the domain configuration of Fig. 2. As in Fig. 1, both longi-

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Fig. 1. Kerr micrograph showing domains in permalloy pat- tern on ceramic substrate. Arrows indicate relation between magnetization direction and Kerr contrast.

0018-9464/90/0900-1331$01.00 @ 1990 IEEE

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Fig. 2. Kerr micrograpa showing domains in Permalloy. S in- i1ar to Fig. 1 except for the presence of an appreciable strain- induced anisotropy.

tudinal and transverse Kerr views are shown; and shaded ar- rows are included as an aid to interpreting the micrographs. In these Kerr micrographs we do not see the horizontally-oriented triangular closure domains of Fig. 1. The magnetization is sub- stantially perpendicular to the edges. Such a configuration is difficult to understand since it seems to imply an unbelievably high demagnetizing energy. The explanation is given by further measurements described below.

The poles of Figs. 1 and 2 were fabricated on a ceramic sub- strate of the type normally used for thin-film heads. To better understand the observed domains, we have fabricated a similar pattern on -0.2 mm-thick glass. The top surface of this pole is shown in the Kerr micrograph of Fig. 3a. Please note the sim- ilarity to Fig. 2a. We assert that the similarity of these figures indicates that the films have similar strain-induced anisotropy. We can therefore use the glass sample to draw conclusions about permalloy films in traditional thin-film heads fabricated on ce- ramic substrates.

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Fig. 3. Kerr mlcrograph showing dornams in permalloy pattern on glass substrate viewed a) from the top and b) from the bot- tom. The b) image is reversed so that the top and bottom may be compared by mentally superimposing the two micrographs.

Figure 3b shows the same sample but is taken through the glass from the other side. This figure has been mirror-imaged to facilitate comparison with Fig. 3a. Thus the reader can see how the magnetization changes from top to bottom of the film by mentally superimposing the two figures. Such a comparison seems to show that the domain walls bend at the edges to place an “up” domain over a “down” domain, thus greatly lowering the magnetostatic energy. Actually, the situation is not quite this simple. A careful look at Fig. 3b reveals that it has light gray areas reminiscent of the triangular closures in Fig. 1. Thus Fig. 3b shows that the magnetization is not as perpendicular to the edge on the bottom as it is on the top. The behavior near the edges is further illustrated by Fig. 4. This figure shows another pair of micrographs like Fig. 3 but taken using the transverse Kerr effect to reveal the orthogonal magnetization component. Figure 4a is similar to Fig. 2b; while Fig. 4b shows a tendency to form triangular closures somewhat like Fig. l b .

DISCUSSION

The Kerr effect indicates the magnetization at a depth of 100A. Thus our results can only tell us what the magne-

tization does near the top and bottom surfaces. We have no direct information about what is occurring in between. Since the component of magnetization perpendicular to the edges of the sample is different on the top than the bottom, we know that the magnetization must rotate from the top to bottom. The exact form of this rotation, i.e. the configuration of this “wall”, is not known. Presumably, the structure adjusts itself to minimize the magnetostatic energy by minimizing the average component of magnetization perpendicular to the edges.

The only way to account for the fact that the magnetization distribution differs from top to bottom is to recognize that the strain also differs. The bottom of the film is firmly attached to the substrate, while the top is free to relax somewhat. Thus at the bottom the strain is relatively uniform; but at the top there can be a substantial difference between the strain perpendicu- lar and parallel to the edge. (The effects of such variations in strain through the film have generally not been included in the strain calculations for thin-film heads that have appeared in the literature.) Since the micrographs indicate that the easy axis is

Fig. 4. Kerr mlcrograph similar to Fig. 3 except with the Kerr microscope adjusted to display magnetization components orthogonal to those shown in Fig. 3.

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perpendicular to the edge on top and since the film has negative magnetostriction, we assert that the net strain at the top edge must be tensile parallel to the edge. The presence of this strain is consistent with the way the samples were prepared and the resultant effects of thermal expansion mismatch [ 111 between the constituent materials. After plating, these samples were heated to -260° (the normal temperature used to hard-bake the photoresist in thin film heads). Apparently the film yields to produce relatively low strain at this temperature, resulting in appreciable tensile stress in the permalloy upon return to room temperature.

