Magnetic anisotropy in metallic multilayers and ultra-thin films

Charles M. Falco, Brad N. Engel*, and J.M. Slaughter

Physics Department and the Optical Sciences Center
The University of Arizona
Tucson, Arizona 85721

As many of the talks at this conference will address, high quality multilayer
films with very thin layers and smooth interfaces are needed for
near-normal-incidence optics at short wavelengths.  For a similar reason, i.e.
the short length scale of the interaction, high quality multilayers also are
needed for studies of magnetism.  As this talk will discuss, the structural
perfection of multilayer thin-film materials can vary considerably; from
"incoherent" multilayers with short range order, to high quality crystalline
superlattices with long range structural coherence in all three dimensions.
Along with x-ray optics, interesting and useful phenomena exhibited by these
materials include new collective magnetic excitations, unusual electronic
properties, elastic property anomalies, and dimensionality crossover effects
in superconductivity.  In this talk we will describe some of our group's
recent results using molecular beam epitaxy (MBE) for the growth of high
quality multilayer materials, as well as provide examples of several of their
magnetic properties.

One property, the magnetic anisotropy, is particularly sensitive to the
perfection of interfaces in the thin-film materials, and hence serves as a
good indirect measure of the quality of the deposition process.  Further, the
fundamental mechanism underlying the phenomena of surface and interface
magnetic anisotropy remains an interesting unresolved problem in modern
magnetism.  This strong anisotropy appears at either a free surface or at the
interface between two different materials.  For thin film systems, it is often
the dominant anisotropy energy, determining the overall magnetization
easy-direction.

Recently, we have been studying the magnetic interface anisotropy of MBE-grown
multilayers and ultra-thin films.  In particular, we have focussed on X/Co/Y
sandwich structures, where X and Y are each one of the non-magnetic metals Ag,
Au, Cu or Pd.  Using in situ polar Kerr effect measurements, we have reported
an anomalous magnetic behavior in the monolayer coverage regime.
Surprisingly, the magnitude of the perpendicular magnetic anisotropy is
strongly peaked at ~1 atomic layer non-magnetic overlayer coverage.

We will report recent work on growth, by MBE, and investigations of the
influence of layering order and composition on this anomalous anisotropy
behavior.  Our measurements indicate that the unusual peak in perpendicular
anisotropy is present regardless of layer order, i.e.  X/Co/Y or Y/Co/X.
However, we find layer-order-dependent behavior when a different material is
deposited at the Co interface, i.e.  X/Co/Y/X or X/Y/Co/X.

Research supported by the Department of Energy Grant No.  DE-FG03-93ER45488

* Present address:  Storage Technology Corporation, Louisville, Colorado 80028