K. Hamacher

Coexistence of different magnetic phases in Dy/Y superlattices caused by growth induced nanostructures

Abstract Epitaxial Dy/Y rare earth superlattices were prepared by molecular beam epitaxy on Ta(211) and Nb(211) buffer layers on α-Al 2 O 3 (1 1 00) and characterized via in situ reflection high-energy electron diffraction (RHEED), in situ scanning tunneling microscopy (STM) and ex situ X-ray and neutron diffraction. The Y buffer and the Dy/Y superlattice are found to grow strongly faceted on the Ta or Nb(211) buffer, and form ‘growth induced’ nanostructures with two different superlattice types on both facet surfaces. Magnetic measurements show two coexisting magnetic phases that are obtained in the temperature range below the Neel temperature. We develop a model to interpret this behavior in connection with the results of structural and magnetic measurements.

Magnetic properties of (11̄02) Dy/Y superlattices

Epitaxial (1102) Dy/Y rare earth superlattices and a thick (1102) Dy film, grown on sapphire with Y/Ta buffer layers, have been prepared by molecular beam epitaxy. Neutron diffraction and SQUID magnetization measurements on a single 200 nm thick (1102) Dy layer showed nearly bulk behavior. However, for the (1102) Dy/Y superlattices we found different magnetic behavior depending on the relative Dy to Y‐layer thickness. The superlattices exhibit both ferromagnetic and helical phases, but with the Neel and Curie temperatures significantly different from bulk Dy. These results differ from previous findings in Dy/Y superlattices grown along [0001].

Neutron inverse optics in layered materials

Neutron specular reflectivity data obtained with a new grazing angle neutron spectrometer (GANS) from a NiC/Ti-multilayer sample were analyzed and modeled for reconstructing the scattering length density profile as a periodic step potential for the layered material. There is some ambiguity in the results due to the uniqueness problem with missing phase information. For more complex layered materials, there is often insufficient knowledge about the layers to use modeling reconstruction without phase information. In the second part, we present a method in which this problem is solved for diffraction data from lipid multilayers: due to changes in chemistry (isomorphous heavy atom method) the phases are determined directly and therefore the density profile of the lipid bilayer can be uniquely determined.

Intercalation of Organic Compounds in Lipid Bilayers

Using neutron diffraction we studied the incorporation of small hydrophobic compounds into bilayers consisting of phosphatidylcholine (PC) and cholesterol. They were found to be localized in a narrow band at the center of the hydrocarbon region, between the two halves of the bilayer. The structures formed by introduction of the compounds are therefore intercalated structures with the long axis of the intercalated molecules lying in the plane of the bilayer. We worked with several bilayers which differed by the length of the hydrocarbon chain of the PC. The quality of the localization depended on the presence of cholesterol, the water content and the PC chain length.

NEUTRON DIFFRACTION STUDY OF MAGNETIC PROPERTIES OF DY/Y SUPERLATTICES

Abstract The magnetic properties of MBE-grown Dy/Y superlattices (SLs) have been studied by neutron diffraction. The SL growth direction is at an angle of approximately 35° to the c -axis instead of being along one of the principal axis directions. The results confirm the coexistence of helical antiferromagnetic order and ferromagnetic order up to the Neel temperature T N ≈ 180 K. This is the first time that ferromagnetism has been observed in Dy/Y superlattices.

Coexisting ferro- and helimagnetism in Dy/Y superlattices

Neutron diffraction studies on Dy/Y superlattices grown with the c axis at an angle of approximately 35° to the film plane reveal a coexistence of helical antiferromagnetic order and ferromagnetic order. The coexistence of magnetic phases has been determined to arise from the development during growth of surface facets of the Y buffer layer, which was grown on bcc (211) Ta or Nb substrates. One of the superlattice facets produces an isotropic expansion of the Dy basal plane which favors the helical antiferromagnetic spin configuration, while the other facet causes an anisotropic contraction of the Dy basal plane that favors ferromagnetism. The net result of the growth anomaly is the apparent coexistence of the two magnetic states as observed by neutron scans made with the scattering vector oriented parallel to the nominal c crystal axis of the superlattice.