Kondo-attractive-Hubbard model for the ordering of local magnetic moments in superconductors

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Metal-insulator transition in Nd1-x Eu-x NiO3 probed by specific heat and anelastic measurements

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Histerese térmica de sistemas magnéticos nanoestruturados

We report a theoretical investigation of thermal hysteresis in magnetic nanoelements. Thermal hysteresis originates in the existence of meta-stable states in temperature intervals which may be tuned by small values of the external magnetic field, and are controlled by the systems geometric dimensions as well as the composition. Two systems have been investigated. The first system is a trilayer consisting of one antiferromagnetic MnF2 film, exchange coupled with two Fe lms. At low temperatures the ferromagnetic layers are oriented in opposite directions. By heating in the presence of an external magnetic field, the Zeeman energy induces a gradual orientation of the ferromagnets with the external field and the nucleation of spin- op-like states in the antiferromagnetic layer, leading eventually, in temperatures close to the Neel temperature, to full alignment of the ferromagnetic films and the formation of frustrated exchange bonds in the center of the antiferromagnetic layer. By cooling down to low temperatures, the system follows a different sequence of states, due to the anisotropy barriers of both materials. The width of the thermal hysteresis loop depends on the thicknesses of the FM and AFM layers as well as on the strength of the external field. The second system consists in Fe and Permalloy ferromagnetic nanoelements exchange coupled to a NiO uncompensated substrate. In this case the thermal hysteresis originates in the modifications of the intrinsic magnetic

Nucleação de vortices em nanoelementos elípticos

The present work reports a theoretical study of the vortex nucleation in elliptical nanoelements of iron with the dimensions of the principal and secondary axes in the range of hundreds of nanometers. It will be divided into three steps: first of all, a general panorama and a justification for the interest of the study of nanosystems and their applications. Second, a explanation about the computational simulations applied for the calculations of the remanent states after the saturation in a external field which value is grater then the exchange field of the material for nanoelements coupled or not to a antiferromagnetic substrate. Systems with that range of axes dimensions and height in the range of a few dozens of nanometers have a natural tendency to nucleate closed magnetic ux, like vortex. Third, we will emphasize the nucleation of double vortex, the main types and the dimensions in which they occur (phase diagram) and the factors that may in uence in the nucleation and control of the remanent state. We shown that we can control specially the distance between the vortex cores changing the value of the interface field. Finally, we present a expectative of continuity of this work with objectives and applications

Superparamagnetismo em Jacobsitas sintéticas

In this experimental study sintetic samples of Jacobsites (MnFe2O4) were synthesized by the Pechini method and calcined within ambient atmosphere and afterwards in the vacuum from 400 to 700ºC, the range of calcination temperatures. The X-Ray Diffraction (XRD) and the Scanning Electronic Microscopy (SEM) analysis have shown that the samples treated at 400ºC temperature are composed by a simple type of spinel phase, with a crystallite size of 8:8nm for the sample calcined in ambient atmosphere and 20; 1nm for the sample treated in the vacuum, showing that the cristallite average size can be manipulated by the atmosphere control. The hysteresis loops for the sample calcined at 400ºC in ambient atmosphere reveal features of superparamagnetic behavior with magnetization 29:3emu=g at the maximum field of 1:2T. The sample calcined in 400oC under vacuum show magnetization = 67emu=g at the maximum field of 1:5T. The sample treated at 500oC, under ambient atmosphere, has shown besides the spinel phase, secondary phases of hematite (Fe2O3) and bixbyite (FeMnO3). The hysteresis loops demonstrate a sharp drop of the magnetization compared to the previous sample. The analysis has revealed that for the samples treated in higher temperatures (600ºC and 700ºC) its observed the absence of the spinel phase and the maintenance of the bixbyite and hematite. The hysteresis loops for those samples in accordance to the external magnetic field are straight lines crossing the origin, consistent with the antiferromagnetic behavior of the phases.The Mössbauer espectroscopy show to the sample calcined at 400ºC within ambiente atmosphere two sextet and one doublet. The two sextets are assigned to the hyperfine fields related to the magnetic deployment in the nuclei of Fe3+ ions, at the tetraedric and octaedric sites. The doublet is assigned to superparamagnetic behavior of the particles with smaller diameter than dc . Now the sample calcined at 400ºC under vacuum only show two sextet

