Surface Segregation in Multicomponent Clusters
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Surface Segregation in Multicomponent Clusters Peter A. Dowben1, Ning Wu1, Natalie Palina2, H. Modrow2, R. Müller3, J. Hormes4, Ya.B. Losovyj1,4 1
Department of Physics and Astronomy and the Center for Materials Research and Analysis, University of Nebraska – Lincoln, Lincoln, NE 68588-0111, [email protected] 2 Physikalisches Institut der Universität Bonn, Nussallee 12, D-53115 Bonn, Germany 3 Institut für Physikalische Hochtechnologie, Albert-Einstein-Straße 9, D-07745 Jena, Germany 4 Center for Advanced Microstructures and Devices, Louisiana State University, 6980 Jefferson Hwy, Baton Rouge, LA 70806, [email protected] ABSTRACT Nanostructured materials are not immune from surface segregation, as can be shown for solid samples made from nanosized BaFe12-2xCoxTixO19 barium ferrite particles and a variety of free clusters. Both theory and experiment provide ample demonstration that very limited dimensions of very small clusters does not necessarily impart stability against surface and grain boundary segregation. In fact, with the larger surface to volume ratio in small clusters and lower average atomic coordination, we anticipate that compositional instabilities in small clusters will readily occur. INTRODUCTION Surface segregation is a long standing problem in materials science with great technological significance. The phenomenon of surface segregation is the preferential enrichment of one component of a multi-component system at a boundary or interface. Atomic size and lattice strain, bond strengths, and even magnetic ordering influence the extent of segregation. Surface segregation indicates that the surface enthalpy is different from the bulk and occurs at finite temperatures (or in the materials growth process) when barriers to diffusion are overcome. The difference in the total free energy of the surface with respect to the bulk is a consequence of the surface truncation to vacuum and the resultant breaking of symmetry. Besides surface segregation, the free energy difference drives other phenomena in order to minimize the total free energy [1], such as surface relaxations and surface reconstructions. All these phenomena are related to changes in crystalline order in the surface, resulting in the creation of different electronic and magnetic properties at the surface with respect to the bulk. These different surface electronic structure signatures can be sometimes exploited to study surface segregation. It is well known that surface segregation plays a crucial role in affecting the surface and interface polarization ferromagnets [2]. This is considered to be very important to many spintronic applications as polarization is strongly influenced by composition [3-8]. Evidence that surface composition affects spin polarization of
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potentially high polarization materials abounds [3,4,6,9-13]. One of the more studied examples of reduced spin-polarization due to surface compositional instabilities of the NiMnSb half-Heusler alloy [2,3,14]. Magnetism plays a role in other ways: segregation
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