Journal of Magnetism and Magnetic Materials 75 (1988) 339-344

North-Holland, Amsterdam 339

THE STRAY-FIELD STRUCTURE OF A HI-B SHEET IN THE VICINITY OF LASER SCRATCHES

Horst SCHAFER and Harald LAMPEY ’

Institut ftir Angewandte Physik der Uniuersitiit Miinster, Corrensstrasse 2/4, D-4400 Miinster, Fed. Rep. Germany

Received 8 July 1988

The stray-field behaviour of laser scratches which are burned into the surface of HI-B sheets in order to reduce the domain-spacing and thus the anomalous eddy-current loss has been measured. The influence of the external applied field on

the stray-field structure is shown and one possible mechanism for the observed loss reduction is favoured. Besides the influence of the charges at the laser scratches on the magnetization reversal of the single sheet the possibility of an interaction

between several stacked sheets due to the stray field at laser scratches is discussed.

1. Introduction

Since the early works of Neurath [l], Gniewek [2] and Williams, Bozorth and Shockley [3] it is well-known that closure domains disappear from the static domain structure of a (110) [OOl] ori- ented SiFe sheet if a tensile stress is applied parallel to the [OOl] direction, thereby reducing the antiparallel domain-wall spacing in order to reduce the stray-field energy. Houze [4] showed that the refined spacing is retained during 60 Hz excitation, thus giving the possibility of decreasing the eddy-current loss in grain oriented (g.0.) SiFe sheets [5,6] by an artificial domain control, while the disturbance of 90” domain complexes in- creases the mobility of antiparallel walls and lowers the coercive force [7].

Yamamoto and Nozawa [8] showed that the reduction in total loss by tensile stress depends upon the degree of grain orientation and becomes larger when the grain orientation is more perfect; the reduction of total loss arises mainly from a reduction in eddy-current loss. The increase in total loss due to surface roughness which acts as

1 Permanent address: Dipl.-Phys. Harald Lampey, Universitat Dortmund, Fachbereich Maschinenbau, Fachgebiet Qualitatskontrolle. Emil-Figge-Strasse 74, D-4600 Dortmund 50, Fed. Rep. Germany.

pinning defects for domain-wall motion can also be suppressed by applying a tensile stress.

A special coating which subjects the material to an isotropic tensile stress is able to take over a function similar to that of an external stress ap- plied parallel to the [OOl]-direction if only the magnetic properties in this direction are consid- ered. This coating is very effective for the im- provement of the magnetic properties of sheets with a high degree of (110) [OOl] texture [9], while the important role of a forsterite coating also has to be taken into account [lo].

Nevertheless a further improvement of the magnetic properties could neither be achieved by metallurgical approaches [ll] nor by increasing the stress from the coating [12]. A further decrease of the eddy-current loss only could be reached by modifications of the stress coating in order to continue the domain-refinement, i.e. by scratching the surface perpendicular to [OOl] in the sheet plane.

2. Modification of the surface coating by scratch- ing

First investigations concerning the loss-reduc- tion by grooving the surface perpendicular to the rolling direction (r.d.) have been carried out on

0304~8853/88/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

340 H. Schiifer, H. Lampey / Stray-field structure of a HI-B sheet

uncoated specimens [11,13,14]. By the scratches a large tensile stress is introduced into the inner part of the sheet [15,16]. At the grooves free magnetic poles occur leading to a strong demagnetizing field which is able to nucleate reverse domains and thus has an important influence on the mag- netization reversal. By this way the scratches are able to take over the function of other imperfec- tions like grain boundaries or even the bounds of the sheets [14,17].

In order to provide a non-contact domain-re- finement technique, the use of laser irradiation has been encouraged [18] and an effective domain control by spark ablation has been proposed [19].

