On the Explanation of the Paramagnetic Meissner Effect in Superconductor/Ferromagnet Heterostructures

An increase of the magnetic moment in superconductor/ferromagnet (S/F) bilayers V(40nm)/F [F$=$Fe(1,3nm), Co(3nm), Ni(3nm)] was observed using SQUID magnetometry upon cooling below the superconducting transition temperature Tc in magnetic fields of 10 Oe to 50 Oe applied parallel to the sample surface. A similar increase, often called the paramagnetic Meissner effect (PME), was observed before in various superconductors and superconductor/ferromagnet systems. To explain the PME effect in the presented S/F bilayers a model based on a row of vortices located at the S/F interface is proposed. According to the model the magnetic moment induced below Tc consists of the paramagnetic contribution of the vortex cores and the diamagnetic contribution of the vortex-free region of the S layer. Since the thickness of the S layer is found to be 3-4 times less than the magnetic field penetration depth, this latter diamagnetic contribution is negligible. The model correctly accounts for the sign, the approximate magnitude and the field dependence of the paramagnetic and the Meissner contributions of the induced magnetic moment upon passing the superconducting transition of a ferromagnet/superconductor bilayer

Magnetic proximity effects in V/Fe superconductor/ferromagnet single bilayer revealed by waveguide-enhanced polarized neutron reflectometry

Polarized neutron reflectometry is used to study the magnetic proximity effect in a superconductor/ferromagnet (SC/FM) system of composition Cu(32nm)/V(40nm)/Fe(1nm)/MgO. In contrast to previous studies, here a single SC/FM bilayer, is studied and multilayer artefacts are excluded. The necessary signal enhancement is achieved by waveguide resonance, i.e. preparing the V(40nm)/Fe(1nm) SC/FM bilayer sandwiched by the highly reflective MgO substrate and Cu top layer, respectively . A new magnetic state of the system was observed at temperatures below 0.7 TC. manifested in a systematic change in the height and width of the waveguide resonance peak. Upon increasing the temperature from 0.7 TC to TC, a gradual decay of this state is observed, accompanied by a 5% growth of the diffuse scattering. According to theoretical studies, such behavior is the result of the magnetic proximity effect. Due to the presence of the thin FM layer the superconducting electrons are polarized and, as a result, near the SC/FM interface an additional magnetic layer appears in the SC with thickness comparable to ksi, the coherence length of the superconductor.Comment: Submitted to the Journal of Superconductivity and Novel Magnetism. 11 pages, 6 figures

A MAS NMR STUDY OF THE STRUCTURE OF AMORPHOUS ALUMINA FILMS

Des films d'alumine amorphe obtenus par dépôts anodiques ont été étudiés par MAS-RMN de Al27. La méthode de préparation fait varier la coordination de l'aluminium de 4 à 6,5 . Environ 20 % de Al se trouve dans des sites comparables à ceux de l'alumine cristalline. Le reste de Al se trouve dans des sites fortement distordus localement. La dispersion sur la longueur des liaisons Al-O est de l'ordre de 10%.Magic Angle Spinning 27Al N.M.R. data on some anodically formed amorphous alumina films are presented. The spectra show that aluminium has 6,5 and 4-fold coordinations in these films whose proportions depend upon the method of preparation of the film. About 20% of the aluminium is at sites with relatively high symmetry comparable with crystalline aluminas, the remaining aluminium has more distorted surroundings. The data imply an Al-O bond length variation of about 10% in these films