Journal of Magnetism and Magnetic Materials 112 (1992) 349-352 North-Holland

Main domains and surface domains in amorphous soft magnetic materials

P. Sch/Snhuber, H. Pfi i tzner , G. Harasko , T. Klinger and K. Futschik

University of Technology Vienna, Gusshausstrasse 27/351, A-1040 Vienna, Austria

Contrary to domain structures typical for extremely anisotropic crystalline materials, amorphous materials show a basically different set-up. At least in the case of pre-annea!ed samples, the inner structure of 180 ° bulk domains of sum~ millimetres width is accompanied by irregular surface ~aze domain patterns with typical width of some 10 pm. The paper presents a comparison of both inner and surface domain structures of Fe-, Co- and FeNi-based types of amorph~,us ribbons received from the magnetotactic bacteria method, the zero-field colloid techoio::e and the destruction-free laser scanning procedure.

1. Introduction

Domain analysis has been established as a valuable tool in the field of magnetic materials research covering the production process as well as applications and magnetic circuit design. While in crystalline material (e.g. grain-oriented 3% sili- con iron) due to its enormous anisotropy several methods of domain analysis make use of surface stray field evaluation (e.g. the colloid method [1], Hall sensors [2], vibrating-!oop magnetometer [3]), this procedure fails in the case of amorphous materials. Here, a continuation of the inner main domain structure - the existence of which has been indicated by classical loss separation mea- surements (e.g. refs. [4,5]) - to the outermnst surface region of the material cannot be expected in a general way. Due to weak or vanishing anisotropy, the development of surface domain structures for the purpose of flux closure is muct: more likely. As demonstrated within this paper, ,t. !_ . . . . L_~- . . . . . ,-~ c o l l u u u l a t e t a by the re- tills ilypUtltlt:;.~l~ ~UUJU be . . . . . . '- . . . . . . suits of respective studies showing irregular

Correspondence to: Dr. P. Sch6nhuber, University of Technol- ogy Vienna, Institute of Fundamentals and Theory. of Elec- trotechnics, Gusshausstrasse 27-351, A-1040 Vienna, Austria. Tel.: +43-1-58801-3934; telefax: +43-1-505-7940; email: peter6~ eteuvl .una.ac.at.

labyrinth- or so-called maze-domains of typically some 10 Ixm width. In general, this surface struc- ture will be coupled with the main domain struc- ture of the bulk material in a complex manner. It was one aim of this paper to demonstrate the simultaneous existence of both types of domains and their basic properties for some typical amor- phous materials.

2. E.~perimental

Surface domains were observed by use of the magnetotactic bacteria method, described in de- tail in [6] or - facultatively - by means of the zero-field colloid technique [1]. Due to the bacte- ria's ability of orienting and propelling them- selves along magnetic field lines in a sign-sensi- tive manner, the former technique yields patterns of double-domain spacing. However, the colloidal ferrofluid is attracted by the Bloch wall's gradient field and therefore showing every Bloch wall and a single-domain spaced pattern.

For the detection of inner main domains, a newly developed Kerr-effect laser scanning tech- nique was applied [7]. An ac magnetization Gf low amplitude and frequency f causes oscillations of the (postulated) inner mobile Bloch walls and synchronous adaption processes of the surface

0304-8853/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved

350 P. Sch6nhuber et al. / Dor,~ain analysis in amorphous soft magnetic materials

structure. Within thi,; study, a flux density value corresponding to about 10% of the saturation polarization J~ of tile respective material was chosen. A dynamic depolarization of the re- flected laser beam with frequency f can be de-

tected by means of a lock-in technique. Magneti- zation was achieved by use of a small C-yoke with pole faces 20 × 10 mm 2 (pole distance 40 mm) carrying different windings for application of ac and dc fields. For the first time, this technique

A3 A4

A1 A2

l i d

B2

' ~ 4 1 ,

B 134 B5 B6

CI A C2 A C3/~

Fig. 1. Overview of surface domain patterns (left) and bulk domain structures (right) of NiFe-based (a, top), Co-based (b, middle) and Fe-based (c, bottom) amorphous ribbons. 1 cm ~ (a, c) 50 gin, (b) 100 ~.m.