Nevertheless every kind of artificial domain control is an encroachment on the surface of the sheet. The role of the grooves or scratches con- cerning their attribute of producing reverse do- mains which facilitate the magnetization reversal can be examined without any modification to the sheet. The important fact is that the scratches can

w 15

ml

Fig. 1. Sketch of the strip with laser scratches. Q--_-D : r.d. ; all dimensions in mm.

cause reverse domains only if they are charged with free magnetic poles which also provide a stray field in regions near the surface of the speci- men. If the dependence of this stray field on the external field parallel to r.d. can be measured, the role of the charges at the scratches concerning the magnetization reversal will become clearer.

3. Experimental procedure and results

In order to determine the stray-field structure in the vicinity of small laser scratches a high

50

H(kA/m)

0

Fig. 2. Stray-field structure of the strip with measuring tracks perpendicular to r.d. at a distance of 40 pm from the surface in different external fields Ho parallel to r.d. (a) Position of the measuring tracks; (b) Ho = 0; (c) Ho = 5 kA/m; (d) Ho = 10 kA/m.

H. Schtifer, H. Lompey / Stray-field structure of a HI-B sheet 341

spatial resolution and a high field resolution of the field-sensor are required. In general these two demands contradict each other so that two differ- ent measuring equipments were used.

The vibrating pickup-loop magnetometer (VPLM [20,21]) yields a spatial resolution of about 20 X 50 km*, a field resolution of about 150 A/m and allows measurements at a minimum distance of 10 pm from the sheet surface. A commercially available Hall sensor [22] which has been modified for our special purposes [23] led to a geometrical resolution of about 200 X 200 pm2 and a simulta- neous field resolution of 1.5 A/m while the minimum measuring distance to the sheet surface is restricted to about 40 pm.

The specimens were cut from commercially available “23ZDKH” [24] coated and laser scribed electrical steel in the shape of circular disks (diam- eter 10 mm) and rectangular stripes (15 X 5 mm2). The external homogeneous field which was ap- plied parallel to r.d. was increased from zero up to 40 kA/m for the measurements taken with the VPLM and from zero up to 10 kA/m for the investigations carried out with the Hall sensor.

Only the stray-field component normal to the sheet surface was measured, but this component already allows us to obtain an instructive image of the magnetic poles at the laser scratch in depen- dence on the external applied field.

From the remanent state up to 10 kA/m the measurements were carried out on a rectangular strip with three laser scratches using the Hall sensor at a distance of 40 pm from the surface. Fig. 1 shows a sketch of the strip with indicated dimensions and the position of the laser scratches. In fig. 2 the stray-field structure of the strip can be seen for the case where the measuring tracks of the sensor are perpendicular to r.d. with reduced distance in the vicinity of the scratches.

Only the stray-field structure resulting from the charges at the bounds which lead to a strongly inhomogeneous demagnetizing field can be seen; no special structure due to the scratches becomes visible. The alternating structure in the middle of the specimen in the remanent state (H,, = 0) might arise from the stray field of Bloch walls [25] because the sensitivity of the Hall sensor is suffi- cient to detect them.

Fig. 3. Stray-field profile of the strip with measuring paths parallel to r.d. at a distance of 40 pm from the surface. (a)

Position of the measuring tracks; (b) H,, = 5 kA/m; (c) H,, = 10 kA/m.

When the measuring tracks are turned parallel to r.d. the stray-field structure changes to the profile shown in fig. 3. Due to the restricted length of the measuring paths, the strong edge peaks are not completely shown but the areas of the laser scratches are registered. On this large scale, no additional stray field from the laser scratches can be seen, the stray-field structure is determined mainly by the strong charges on the surface due to an inhomogeneous demagnetizing field.

Fig. 4 shows the vicinity of the laser scratch in the middle of the specimen on a reduced scale.

342 H. &h&r, H. Lampey / Stray-field structure of a HI-B sheet

a

a I>

0.75

1 0.

5 ,

b

, “y .

