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Discontinuous metal-insulator multilayers (DMIMs) are a special type of nanostructures with a layered arrangement of metallic particles sandwiched between continuous insulating layers. DMIMs exhibit moderate tunneling magnetoresistance (TMR) ratio but enhanced low-field sensitivity, which makes them promising candidates for magnetic field sensors. Recently we have grown epitaxial Fe/MgO DMIMs on MgO (0 0 1) single crystal substrates at different deposition conditions. Here, based on the analysis of magnetic isotherms in a broad temperature range, the effect of deposition temperature (TS) on microstructure of DMIMs is being studied and compared with the results of Transmission Electron Microscopy. It is shown that metallic layers consist of flat nanoparticles whose average size decreases, and their crystallinity improves with the increase of TS., Portuguese team acknowledges Network of Extreme Conditions Laboratories-NECL and Portuguese Foundation of Science and Technology (FCT) support through the projects NORTE-01-0145-FEDER-022096, EXPL/IF/00981/2013 (G.N.K), and grant SFRH/BPD/81710/2011 (A.G.-G.). This work was partially supported by Spanish Ministerio de Economía y Competitividad through project MAT2017-82970-C2, and from regional Gobierno de Aragón through project E26 including FEDER funding.

Investigation of crystal structure, dielectric, magnetic and local ferroelectric properties of the diamagnetically substituted Bi 1− x A x FeO 3− x /2 (A=Ca, Sr, Pb, Ba; x =0.2, 0.3) polycrystalline samples has been carried out. It has been shown that the heterovalent A 2+ substitution result in the formation of oxygen vacancies in the host lattice. The solid solutions have been found to possess a rhombohedrally distorted perovskite structure described by the space group R 3 c . Piezoresponse force microscopy has revealed signs of existence of the ferroelectric polarization in the samples at room temperature. Magnetization measurements have shown that the magnetic state of these compounds is determined by the ionic radius of the substituting elements. A-site substitution with the biggest ionic radius ions has been found to suppress the spiral spin structure of BiFeO 3 giving rise to the appearance of room-temperature weak ferromagnetism.

Polycrystalline samples Bi 4 − x Gd x Ti 3 O 12 ( x = 1, 1.5, 2) were investigated by X-ray diffraction, piezoresponse force microscopy and SQUID-magnetometry techniques. Increasing the gadolinium content was shown to suppress the spontaneous polarization in Bi 4 − x Gd x Ti 3 O 12 , resulting in a polar-to-nonpolar phase transition near x = 1.5. In contrast to previous expectations, all these samples were found to be paramagnets. It was thus proven that introducing magnetically-active Gd ions into the lattice of the ferroelectric Aurivillius-type compound should not be considered as an effective way to achieve multiferroic behavior.

A scanning tunneling microscope, operated in the near field emission regime, is used to obtain the phases of very low energy electrons reflected from a sample surface. A simple theoretical model shows that the spectrum of the electron standing waves, formed in the vacuum gap between the tip probe and the sample, is directly related to the complex amplitudes of the reflected electron waves. The surface sensitivity of the interferometric spectra is demonstrated in the analysis of different reconstructions of the Pb/Si(111) system. DGCyT PB950061 and PB95-0169 DGCyT FPI Grant IN92-D00830413 Teoría de la Señal y Comunicaciones

Investigation of crystal structure, magnetic and local ferroelectric properties of the diamagnetically-doped Bi1 − xAxFeO3 (A = Ca, Sr, Pb, Ba; x = 0.2, 0.3) ceramic samples has been carried out. It has been shown that the solid solutions have a rhombohedrally distorted perovskite structure described by the space group R3c. Piezoresponse force microscopy data have revealed the existence of the spontaneous ferroelectric polarization in the samples at room temperature. Magnetization measurements have shown that the magnetic state of these compounds is determined by the ionic radius of the substituting elements. A-site substitution with the biggest ionic radius ions has been found to suppress the spiral spin structure of BiFeO3 and to result in the appearance of weak ferromagnetism. The magnetic properties have been discussed in terms of doping- induced changes in the magnetic anisotropy.

In an effective continuum approach alike the phenomenological Landau theory, we study low energy excitations in a square lattice of dipolar coupled magnetic moments μ , over continuously degenerate microvortex (MV) ground states defined by an arbitrary angle 0 v =μ (cos θ, sin θ) and the ferromagnetic (FM) vector f = 1 2 (∂yvx, −∂xvy). The excitation energy density ∼f2 leads to a non-linear Euler equation. It allows, besides common linear waves of small amplitude, also non-linear excitations with unlimited (but slow) variation of θ( r ) . For plane wave excitations θ( r )=θ( n · r ) propagating along n = (cos ϕ, sin ϕ), exact integrals of Euler equation are found. The density of excitation states turns anisotropic in θ, conforming to the enhanced occurrence of MV-like states with θ close to 0 or π/2 in our Monte Carlo simulations of this system at low excitation energies

Considering chains of classical spins coupled by long-range interactions ∼ r − σ , we show a qualitative difference between the critical behaviour of systems with isotropic and anisotropic interactions, as seen from the analysis of the Binder cumulant by Monte Carlo simulations. Unlike the known absence of long range order in isotropic case for σ >2, we found indications of finite critical temperatures in the anisotropic case even for σ as high as 3, related to dipolar coupling.

An extension of the RKKY mechanism is obtained for indirect magnetic interaction between nanogranules of magnetic metal in non-magnetic metallic matrix. The coherent scattering by all the spins in a granule essentially modifies the common RKKY law for point-like magnetic scatterers. In 3D system, the magnitude of RKKY oscillations is strongly suppressed, due to coherent narrowing of scattering angle to ∼1/( k F r 0 ) 2 . No such strong suppression occurs for the 2D analog of this system, in-plane magnetized discs over a 2DEG layer, but the RKKY period and the spatial decay law are modified.