Search

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.

Investigation of crystal structure and magnetic properties of the diamagnetically- substituted Bi1-xAxFeO3-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 A2+ 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 R3c. 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 giving rise to the appearance of 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

3 páginas, 3 figuras.-- et al., Epitaxial discontinuous Fe/MgO multilayers have been grown by pulsed laser deposition on MgO(001) single-crystal substrates. The multilayers with 0.6 nm nominal Fe layers thickness are superparamagnetic and demonstrate tunneling magnetoresistance (TMR) in the current-in-plane configuration. Increasing deposition temperature causes an improvement in crystal quality and is accompanied by higher TMR ratios. The maximum value (9.2% TMR at room temperature and 18 kOe magnetic field) trebles that of polycrystalline samples deposited simultaneously on glass substrates. A model formula for TMR ratio that includes both spin-dependent tunneling and spin-filtering effect is proposed to explain these results., Financial support by Spanish Ministry of Science (through project No. MAT2008-06567-C02 including FEDER funding) and by the Aragón Regional Government (through projects E26 and PI059/08) is acknowledged. Work of A.V. and G.N.K. was supported by Portuguese FCT through “Ciencia 2008” and “Ciencia 2007” programs, respectively.

Multiferroics, where magnetic order and ferroelectric polarization might be combined in a single phase, have been an object of the growing interest. However, most magnetic ferroelectrics tend to have low magnetic ordering temperature and are often antiferromagnets, in which the magnetoelectric effect is intrinsically small. BiFeO3 perovskite seems to be the most suitable object for multiferroic research in view of their high magnetic and ferroelectric ordering temperatures. On the other hand, spatially modulated G-type antiferromagnetic spin structure of BiFeO3 prevents any net magnetic signal and inhibits the observation of linear magnetoelectric effect. A-site doping of BiFeO3 has been found to be one of the way to suppress the spin modulation, however main regularities of changes of the magnetic and ferroelectric properties of the BiFeO3-based multiferroics under the A-site doping are still unclear. To reveal them, we have carried out complex investigation of crystal structure, magnetic and ferroelectric behavior of A= Ca, Sr, Pb and Ba- substituted multiferroic perovskites. The Bi1-xAxFeOy (x= 0.2, 0.3) solid solutions have been prepared and characterized by x-ray diffraction, magnetic, dielectric, Mossbauer spectroscopy and PFM measurements. It has been found that the heterovalent A2+ substitution in Bi1-xAxFeOy is realized through the formation of the oxygen vacancies. It has been shown that the crystal structure of the compounds is described within the space group R3c, permitting the spontaneous polarization, whose existence is confirmed by the PFM data. Magnetic properties of the solids have been found to be determined by the ionic radius value of the substituting element. Experimental results suggest that the A-site substitution with the biggest ionic radius ions effectively suppress the spiral spin structure of BiFeO3 and result in the net magnetization appearance.

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.

We report the magnetization measurements in paramagnetic regime for La2/3Ca1/3MnO3. Close to ferromagnetic transition (T - T c < 20K) at applied fields below ~ 50 Oe, a series of discontinuous steps in the Curie constant as a function of temperature are observed. These and related peaks in susceptibility as a function of magnetic field are interpreted as evidence for consecutive formation of spin clusters of definite increasing size at approaching T c .

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.

The mechanism of the formation of heterovalent-substitution-induced defects as well as their influence on the magnetic properties of BiFeO3-based multiferroics has been studied. It has been shown that heterovalent A2+ substitution results in the formation of oxygen vacancies in the host lattices of both antiferromagnetic and weak ferromagnetic Bi1−xAxFeO3 (A = Ca, Sr, Pb, Ba; x = 0.2, 0.3) compounds, thus indicating the intrinsic (i.e. not related to defects themselves) mechanism of doping-induced enhancement of magnetization. A correlation between the ionic radius of the substituting element and the value of the spontaneous magnetization of the corresponding solid solution has been found. The experimental results suggest that A-site substitution with the biggest ionic radius ions effectively suppresses the spiral spin configuration of antiferromagnetic BiFeO3.