Propriedades magnéticas de ortoferritas de lantânio dopadas com estrôncio

Samples of lanthanum Ortoferrites doped with strontium were synthesized in a single phase by the sol-gel method. Two samples were prepared, one by varying the concentration of strontium in lanthanum ortoferrites La1−xSrxFeO3−δ with (0 ≤ x ≤ 0.5), and another batch of samples of type, La1/3Sr2/3FeO3−δ, now varying only the temperature of calcination. Our samples were obtained by Pechini method and sintered in air and oxygen atmospheric. Their crystal structures were determined by x-ray diraction (XRD), scanning electron microscopy (SEM), where we observed that the samples (0 ≤ x ≤ 0.3) have orthorhombic symmetry and the volume of the single cell decreases with the increasing of concentration of strontium. For x = 0.5 it is only observed the simple phase when that is sintered in O2 atmospheric. Their magnetic characteristics were obtained by the Mössbauer spectroscopy and magnetic measurements. The magnetization measurements for samples La1−xSrxFeO3−δ with (0 ≤ x ≤ 0.5) revealed that the magnetization decreases with increasing concentration of strontium, but for the sample x = 0.4 the magnetization shows a high coercive field and a ferrimagnetic behavior, which is attributed to a small amount of strontium hexaferrite. As for the samples La1/3Sr2/3FeO3−δ calcined between 800 oC e 1200 oC. The hysteresis curves revealed two distinct behaviors: an declined antiferromagnetic behavior (Canted) for samples calcined between 800 oC and 1000 oC and a paramagnetic behavior for the samples calcined at 1100 oC e 1200 o C. Thermal hysteresis and sharp peaks around the Néel temperature (TN), over the curves of specific heat as a function of temperature was only observed in calcined samples with 1100 oC and 1200 oC. This eect is attributed to the charge ordering. These results indicate that the charge ordering occurs only in the samples without oxygen deficiency. Magnetic measurements as a function of temperature are also in agreement with this interpretation

Nucleação de vortices e paredes de domínio em nanoestruturas magnéticas

We report a theoretical investigation of the magnetic phases and hysteresis of exchange biased ferromagnetic (F) nanoelements for three di erent systems: exchange biased nanoparticles, exchange biased narrow ferromagnetic stripes and exchange biased thin ferromagnetic lms. In all cases the focus is on the new e ects produced by suitable patterns of the exchange energy coupling the ferromagnetic nanoelement with a large anisotropy antiferromagnetic (AF) substrate. We investigate the hysteresis of iron and permalloy nanoparticles with a square basis, with lateral dimensions between 45 nm and 120 nm and thickness between 12 nm and 21 nm. Interface bias is aimed at producing large domains in thin lms. Our results show that, contrary to intuition, the interface exchange coupling may generate vortex states along the hysteresis loop. Also, the threshold value of the interface eld strength for vortex nucleation is smaller for iron nanoelements. We investigate the nucleation and depinning of an array of domain walls pinned at interface defects of a vicinal stripe/AF bilayer. The interface exchange eld displays a periodic pattern corresponding to the topology of the AF vicinal substrate. The vicinal AF substrate consists of a sequence of terraces, each with spins from one AF subalattice, alternating one another. As a result the interface eld of neighboring terraces point in opposite direction, leading to the nucleation of a sequence of domain walls in the ferromagnetic stripe. We investigated iron an permalloy micrometric stripes, with width ranging from 100 nm and 300 nm and thickness of 5 nm. We focused in domain wall sequences with same chirality and alternate chirality. We have found that for 100nm terraces the same chiraility sequence is more stable, requiring a larger value of the external eld for depinning. The third system consists of an iron lm with a thickness of 5 nm, exchange coupled to an AF substrate with a periodic distribution of islands where the AF spins have the opposite direction of the spins in the background. This corresponds to a two-sublattice noncompensated AF plane (such as the surface of a (100) FeF2 lm), with monolayer-height islands containing spins of one sublattice on a surface containing spins of the opposite sublattice. The interface eld acting in the ferromagnetic spins over the islands points in the opposite direction of that in the spins over the background. This a model system for the investigation of interface roughness e ects. We have studied the coercicivity an exchange bias hysteresis shift as a function of the distance between the islands and the degree of interface roughness. We have found a relevant reduction of coercivity for nearly compensated interfaces. Also the e ective hysteresis shift is not proportional to the liquid moment of the AF plane. We also developed an analytical model which reproduces qualitatively the results of numerical simulations

