Depth profiling multilayer domain structures

Magnetic multilayers have become increasingly popular as magnetic field sensors. The resistance of these structures depends directly on the relative alignment of the magnetization between adjacent layers, which, in turn, depends on the ambient magnetic field. Imaging the magnetic structure of the buried layers in the multilayer is therefore valuable in understanding how these devices work and how to optimize their performance. By combining SEMPA with ion milling we were able to image the magnetic domain structure of individual Co layers in a [Co(6nm)/Cu(6nm)]₂₀ multilayer sensor material. This work which involved a collaboration with Michigan State University Center for Sensor Materials and the NIST neutron condensed matter science group resolved long standing questions regarding the size and relative alignment of the magnetic domains in these multilayers.

Of particular interest is the as-prepared state of this multilayer which has a large magnetoresistance value that is irreversibly lost after applying a magnetic field. SEMPA images of the magnetic domain structure and topography of the as-prepared top Co film are shown in Figures (a) and (b), respectively. Figure (a) shows irregular magnetic domains with feature sizes generally on the order of a micron, along with Néel-like domain walls about 0.2 µm wide with random chirality. Figure (c) shows a SEMPA image from the second Co layer after removing the top Co and Cu layers. The domain structure of this layer is strongly anti-correlated with that of the outermost layer in Figure (a). The anti-correlation even extends to such small features as the domain walls, which preserve chirality in the adjoining layer. The degree of correlation is quantified in a histogram shown in Figure (d) of the difference in magnetization direction, δ Φ, between the two Co layers. The histogram shows that about 60% of the domains are aligned antiparallel, while the rest are uncorrelated.

These images along with polarized neutron reflectivity measurements of the average magnetization show that the high magnetoresistance of the as-prepared multilayer is associated with a strong antiparallel magnetic alignment between adjacent Co layers that cannot be recovered after magnetizing the multilayer.

SEMPA images of the topmost Co layer magnetization (a) and topography (b) and second Co layer
magnetization (c) in the [Co(6 nm)|Cu(6 nm)]₂₀ sample. The magnetization direction is mapped into
color as indicated by the colorwheel in the center. A histogram of the difference in the magnetization
direction between the two layers, δ Φ, is shown in (d).

Observation of antiparallel magnetic order in weakly coupled Co/Cu multilayers

John Unguris
Robert J. Celotta

Michael H. Kelley - NIST
David Tulchinsky - Naval Research Laboratory

J.A. Borchers - NIST
J.A. Dura - NIST
C.F. Majkrzak - NIST
W.P. Pratt Jr. - Michigan State University
J. Bass - Michigan State University

Supported in part by the Office of Naval Research

Online: July 1999
Last Updated: February 2008

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