P. Sch6nhuber et al. / Domain analysis in amorphous soft magnetic materials 351

allowed a completely destruction-free investiga- tion of the bulk structure. Moreover, there are no specific restrictions with respect to sample size, geometry or pre-treatment of the ribbon.

All samples were demagnetized with fd = 30 Hz and magnetized with f = 300 Hz. The scan- ning rate, which can be chosen freely as any multiple of 10 I~m, was 80 lxm. In detail, ribbons of about 25 Ixm thickness of the following materi- als were investigated: A NiFe-based composition (Metglas 2826MB or

Vitrovac 4040), B Co-based composition (Vitrovac 6030), C Fe-based composition (Metglas 2605CO or

Vitrovac 7505). The ribbons had a length of about 150 mm. Samples A and C, both 25 mm wide, were pre-an- nealed at 350°C for2 h in 800 A m-1 longitudinal field. Sample B of 15 mm width was investigated as supplied by Vacuumschmelze.

Preliminary examinations had demonstrated the necessity of elimination of effects of the earth's field. Within this work this was achieved by active shielding. Generally, the effect of exter- nal dc field components on the magnetization status is discussed-in section 3.

3, Results and discussion

An overview of typical surface and bulk do- main structures of the materials mentioned above is given in fig. 1. Surface structures on the left side are put opposite to inner domains of the bulk on the right part of the figure. Considering the surface structures, distinct differences be- tween the materials can be recognized. Sample A exhibits irregular maze domain patterns, partly similar to fingerprints, of typically 5 I~m domain width. For sample B, n~rrow lines of bacteria concentration of typically 25 ~m width almost perpendicular to the ribbon direction are pre- dominant. Obviously these lines (as well as for- mations in rib Don direction) are due to surface topography. Sample C showed similar surface structures to A, but of higher extent of order.

Concerning the bulk domain structures, here, sharp peaks represent mobile Bloch walls (includ-

H f b)~ [A/m]lo a)

Fig. 2. Influence of external dc field Hf on bulk domain structure of sample B-type ribbon. (a) H t = 0;(b) Hf = 10

A m -I.

ing domains of 1 mm and 2.5 mm width, respec- tively) almost parallel to the ribbon direction. At first sight, these well oriented domains four mil- limetres wide can be considered as a characteris- tic property of all three materials. Widths cf 1 mm to 2 mm seem to be typical for B and C, but reaching values up to 4 mm in A. Depending on the actual flux density value, the height or base width of the peaks give hints on the mobility of the respective wall. From fig. 1, walls A1 and B4-B6 appear somewhat less compliant than the rest of the respective ribbon. The walls of sample C show fairly uniform mobility.

Results of dc magnetization are shown in fig. 2. Starting out from a carefully demagnetized state (fig. 2a) showing six almost equidistant Bloch walls and respective domain widths of about 1.5 mm, an external longitudinal field H r of only 10 A m-~ was applied. Consequently, the domains parallel to Hf were enlarged at the cost of those antiparallel to Hf (fig. 2b).

4. Conclusions

The results of this study allow the following conclusions: (a) Soft magnetic amorphous materials in the

pre-annealed state exhibit at least two differ- ent domain structures: narrow in spaced sur- face domain structures and some orders of magnitude wider inner domains of the bulk.

352 P. Sch6nhuber et al. / Domain analysis #~ amorphous soft magnetic materials

(b) The main domains of the investigated materi- als exhibit widths which were postulated by global loss separation measurements. These structures are mainly responsible for the global magnetic properties of the materials and, consequently, their technical J'elevance.

(c) Patterns ~f surface domain structure result from flL~ closure mechanisms for stray field minimization and local stresses, which might occur from surface roughness, local deforma- tions, etc.

(d) In accordance with the crystalline material's behaviour, also Bloch walls of amorphous ma- terial were found to be of variable m6bility.

(e) Due to the rather soft magnetic behaviour of the ribbons investigated, consideration of the influence of the earth's field is a prerequisite for effective investigations.

Acknowledgements

Financial support of the "Fonds zur F6rderung der wissenschaftlichen Forschung in Osterreich" gratefully acknowledged (projects P7348-PHY and P7575-TEC).

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