5 6.75 7 7.25

Y (mm>

.5

Fig. 4. Modification of the stray-field profile of the strip due to the laser scratch in an external field of Ho = 10 kA/m at a distance of 40 pm from the surface. (a) Position of the measuring track relative to the laser scratch; (b) -: measured stray field in the

vicinity of the scratch, - - - - - -: expected stray field without a scratch.

Thus the deviation of the stray-field profile from the one without scratch becomes visible and dem- onstrates that the scratch is only slightly charged; the maximum stray field arising from the scratch at this external field is considered to be about f 250-500 kA/m and thus is nearly negligible in comparison to stray-field peaks at the edges and at grain boundaries [27-291. At higher external fields parallel to r.d. the amplitude of the stray field at the scratch is only slightly increased.

The following measurements were carried out on a circular disk with one single laser scratch in the center of the specimen using the VPLM at a distance of 10 pm from the surface. Fig. 5 shows the stray-field structure in an increasing external field parallel to r.d.

In low external fields up to 10 kA/m only a small structure comparable to fig. 4 becomes visi- ble at the scratch. The disturbance by the scratch becomes more pronounced if the external field increases. At 30 kA/m (fig. 5d) the scratch-field shows its maximum amplitude of about + 4 kA/m superimposed to the stray field arising from surface charges. The interaction of the stray field from the

scratch with the charged surface seems to play an important role concerning the development of the stray-field structure of the whole sheet. As can be seen from fig. 5e, the stray field from the scratch disappears if the external field is increased up to 40 kA/m, presumably due to this interaction.

Nevertheless the scratch introduces an ad- ditional charged area into the sheet which may take over the function of other imperfections like grain boundaries and thus facilitates the magneti- zation reversal [30] by introducing reverse do- mains.

On the other hand it has been shown that stray fields with normal components may have an in- fluence on the magnetic behaviour of a stack of several sheets and thus can influence the complete core [23,26,27,29,31-341. It can be shown that for g.o. SiFe sheets a remarkable coupling due to stray-fields which influence the magnetic be- haviour of the neighbouring sheets in a stack has to be expected only if the normal component reaches the value of the anisotropy field which is about 35 kA/m [23,29,35,36]. Thus it can be estimated that other stray fields arising from the

H. Schiifer, H. Lumpey / Stray-field structure of a HI-B sheet 343

H (kA/ml

Ho=lOkA/m Ho: 20 kA/m

lb

d -15

Ho:30kA/m e 1 Ho I 40 kA/m

-15

Fig. 5. Stray-field structure of a circular disk with one single laser scratch in the center at a distance of 10 pm from the surface. (a) Sketch of the specimen with measuring tracks; (b) H, = 10 kA/m; (c) H,, = 20 k-A/m; (d) Ho = 30 kA/m; (e) Ho = 40 kA/m.

bounds of a sheet or from an air gap [23,37] play a more important role concerning a coupling effect due to stray fields than laser scratches. This pre- sumption is confirmed by the results of Holle and Schoen [38] who showed that the loss reduction of a single sheet by laser scribing can be retained if a

core loss of single sheets compared with a com- plete core remains nearly unchanged by laser scribing.

4. Conclusions

model core is built up from this sheets, i.e. the From our measurements it is concluded that destruction factor which describes the ratio of the the main effect of reducing the core loss by laser

344 H. H. Lompey Stray-field structure a HI-B

scribing arises from the charges at the scratches which take over the function of charged grain boundaries and imperfections by supporting the magnetization reversal.

The stray field arising from the scratches is low compared with the stray field arising at charged grain boundaries and at the bounds of a sheet. A coupling effect between stacked laser scribed sheets due to the stray field from the laser scratches has to be questioned, because the stray field at a laser scratch is not able to reach the value of the anisotropy field even in high external fields paral- lel to r.d. (the rolling direction).

Acknowledgement

The authors wish to thank Prof. Dr. W. Hampe for many helpful discussions.

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