Transmission of information using the spin of the electron as well as its charge requires a high degree of spin polarization at surfaces. However, at surfaces this degree of polarization can be quenched by competing interactions. Using a combination of surface-sensitive X-ray and tunnelling probes, we show for the quasi-twodimensional bilayer manganites that only the outermost Mn-O bilayer is affected: it is a 1-nm-thick insulator that exhibits no long-range ferromagnetic order, whereas the next bilayer displays the full spin polarization of the bulk. Such an abrupt localization of the surface effects is due to the two-dimensional nature of the layered manganite, and the loss of ferromagnetism is attributed to weakened double exchange in the reconstructed surface bilayer and a resultant antiferromagnetic phase.The creation of a well-defi ned surface insulator atop a fully spin-polarized bulk demonstrates the ability of two of the most demanding components of an ideal magnetic tunnel junction to self-assemble naturally.
High spin polarization at surfaces and interfaces is a key component for transport of information using the spin degree of freedom of the electron 1 . Unfortunately, the physics and chemistry of surfaces can reduce this spin polarization through chemical inhomogeneity 2,3 , strain or surface reconstruction. Many half-metallic ferromagnetic oxides exhibit very high bulk magnetization, but the spin polarization at surfaces and interfaces declines more rapidly than the bulk magnetization as the Curie temperature, T C , is approached 4 . Magnetic tunnel junctions 5 as well as other probes of surface polarization 6-10 in perovskite-based manganites indicate the presence of a non-ferromagnetic surface region, often called a 'dead layer,' of thickness ~5 nm.We show for the quasi-two-dimensional bilayer manganites that the outermost Mn-O bilayer, alone, is affected: this 1-nm-thick intrinsic nanoskin is an insulator with no long-range ferromagnetic order while the next bilayer displays the full spin polarization of the bulk. This unexpectedly abrupt termination is probably due to the reduced dimensionality of the crystal structure. That is, the electronic and magnetic coupling between the bilayers is markedly weaker than the corresponding intrabilayer couplings or the isotropic couplings found in non-layered ferromagnetic oxides.A series of La 2−2x Sr 1+2x Mn 2 O 7 single crystals were prepared by the fl oating zone method 11 between x = 0.36 and x = 0.5. Sample preparation involved cleaving crystals in air under ambient conditions just before they were mounted in the ultra-high-vacuum measurement chamber. The natural cleavage planes between the bilayers (see Fig. 1) provide a clear advantage, and both atomic force microscopy and soft-X-ray rocking curves showed large, fl at terraces. Surface-sensitive absorption measurements compare well with bulksensitive measurements, indicating that the surface is not degraded. These experiments involve data from several subsequent runs with different crystal batches ...
Resistivity and I-V measurements have been performed on thin amorphous Nb3Ge films in perpendicular fields. In the vicinity of the mean-field-transition line, pure flux-flow behavior occurs due to melting of the vortex lattice. The dependence of the melting field on temperature and thickness is well described by the melting theory for a two-dimensional lattice. In the narrow crossover regime between melting and weak pinning, dislocation-mediated flux creep occurs prior to melting.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.