Evidence for nonmonotonic magnetic field penetration in a type-I superconductor

Polarized neutron reflectometry (PNR) provides evidence that nonlocal electrodynamics governs the magnetic field penetration in an extreme low-k superconductor. The sample is an indium film with a large elastic mean free path (11 mkm) deposited on a silicon oxide wafer. It is shown that PNR can resolve the difference between the reflected neutron spin asymmetries predicted by the local and nonlocal theories of superconductivity. The experimental data support the nonlocal theory, which predicts a nonmonotonic decay of the magnetic field.Comment: 5 pages, 4 figures, LaTex, corrected typos and figure

Orientation of Vortices in a Superconducting Thin-Film: Quantitative Comparison of Spin-Polarized Neutron Reflectivity and Magnetization

We present a quantitative comparison of the magnetization measured by spin-polarized neutron reflectivity (SPNR) and DC magnetometry on a 1370 \AA\ -thick Nb superconducting film. As a function of magnetic field applied in the film plane, SPNR exhibits reversible behavior whereas the DC magnetization shows substantial hysteresis. The difference between these measurements is attributed to a rotation of vortex magnetic field out of the film plane as the applied field is reduced. Since SPNR measures only the magnetization parallel to the film plane whereas DC magnetization is strongly influenced by the perpendicular component of magnetization when there is a slight sample tilt, combining the two techniques allows one to distinguish two components of magnetization in a thin film.Comment: 12 pages, 8 figures, It will be printed in PRB, Oct. 200

Magnetic phases and reorientation transitions in antiferromagnetically coupled multilayers

In antiferromagnetically coupled superlattices grown on (001) faces of cubic substrates, e.g. based on materials combinations as Co/Cu, Fe/Si, Co/Cr, or Fe/Cr, the magnetic states evolve under competing influence of bilinear and biquadratic exchange interactions, surface-enhanced four-fold in-plane anisotropy, and specific finite-size effects. Using phenomenological (micromagnetic) theory, a comprehensive survey of the magnetic states and reorientation transitions has been carried out for multilayer systems with even number of ferromagnetic sub-layers and magnetizations in the plane. In two-layer systems (N=2) the phase diagrams in dependence on components of the applied field in the plane include ``swallow-tail'' type regions of (metastable) multistate co-existence and a number of continuous and discontinuous reorientation transitions induced by radial and transversal components of the applied field. In multilayers (N \ge 4) noncollinear states are spatially inhomogeneous with magnetization varying across the multilayer stack. For weak four-fold anisotropy the magnetic states under influence of an applied field evolve by a complex continuous reorientation into the saturated state. At higher anisotropy they transform into various inhomogeneous and asymmetric structures. The discontinuous transitions between the magnetic states in these two-layers and multilayers are characterized by broad ranges of multi-phase coexistence of the (metastable) states and give rise to specific transitional domain structures.Comment: Manuscript 34 pages, 14 figures; submitted for publicatio

Memory effects in annealed hybrid gold nanoparticles/block copolymer bilayers

We report on the use of the self-organization process of sputtered gold nanoparticles on a self-assembled block copolymer film deposited by horizontal precipitation Langmuir-Blodgett (HP-LB) method. The morphology and the phase-separation of a film of poly-n-butylacrylate-block-polyacrylic acid (PnBuA-b-PAA) were studied at the nanometric scale by using atomic force microscopy (AFM) and Time of Flight Secondary Ion Mass Spectrometry (TOF-SIMS). The templating capability of the PnBuA-b-PAA phase-separated film was studied by sputtering gold nanoparticles (NPs), forming a film of nanometric thickness. The effect of the polymer chain mobility onto the organization of gold nanoparticle layer was assessed by heating the obtained hybrid PnBuA-b-PAA/Au NPs bilayer at T >Tg. The nanoparticles' distribution onto the different copolymer domains was found strongly affected by the annealing treatment, showing a peculiar memory effect, which modifies the AFM phase response of the Au NPs layer onto the polar domains, without affecting their surfacial composition. The effect is discussed in terms of the peculiar morphological features induced by enhanced mobility of polymer chains on the Au NPs layer

Morphology selection of nanoparticle dispersions by polymer media

A systematic theory of ultrathin polymer films as organizing media to achieve 2D nanoparticle arrangements was developed. The key physical variables to achieve nanoparticle dispersions and control morphology were determined.open727

Transverse and lateral structure of the spin-flop phase in Fe/Cr antiferromagnetic superlattices

Direct evidence of the nonuniformly canted state of the spin-flop phase induced by a magnetic field applied to Fe/Cr(100) superlattices is obtained by polarized neutron reflectometry. It is unambiguously demonstrated that the magnetization of the alternating Fe layers is twisted through the multilayer stack proving a stable noncollinear configuration. The maximal tilt at the end layers progressively reduces towards the center of the multilayer. The set of tilt angles is deduced from a model-free data evaluation employing the supermatrix routine. Spin-flip off-specular scattering is determined by the in-plane magnetization fluctuations and is fitted by a theoretical model of domains