One can speculate on the influence these strain-induced do- main configurations have on the performance of thin-film heads. In the wide part of the pole, flux tends to beam up the center 112,131. Hence, in this region, the newly discovered configura- tion at the edges probably has little direct effect on flux con- duction. However, there may be some indirect effect on flux beaming due to the resultant curvature of the domain walls.

Also of interest is the domain configuration in the pole-tip region. To aid in evaluating the behavior in this region, our pole patterns are made so they actually extend to the right beyond what is shown in the previous figures. Here they widen out so that there is a relatively wide flux-gathering structure on both sides of the pole “tip”. Viewed from the top, the domain config- uration in this region is as shown in Fig. 5 . Here we see several transverse domains with small closures at the edge. The 180” domain walls separating these transverse domains are promi- nently displayed in Fig 5b. This type of domain structure is favorable for flux conduction, as long as the effective anisotropy induced by the strain is not so large that it significantly reduces the permeability.

As shown by Fig. 6, the situation at the bottom is some- what different. The magnetization in the center of the tip is not aligned in a transverse direction but lies at some intermediate angle. (There is actually a net component along the bar, pre- sumably due to the effect of some small residual magnetization of the large “bulbs” on each end of the region shown.) Here we are again seeing the effects of the smaller anisotropy at the bottom of the film.

CONCLUSIONS

We have reported here a newly-observed domain configura- tion in pole pieces of the type used in thin-film heads. There

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Fig. 6. Kerr micrograph which is the same as Fig. 5 except that these images are of the bottom of the film. These images are not reversed like those of Figs. 3b and 4b.

is nothing unusual about the composition of these permalloy films; the Ni/Fe ratio does not differ greatly from the zero- magnetostriction value. Thus the domain configuration shown here is probably quite common in thin-film heads, even though it has not been previously reported.

ACKNOWLEDGMENTS

We would like to thank Jeff Barnum for supplying the permal- loy samples on glass. We also acknowledge many useful discus- sions with Michael Mallary and Hal Shukovsky.

REFERENCES

111 R. E. Jones, Jr., IEEE Trans. MAG-15, 1619 (1979). (21 R. D. Hempstead and J. B. Money, U. S. Patent 4,242,710, (1980). [3] M. L. Mallary, J . Appl. Phys. 57, 3952 (1985). [4] E. Klokholm and S. Krongelb, abstract 272, Electrochemical Society fall meeting, (1989). 151 F. Schmidt, W. Rave, and A. Hubert, IEEE Trans. MAG- 21, 1596 (1985). [6] B. E. Argyle, B. Petek, and D. A. Herman, Jr., J. Appl. Phys. 61, 4303, (1987). [7] A. B. Smith, Digital Tech. J . No. 8, 74 (February, 1989). IS] W. Rave, R. Schafer, and A. Hubert, J. Magnetism and Mag- netic Materials 65, 7 (1987). [9] P. Kasiraj, R. M. Shelby, J. S. Best, and D. E. Horne, IEEE Trans. MAG-22, 837 (1986). [lo] B. E. Argyle, B. Petek, M. E. Re, F. Suits, and D. A. Her- man, J. Appl. Phys. 63, 4033 (1988). [I11 H. Koyanagi, R. Arai, K. Mitsuoka, 11. Fukui, S. Narishige, and Y. Sugita, J . Mag. Soc. Japan 13, 103 (1989). [12] M. Mallary and A. B. Smith, IEEE Trans. MAG-24, 2374 (1988). (131 M. Mallary, A. Torabi, and S. Batra, paper CA-06, Mag- netism and Magnetic Materials Conference, Boston, (1989), to be published in J. Appl. Phys. (1990).

Fig. Kerr micrograph of pole “tip” region of the same permalloy pattern as Figs. 3 and 4. Both a) and b) are im- ages of the top c l the film.

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