Hysteresis and domains in magnetic multilayers

Journal of Magnetism and Magnetic Materials 148 (1995) 244-246

Journal of mauneUsm and mat]netic materials

ELSEVIER

Hysteresis and domains in magnetic multilayers J. McCord a,*, H. Brendel a A. Hubert a, S.S.P. Parkin b a Institute of Materials Science, University ofErlangen-Niirnberg, Martensstr. 7, 91058 Erlangen, Germany b IBMAImaden Research Center, San Jose, CA 95120-6099, USA

Abstract We integrated an optical magnetometer into a conventional Kerr microscope, which gives us the possibility to measure hysteresis loops and observe magnetic domains simultaneously. An application is shown, the investigation of an exchange coupled N i s i F e l g / R u / N i s l F e 1 9 sample in which the top ferromagnetic layer is wedge shaped.

1. Introduction

oscillating with the interlayer thickness [2]. Even if optical experiments were used in these investigations, they were performed with low lateral resolution. The usefulness of high resolution was first demonstrated in the discovery of the biquadratic coupling effect [3]. Refinements of these techniques were presented in Refs. [4,5]. Particularly on polycrystalline multilayers with a higher coercivity level it is sometimes difficult to derive the coupling mechanism from domain observations alone, A quantitative determination of the coupling stret~gth from the observed domains is rarely possible. Carefully mea-

Interest in magnetic multilayers was greatly enhanced with the discovery of antiferromagnetic exchange between two ferromagnetic layers separated by a n0n-ferromagnetic l~yer [1] and the discovery of an exchange interaction

* Corresponding author. Fax: 4- 49-9131-857472. Email: [email protected].

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J. McCord et aL I Journal of Magnetism and Magnetic Materials 148 (1995) 244-246

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Fig. 2. Overview domain observation of the first part of the sample. The magmtization directions in both layers are indicated. The transition at the left end is irrelevant. sured and evaluated magnetization curves are therefore important, but a clarification of coercivity mechanisms and the origins of noise can only be achieved by direct observation of the domains. We developed the possibility of a combination of both techniques.

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2. Integraeed magnetometer A flexible microscopical loop tracer using the transverse magnetooptical Kerr effect [6] is integrated into a conventional Kerr microscope, which is run under conditions of the longitudinal Kerr effect. The optical magnetometer is operated by a modulated polarized red diode laser which is coupled into and out of the microscope by a dichroitic mirror (see Fig. 1). The area over which the magnetometer averages can be optically adjusted between 1 micron width and the full viewing field.

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3. Exchange eoulPling in a magnetic layer wedge The usual type of wedge samples on which the oscillat- ' ing exchange phenomenon can be demonstrated have a variable non-magnetic interlayer thickness. In our sample the top magnetic layer of Pa = NisaFe19 is wedge shaped. The sample, prepared by dc magnetron sputtering, consists of: S i / P a / F e s o Mns0/Pa (10 n m ) / R u (1.2 n m ) / P a ( 0 - 1 8 rim).

Overview domain pictures (Fig. 2) reveal a coupling transition from antiferromagnetic (AF) to ferromagnetic (F) coupling at a ferromagnetic film thickness of 1.6 nm. From magnetization curves of the top layer the strength of Fig. 4. Domain behavior together with hysteresis loops at three different locations of the wedge (see Fig. 3).

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the coupling can be derived immediately frona the horizontal shifting H s of this curve. Also of izterest is the variation of the coercivity H c (Fig. 3). As shown in Fig. 4b, a maximum o f coercivity in the neighborhood of the A F - F transition is accompanied with the occurrence of 360 ° walls [5]. Acknowledgments: This work was suppc)rted by the Deutsche Forschungsgemeinschaft. Thanks are due to Rudi Sch~fer for helpful discussions.

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J. McCord et al. /Journal of Magnetism and Magnetic Materials 148 (1995) 244--246

References [I] P. Grilnberg, R. Schreiber, U.W.Y. Pang, M.B. Brodsky, H. Sowers, J. Appl. Phys. 61 (1987) 3750. [2] S.S.P. Parkin, N. More, KP. Roche, Phys. Rev. Lett. 64 (1990) 2304. [3] M. Riihrig, R. Sch~ifer, A. llubert, R. Mosler, J.A. Wolf, S. Demokritov, P. Griinberg, Phys. Stat. Sol. (a) 125 (1991) 635.

[4] J. McCord, A. Hubert, R. Schiller, A. Fuss, P. Griinberg, IEEE Trans./dagn. 29 (1993) 2735. [5] R. SehSfer, A. Hubert, S.S.P. Parkin, IEEE Trans. Magn. 29 (1993) 2738. [6] M. Neudecker, K. Booekmann, A. Hubert, IEEE Trans. Magn. 26 (1990) 2664.