Alfonso Fondado

• University of Santiago de Compostela

With the recent development of nanotechnology, magnetic nanoparticles (mNPs) have received increasing attention as potential heterogeneous Fenton catalysts in wastewater treatment applications, as an alternative to the conventional Fenton process using dissolved iron salts. Due to their superparamagnetic properties, Fe3O4 mNPs can be easily recover...

In this research, a new sequential batch reactor (SBR) coupled to an internal magnetic separator was developed. The separator consists of a set of axially magnetized permanent toroidal magnets, distributed along a non-magnetic steel rod, uniformly spaced with alternate polarity, which provide an external magnetic field of up to 1.2 T. The feasibili...

In this work we further the structural characterization of the recently discovered (C3N2H5)[Mn(HCOO)3] metal-organic framework with perovskite-like structure, and we present its magnetic and dielectric properties up to 350 K. At low temperature, the C3N2H5+ imidazolium cations, that sit oblique within the cavities of the [Mn(HCOO)3]- framework stru...

En este articulo se examinan los efectos intergranulares sobre el transporte electrico en muestras ceramicas de perovskitas de manganeso que presentan magnetorresistencia colosal. La progresiva reduccion del tamano de particula que nos permite la tecnologia sol-gel hace que estos materiales muestren nuevas e inesperadas propiedades. Asi, en la zona...

In this work the dielectric behavior of semiconducting La-manganites in an intermediate Ca-doping regime is studied. Ceramic samples were prepared by conventional solid-state reaction, using elemental oxides as starting reactants. A chemical, structural and microstructural characterization of the prepared manganites was performed by iodometric titr...

We report the optimization of the dielectric properties of La2/3Ca1/3MnO3. This is achieved by synthesizing this material with small particle size, strategy that reduces the conductivity of the sample and its dielectric losses, while retaining a reasonably high dielectric permittivity and a good magnetocapacitive response. In order to further impro...

We measured the time decay of the electric field in two dielectric non-crystalline solids and the data were analyzed in the framework of a new theoretical approach. It is observed that the application of this approach allows identifying different regimes in the relaxation process. At long times the dynamics end up by following a power law decay, wh...

The influence of different restorative materials on in vitro dielectric properties of sound dentine was investigated. The studied samples were three-layer materials consisting of successive disks of dentine and silver amalgam or nanohybrid composite resin. Before being tested, the samples were maintained in physiological solution never more than 48...

The correct measurement of electrical parameters is a fundamental requirement for the accomplishment of suitable regulatory and protective actions in electric arc furnaces. The Spanish ferroalloy company Ferroatlantica has devoted many efforts during the last years to the development and improvement of an efficient silicon production technology, ba...

Computer simulations of first-order relaxation processes show that the spatial configurations of the system acquire an invariant shape once the stationary regime is attained. Inspired by them we find that, in any first-order relaxation process, if the interaction that governs the system fulfils a simple scale property, then the relaxation will end...

The dielectric response of polycrystalline La0.5Ca0.5MnO3 as a function of frequency and temperature was studied. The semiconducting nature of this oxide gave rise to very high dielectric constant values, which were mainly attributed to interfacial polarization (grain boundaries and contacts sample-electrode). However, an increase of the permittivi...

In this work the dielectric behavior of semiconducting La-manganites in an intermediate Ca-doping regime is studied. Ceramic samples were prepared by conventional solid-state reaction, using elemental oxides as starting reactants. A chemical, structural and microstructural characterization of the prepared manganites was performed by iodometric titr...

In this work, we study the dielectric properties of the layered perovskites La3/2Sr1/2NiO4 and La5/3Sr1/3NiO4 in the frequency range 20 Hz-1 MHz and in the temperature range 110-350 K. For this study, we have synthesized singe phase polycrystalline samples by a conventional solid-state reaction. These nickelates show very high values of the dielect...

We present in this paper a detailed investigation of the dielectric properties of the mixed oxide CaMn7O12 that shows a charge-ordering transition at TCO = 440 K. Surprisingly, this compound presents a high dielectric constant at room temperature. Data taken at several frequencies and temperatures point to relaxor dielectric behaviour, that can be...

We report a rather high dependence of the dielectric permittivity on the magnetic field in La2/3Ca1/3MnO3. The variation is maximum at around 270 K, little above the Curie temperature, TC, and it reaches a 30% under only 0.5 T. We attribute this phenomenon to the space-charge or interfacial polarization produced between the insulator and the metall...

In this work, we study the dielectric properties of the layered perovskites La3/2Sr1/2NiO4 and La5/3Sr1/3NiO4 in the frequency range 20 Hz- 1 MHz and in the temperature range 110-350 K. For this study, we have synthesized single phase polycrystalline samples by a conventional solid-state reaction. These nickelates show very high values of the diele...

In this work, we study the dielectric properties of the layered perovskites La 3/2 Sr 1/2 NiO 4 and La 5/3 Sr 1/3 NiO 4 in the frequency range 20 Hz-1 MHz and in the temperature range 110-350 K. For this study, we have synthesized single phase polycrystalline samples by a conventional solid-state reaction. These nickelates show very high values of...

