Direct Experimental Study of Domain Structure In Magnetic Multilayers
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V.I. Nikitenko*, V.S. Gornakov*, L.M. Dedukh*, L.H. Bennett**, R.D. McMichael**, L.J. Swartzendruber**, S. Hua***, D.L. Lashmore***, and A.J. Shapiro** *Institute of Solid State Physics, Russian Academy of Sciences, Chernogolovka, Moscow Dist., 142432, Russia "**National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA ***Materials Innovation, West Lebanon, NH 03784, USA
ABSTRACT We have applied a polarized light optical microscope in reflective mode with a Bi-substituted yttrium-iron-garnet indicator film with in-plane anisotropy for visualization of the magnetostatic fields produced by nanostructured magnetic CoNiCu/Cu electrodeposited multilayers. By analysis of the magneto-optical stray field image, detailed information is obtained not only on the as-grown multilayer magnetic structure but on its change during the magnetization reversal processes. The influences of crystal lattice defects and nonuniformity of the nonmagnetic spacers thicknesses on the domain wall nucleation and motion are studied. Peculiarities of the re-magnetization of antiferromagnetically exchange coupled multilayers are discussed, including real-time observations of domain wall creep in a constant applied field. INTRODUCTION Multilayer systems composed of ferromagnetic layers separated by nonmagnetic metallic spacers (with nanoscale range thicknesses) have become a subject of great interest as a new type of material in the last few years 1,2. It has been discovered that such artificially modulated structures can possess unique properties. The exchange interaction between the layers can oscillate from ferromagnetic to antiferromagnetic with the thickness of the spacers. Inversion of the magnetization direction in adjacent layers leads to the giant magnetoresistance effect (GMR). Utilization of this effect opens up perspectives in development of new devices based on noninductive reading of magnetically recorded information for future generations of computers. The character of the spin distribution in antiferromagnetically coupled multilayers can be disturbed by nonuniformities in the nonmagnetic spacer thicknesses, crystal structure defects, and by magnetization reversal processes. As a result, peculiar magnetic domain structures with unusual walls can be formed. Some examples were discussed in reference 3. If domain walls are nucleated in only one or in only a few layers, their motion is accompanied by changes in the area of "pseudo-domain boundaries" (parallel to the interfaces) and therefore the value of the exchange coupling energy between layers. Changes in "pseudo-domain boundary" area also give rise to changes in resistance through the GMR effect. All 277
Mat. Res. Soc. Symp. Proc. Vol. 384 @1995 Materials Research Society
of this emphasizes the necessity of the development of nondestructive methods for characterization of the multilayer microstructure and real time investigation of the magnetization processes. Recently we have shown the capabilities of the magnetooptical indicator film (MOIF) method in sol
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