Simultaneous observation of the magnetic and electric behavior in a correlated system near a metal-semiconductor transition: ESR in pellets of conducting polymers

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Order-disorder transformation in the Ni-4.49 at.% Al alloy

The order-disorder transformation in the Ni-4.49 at.% Al alloy was studied using electrical resistivity measurements, microhardness measurements, differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). The results confirmed the ordering behavior expected for Ni-Al dilute alloys and the suggested relation between resistivity changes and microhardness changes with antiferromagnetic spin ordering. The higher value obtained for the activation energy of vacancy migration was associated with a decrease in the Al concentration gradient near solute-depleted regions. (c) 2005 Elsevier B.V. All rights reserved.

Spin reversal and magnetization jumps in ErMexMn1-xO3 perovskites (Me = Ni, Co)

The erbium-based manganite ErMnO3 has been partially substituted at the manganese site by the transition-metal elements Ni and Co. The perovskite orthorhombic structure is found from x(Ni) = 0.2-0.5 in the nickel-based solid solution ErNixMn1-xO3, while it can be extended up to x(Co) = 0.7 in the case of cobalt, provided that the synthesis is performed under oxygenation conditions to favor the presence of Co3+. Presence of different magnetic entities (i.e., Er3+, Ni2+, Co2+, Co3+, Mn3+, and Mn4+) leads to quite unusual magnetic properties, characterized by the coexistence of antiferromagnetic and ferromagnetic interactions. In ErNixMn1-xO3, a critical concentration x(crit)(Ni) = 1/3 separates two regimes: spin-canted AF interactions predominate at x < x(crit), while the ferromagnetic behavior is enhanced for x > x(crit). Spin reversal phenomena are present both in the nickel- and cobalt-based compounds. A phenomenological model based on two interacting sublattices, coupled by an antiferromagnetic exchange interaction, explains the inversion of the overall magnetic moment at low temperatures. In this model, the ferromagnetic transition-metal lattice, which orders at T-c, creates a strong local field at the erbium site, polarizing the Er moments in a direction opposite to the applied field. At low temperatures, when the contribution of the paramagnetic erbium sublattice, which varies as T-1, gets larger than the ferromagnetic contribution, the total magnetic moment changes its sign, leading to an overall ferrimagnetic state. The half-substituted compound ErCo0.50Mn0.50O3 was studied in detail, since the magnetization loops present two well-identified anomalies: an intersection of the magnetization branches at low fields, and magnetization jumps at high fields. The influence of the oxidizing conditions was studied in other compositions close to the 50/50 = Mn/Co substitution rate. These anomalies are clearly connected to the spin inversion phenomena and to the simultaneous presence of Co2+ and Co3+ magnetic moments. Dynamical aspects should be considered to well identify the high-field anomaly, since it depends on the magnetic field sweep rate. (C) 2006 Elsevier B.V. All rights reserved.

Oxygen stoichiometry effects in spinel-type NiMn2O4-delta samples

We report the effects of oxygen stoichiometry in the structure and magnetic response of spinel-type NiMn2O4-delta polycrystalline samples controlled by heat-treatments in different atmospheric conditions.The post-annealed samples were analyzed by Scanning Electron Microscopy associated to X-ray Energy Dispersion Spectrometry, X-ray Photoelectron Spectroscopy and AC/DC magnetic measurements.Results indicate that the oxygen stoichiometry highly influences the magnetic interactions between the ferromagnetic, and antiferromagnetic sublattices in these compounds due to the presence of manganese in three possible valence states. (c) 2005 Elsevier Ltd. All rights reserved.