Polycrystalline samples of the charge ordered mixed oxide LaMn0.5Co0.5O3−δ (TCO = 400 K) have been prepared by the nitrate decomposition method. These samples are biphasic, according to XRPD, and except one, oxygen defficient (δ ≈ 0.04-0.05). The study of the dielectric properties of these samples reveal that LaMn0.5Co0.5O3−δ displays a high dielec...

The dielectric properties of mixed-oxide CaMn7O12 are reported. This compound exhibits a high dielectric constant at room temperature (ε�r> 104), for frequencies up to 104 Hz, values that make it very attractive for potential applications provided that its rather high losses can be minimized. Complex plane analysis of the obtained data reveal an im...

Starting from a simple definition of stationary regime in first-order relaxation processes, we obtain that experimental results are to be fitted to a power-law when approaching the stationary limit. On the basis of this result we propose a graphical representation that allows the discrimination between power-law and stretched exponential time decay...

We are reporting the dielectric response of La1.5Sr0.5NiO4, a system that presents a charge-ordered state above room temperature and a rearrangement of its charge-order pattern in the temperature region 160-200 K. A careful analysis of the role of the electric contacts used, sample thickness and grain size on the experimental data allows us to dete...

We have focused on the magnetic and electrical properties of a material of nominal composition Pr0.6Ba0.4CoO3−δ. Magnetic and EPR measurements, together with powder X-ray diffraction reveal that this material segregates into two closely related phases with different Curie temperatures. The observed results are discussed in the light of previously r...

The last sentence in the abstract reads: ''The phenomenon appears to be similar to that of the colossal magneto-resistance response observed in the Mg perovskite family.'' The sentence should be: ''The phenomenon appears to be similar to that of the colossal magnetoresistance response observed in the Mn perovskite family.'' The comparison is made t...

MnAs exhibits a first-order phase transition from a ferromagnetic, high-spin metal hexagonal phase to a paramagnetic, lower-spin insulator orthorhombic phase at T(C)=313 K. Here, we report the results of neutron diffraction experiments showing that an external magnetic field, B, stabilizes the hexagonal phase above T(C). The phase transformation is...

A complete characterization of the magnetotransport and magnetoelastic properties of two classical examples of half doped manganites, Nd0.5Sr0.5MnO3 (NSMO) and La0.5Ca0.5MnO3 (LCMO), was carried out by electrical resistivity, magnetoresistance, thermoelectric power, electron spin resonance (ESR), and thermal expansion measurements. We have identifi...

The electronic transport in the paramagnetic range of mixed valence manganites is studied here through resistivity and thermopower of Gd3+ doped La2/3Ca1/3MnO3. The analysis of the experimental results of the high temperature limiting value for the thermopower reveals a discrepancy with theory, which could be interpreted as a consequence of an incr...

We introduce the magnetotransport properties of Gd1-xSrxCoO3 (0 < x ≤ 0.30) materials. These compounds are semiconducting and show a magnetic transition at Tc ≈ 160 K associated with the onset of magnetic order of cobalt ions within superparamagnetic clusters, which have a freezing point TB < Tc. Very remarkably, the more resistive samples showed r...

A systematic study of the magnetic and transport properties of the system (La1-xGdx)(0.67)Ca0.33MnO3 (0less than or equal toxless than or equal to1) is presented in this article. Through this series, a great crystallographic distortion is achieved but keeping constant the electronic density. At low Gd3+ content, these ions behaves in a paramagnetic...

MnAs exhibits a first-order phase transition from a ferromagnetic, high-spin metal NiAs-type hexagonal phase to a paramagnetic, lower-spin insulator MnP-type orthorhombic phase at T_C = 313 K. Here, we report the results of neutron diffraction experiments showing that an external magnetic field, B, stabilizes the hexagonal metallic phase above T_C....

We have used the “Pechini” method for the efficient preparation of Pr1-xBxCoO3 (B2+ = Ba2+, Ca2+) samples. According to the XRD data, the so-obtained samples present an orthorhombic symmetry for x≤0.30 (B2+ = Ba2+, Ca2+), while the Pr0.5Ba0.5CoO3 compound has a tetragonal symmetry (a = b ≅ ac and c ≅ 2ac.) and Pr0.6Ba0.4CoO3 is a mixture of an orth...

We have prepared polycrystalline, single-phase Nd1−xSrxCoO3 materials (0≤x≤0.40) using the nitrate decomposition method and we have studied how their structural, magnetic and transport properties change upon Sr2+ doping. In this context we find that these series of compounds crystallize in GdFeO3-type perovskite structures whose cell volume increas...

In this work magnetic and magnetotransport experimental data in well-characterized small particles of La0.67Ca0.33MnO3 are presented. Grain size reduction leads to a larger resistivity and a decrease in metal–insulator transition temperature. Intrinsic colossal magnetoresistance (CMR) is destroyed while intergranular one is promoted to larger value...

