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Crystalline erbium thin films and Er/Y superlattices with varying Er-layer thicknesses have been grown by molecular-beam epitaxy. The magnetic and structural properties of these samples have been analyzed by x-ray-scattering, bulk magnetization, and neutron-diR'raction techniques.From a comparison of the data for the two systems, the importance of interfacial strain relative to artificial modulation in shaping the magnetic behavior has been determined. Though the basic nature of the erbium magnetic order is not qualitatively altered in either the thin films or superlattices, the conical ferromagnetic phase is suppressed in all of the samples considered. The enhanced critical fields exhibit a systematic dependence on Er-layer thickness. These effects appear to follow directly from the epitaxial basal-plane strain which is measurable in films over 14000 A thick. This strain, along with a "clamping" of the Er thermal expansion to the Y lattice, leads to a reduction of the magnitude of the magnetoelastic energy that drives the ferromagnetic transition. The dependence of the magnetoelastic energy on the epitaxial strain is described by a model which accounts for the elastic coupling of the erbium lattice to the yttrium.

We describe a novel system inwhich low-angle, asymmetric tilt boundaries organise spontaneously under conditions of molecular beam epitaxial growth. In the cases studied, the (1072) surfaces of hexagonal rare earths grow coherently but tilted on the (21 1) surfaces of BCC transition metals. The main driving forces are interfacial coherency and the relief of long-range epitaxial strain.

The magnetic ordering of highly perfect single-crystal multilayer films of alternate layers of magnetic Dy and nonmagnetic Y prepared by molecular beam epitaxy has been studied by neutron diffraction. Results on a series of films with Dy thicknesses of approximately 16 atomic planes (≊45 Å) and Y thicknesses ranging from 10 to 22 planes have confirmed the existence of long-range helimagnetic ordering of the Dy 4f spins which is propagated through the intervening Y layers in phase coherence. The propagation vectors in both Dy and Y layers have been calculated from the wave vector of the magnetic satellites and the intensity of the bilayer harmonics. The propagation vector for Dy decreases continuously with temperature, while that in the Y is temperature independent and equal to 0.31 Å−1. The nature of the ordering and the noninteger multiple of π for the phase change of the propagation wave vector across the Y, suggests that the mechanism of long-range coupling is a conduction band spin-density wave in both Y and Dy stabilized by the 4f spins of the Dy. No intrinsic ferromagnetic transition is observed due to the clamping effect of the Y layers on the Dy magnetostriction. The application of a field along basal plane directions destroys the helical order and produces a ferromagnetic state with all spins aligned along the field direction. The moment in the induced ferromagnetic state is 10 μB, while that calculated for the zero-field helical state ranges from 7.5μB to 9.5μB for different samples, suggesting a possible static disorder of some Dy spins superimposed on the intrinsic long-range helical state.

We have successfully grown high-quality Dy/Y rare-earth superlattices with their a or b axes perpendicular to the growth plane, at choice. Earlier eA'orts by molecular-beam-epitaxy methods produced only growth along the c axis. In other research, we have grown almost freestanding superlattices of Dy/Y and other hexagonal rare-earth superlattices. These new configurations make a variety of significant new experiments accessible.Interesting magnetic phenomena are revealed by recent studies of magnetisrn in rare-earth superlattices and films.In earlier work with collaborators, we have examined the helical antiferroma, gnetism of Dy multilayers separated by nonmagnetic Y. An unexpected result is that the Dy antiferromagnetism is transmitted coherentry through the Y so that even the chirality of the helical order is preserved. 2 We observe analogous transmission for the caxis-modulatedEr structure in superlattices with Y. 3These effects appear to be consequences of the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction throu, gh the Y, linking spins in neighboring magnetic layers. 2' All the materials in this work have employed molecular-beam-epitaxy