Charge-orbital ordering and phase separation in the two-orbital model for manganites: Roles of Jahn-Teller phononic and Coulombic interactions

The main properties of realistic models for manganites are studied using analytic mean-field approximations and computational numerical methods, focusing on the two-orbital model with electrons interacting through Jahn-Teller (JT) phonons and/or Coulombic repulsions. Analyzing the model including both interactions by the combination of the mean-field approximation and the exact diagonalization method, it is argued that the spin-charge-orbital structure in the insulating phase of the purely JT-phononic model with a large Hund couphng J(H) is not qualitatively changed by the inclusion of the Coulomb interactions. As an important application of the present mean-held approximation, the CE-type antiferromagnetic state, the charge-stacked structure along the z axis, and (3x(2) - r(2))/(3y(2) - r(2))-type orbital ordering are successfully reproduced based on the JT-phononic model with large JH for the half-doped manganite, in agreement with recent Monte Carlo simulation results. Topological arguments and the relevance of the Heisenberg exchange among localized t(2g) spins explains why the inclusion of the nearest-neighbor Coulomb interaction does not destroy the charge stacking structure. It is also verified that the phase-separation tendency is observed both in purely JT-phononic (large JH) and purely Coulombic models in the vicinity of the hole undoped region, as long as realistic hopping matrices are used. This highlights the qualitative similarities of both approaches and the relevance of mixed-phase tendencies in the context of manganites. In addition, the rich and complex phase diagram of the two-orbital Coulombic model in one dimension is presented. Our results provide robust evidence that Coulombic and JT-phononic approaches to manganites are not qualitatively different ways to carry out theoretical calculations, but they share a variety of common features.

Magnetic doping in Zn7-xMxSb2O12 spinels (M = Ni and Co)

In this article we present a concise report on our studies on the magnetic behavior and structural arrangements of the inverse spinel Zn7-xMxSb2O12 system (M = Ni, Co). Studies on the temperature dependence of the magnetization (M) of several samples in this system showed the occurrence of a spin-glass-like state in temperatures around 10 K. The capability of this system to hold magnetic ions in either octahedral and/or tetrahedral positions is responsible for the occurrence of competing ferromagnetic and antiferromagnetic interactions. This condition is likely to cause the appearance of the observed spin-glass-like behavior. (C) 2002 Elsevier B.V. B.V. All rights reserved.

Effect of presence of an acid catalyst on structure and properties of iron-doped siloxane-polyoxyethylene nanocomposites prepared by sol-gel

In this work we investigate the effect of hydrochloric acid (HC) addition on the structure and thermal and magnetic properties of iron-doped siloxane-polyoxyethylene (POE) hybrids prepared by the sol-gel route. X-ray powder diffraction (XRD) and X-ray absorption near edge structure (XANES) results reveal the dominance of ferrihydrite nanoparticles and a mixture of this phase with FeCl4- species in the hybrid prepared without and with HCl, respectively. Thermal analysis reveals the existence of two crystalline polymeric phases in the hybrid prepared with HCl whereas hybrids prepared without HCl are amorphous. The 105 and 60 Angstrom sized ferrihydrite nanoparticles were detected by SAXS analysis of the composite prepared without and with HCl, respectively. The magnetic results suggest that in both samples antiferromagnetic nanoparticles coexist with small clusters/isolated ions. In the sample without HCl addition, larger particles dominate the magnetic behavior, while the opposite occurs for the sample prepared using HCl catalyst. (C) 2004 Elsevier B.V. All rights reserved.

A Chloro-Bridged Linear Chain Imine-Copper(II) Complex and Its Application as an Enzyme-Free Amperometric Biosensor for Hydrogen Peroxide

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