We report DC magnetization and electron paramagnetic resonance (EPR) measurements in ceramic samples of La0.67Ca0.33Mn1-yCryO3 (0 less than or equal to y less than or equal to 0.1). We have observed a decrease of T-C with doping, which reveals a continuous dilution of the ferromagnetic exchange interaction. On the other hand, although the EPR linew...

The magnetic and electrical properties of R1-xSrxCoO3 materials (R=La, Pr, Nd, Gd) (0 less than or equal to x less than or equal to 0.50) are studied. Rare-earths with angular momentum L not equal 0 enhance coercive field values. They are maximum for R = Nd (the highest L) and much smaller for Gd and La (L = 0). The result is interpreted as the con...

Intergranular magnetotransport effects are studied on polycrystalline manganites. Reducing grain size through sol-gel technology allows us to show new results on these materials. Intrinsic colossal magnetoresistance is destroyed in metal-insulator phase transition for grain size smaller than 150 nm. In low temperature region, that is, T<30K, semico...

The transport properties of Pr2/3Sr1/3MnO3 and La2/3Sr1/3MnO3 perovskites have been studied by means of thermoelectric power measurements in high-quality single crystals. It was observed that, whereas Pr2/3Sr1/3MnO3 exhibits anisotropic conducting properties, La2/3Sr1/3MnO3 does not. The former crystallizes in an orthorhombic structure, with differ...

We refer here to the electrical and magnetic properties of the Ln1−xMxCoO3 systems (Ln3+: La3+, M2+: Ca2+, Sr2+, Ba2+; Ln3+: Nd3+, M2+: Sr2+), paying special attention to those ferromagnetic compounds that display M–I transitions as temperature rises: La1−xMxCoO3 (M2+: Ca2+, Sr2+, Ba2+) in the compositional interval x=0.2–0.3, and Nd1−xSrxCoO3, wit...

Charge ordering and antiferromagnetic transitions in Pr0.67Ca0.33MnO3 have been studied by resistivity, thermopower, magnetic susceptibility and capacitance measurements. Strong magnetic competition arising from the distorted perovskite structure leads to a complex behavior. The appearance of the charge ordering coincides with antiferromagnetism at...

We prepared in this work La1−xBaxCoO3−δ (0<x≤0.50) compounds using the nitrate decomposition method, and focus in the study of the transport properties of the ferromagnetic compounds (x>0.15, Tc≈200 K) in the temperature range 77 K≤T≤300 K. We find that while for x<0.20 these materials show semiconducting behavior, their electrical conduction being...

In this article we present d.c. resistivity, a.c. impedance, magnetization and dielectric permittivity measurements in Pr0.67Ca0.33MnO3. A rise in the imaginary part of impedance was detected at the charge ordering transition temperature (T=225K). This indicates an intrinsic capacitive behavior of the sample in the charge ordering state. The a.c. i...

We have used three soft-chemistry methods for the efficient preparation of Ln1−xSrxCoO3 samples, adapting the combustion and the liquid-mix methods for the synthesis of the Ln = La and Gd compounds, respectively, and the preparation of Nd1−xSrxCoO3 by the nitrate decomposition method. We report the magnetic and electrical properties of these relat...

Co fine particles coated with Ag have been synthesized through the microemulsion method in an inert atmosphere. The size of the particles is controlled by the water droplets of the microemulsions. Fine particles prepared by this method, consist of a magnetic core of Co covered by a layer of Ag. Samples containing from 3.3 to 40.5 vol % Co have been...

Measurements of electron paramagnetic resonance (EPR) at 9 and 35 GHz between 2 and 300 K in single crystals and powder samples of CuGeO 3 are presented. Below 14 K a large decrease in the intensity of the Cu²⁺ EPR signal is observed. The data can be interpreted as due to a spin‐Peierls transition. However, an alternative interpretation...

We report the results obtained by x‐ray, dc magnetization, ac magnetic susceptibility, microwave magnetic absorption, and electron paramagnetic resonance of the magnetic and superconductive properties of a large series of compounds of the form A 2-x-y B x Ce y CuO 4 with A,B≡La, Pr, Nd, Sm, Eu, Gd, and Y. For y=0 antiferromagnetic ordering of the c...

A procedure is described for the calculation of the amplitudes and times which fit a sequence of data items, equally spaced in time, to a discrete sum of exponentials by minimizing the sum of the squares of the errors. This procedure consists in the calculation of the Fourier transform of the sequence and its fitting by means of a rational function...

MnAs, a commercially available material first studied a century ago, exhibits a first-order phase transition from a ferromagnetic, high-spin metal NiAs-type hexagonal phase to a paramagnetic, lower-spin insulator MnP-type orthorhombic phase at T_C= 313 K. We report the results of neutron diffraction experiments showing that an external magnetic fie...

We report DC magnetization and electron paramagnetic resonance (EPR) measurements in ceramic samples of La Ca Mn \W Cr W O (0)y)0.1). We have observed a decrease of ¹ ! with doping, which reveals a continuous dilution of the ferromagnetic exchange interaction. On the other hand, although the EPR linewidth (H) depends on the doping level, its high-t...