Raul Valenzuela

Universidad Nacional Autónoma de México | UNAM · Materials Research Institute (IIM-UNAM)

Magnetic materials are crucial for the efficiency of the conversion-storage-transport-reconversion energy chain, and the enhancement of their performance has an important impact on technological development. The present work explores the possibility of preparing hetero-nano-structured ceramics based on magnetic oxides, by coupling a ferrimagnetic p...

Hetero-nanostructures based on magnetic contrast oxides have been prepared as highly dense nanoconsolidates. Cobalt ferrite-cobalt oxide core-shell type nanoparticles (NPs) were synthesized by seed mediated growth in polyol and subsequently consolidated by Spark Plasma Sintering (SPS) at 500 °C for a few minutes while applying a uniaxial pressure o...

Cobalt ferrite ferrimagnetic nanoparticles (NPs) are prepared and used in this work as seeds to grow a thin antiferromagnetic poly‐ and nanocrystalline CoO shell. The major purpose is to study systematically the characteristics of the as‐produced powders, making emphasis on their internal crystallographic arrangement and their magnetic properties....

Magnetically contrasted granular hetero‐nanostructures are prepared by seed‐mediated growth in polyol, properly combining two oxide phases with different magnetic order, ferrimagnetic (F) partially oxidized magnetite Fe3−xO4 and antiferromagnetic (AF) cobalt oxide. Spinel Fe3−xO4 nanoparticles are first synthesized and then used as seeds for rock s...

There is a great interest to synthesize ferroelectric ceramics both with fine grain size and significant electric properties. Here, we report the preparation of nanostructured tetragonal barium titanate by combining forced hydrolysis of metallic salts in polyol, soft annealing and 650 °C spark plasma sintering under uniaxial pressure of 120 MPa for...

Ferrites are a well-known class of ferrimagnetic materials. In the form of nanoparticles (NPs), they exhibit novel and fascinating properties, leading to an extremely wide variety of applications in electronics, biomedical and environmental fields. These applications result from nanoscale effects on physical properties, particularly magnetic proper...

Barium hexaferrite (BaFe12O19) ceramics were obtained by combination of polyol-synthesized precursors subsequently consolidated by spark plasma sintering (SPS) at a low temperature (800 ∘C). X-ray powder diffraction studies showed the influence of the experimental parameters to produce a virtually single- or multiphase material. The room temperatur...

Granular BaTiO3-CoFe2O4 (BTO-CFO) nanocomposites were prepared by combining polyol synthesis and spark plasma sintering (SPS). This method allows samples with a high density and a very small grain size (less than 150 nm), and thus a large interface area between phases. In order to study the involved magnetoelectric effects, the impedance response o...

The partial substitution of Fe3+ by Sc3+ in barium hexaferrite has shown to be an effective method to tailor anisotropy for many novel microwave applications. Some basic studies have revealed that this substitution leads to unusual interactions among the magnetic sublattices of the ferrite. In order to investigate these interactions, samples with f...

We present a comparative study of synthesis of M-type strontium hexaferrite from strontium and iron single oxides mechanically activated by high-energy ball milling for 5 h, and assisted by two different methods: (a) conventional heat treatment and (b) Spark Plasma Sintering (SPS), both at relative low temperatures ( 900 °C). Although it was found...

Ferrite magnetic nanoparticles in the composition Zn0.7Ni0.3Fe2O4 were synthesized by the polyol method, with an average size of 8nm. Electron spin resonance (ESR) measurements were carried out at a frequency of 9.45 GHz in the 100–500K temperature range. Obtained results exhibited a characteristic ESR signal in terms of resonance field, Hres, linew...

Magnetic nanoparticles (NPs) of magnetite and Ni–Zn ferrites, synthesized by the forced hydrolysis in a polyol method, were used as the starting material to consolidate them by means of the spark plasma sintering (SPS) method. The microstructural changes were analyzed by X-ray diffraction, transmission electron microscopy (TEM), magnetic hysteresis...

Magnetite nanoparticles about 10 nm sized were synthesized by the polyol method. Zero-field-cooled (ZFC)-FC measurements showed a blocking temperature ̃170 K and the absence of the Verwey transition. They were subsequently consolidated by spark plasma sintering at 750 °C for 15 min, leading to a high density (92% of the theoretical density), solid...

M-type strontium hexaferrite was prepared by mechanosynthesis using high-energy ball milling. The influence of milling parameters, hematite excess and annealing temperature on magnetic properties of SrFe12O19 were investigated. Commercial iron and strontium oxides were used as starting materials. It was found that mechanical milling followed by an...

Spinel ferrite nanoparticles (NPs) have raised interest due to their potential technological applications in fields as varied as high frequency electronic device components, soil remediation, and medical diagnosis and treatments. In this paper, we present a brief review of the magnetic properties of spinel ferrite NPs (Ni-Zn, Co, and magnetite) syn...

Stoichiometric mixtures of FeO and Y2O3 were milled and heat treated to obtain yttrium iron garnet, Y3Fe5O12. Two types of heating systems were used: one, a spark plasma sintering machine and the second, an electrical oven. The magnetic properties of the resulting specimens have been analyzed and discussed as a function of the grain size and the pa...

We report the synthesis of yttrium iron garnet (YIG) combining soft chemistry route, namely the polyol process, and spark plasma sintering (SPS) technique. The polyol process produced an intermediary amorphous phase containing both iron and yttrium cations in the desired ratio. They were annealed at 400°C in air to decompose the organic content of...

Solid solutions of yttrium-gadolinium orthoferrites GdxY1-xFeO3 (0 x 1) were prepared by high-energy ball milling. The aim of this work was to study the influence of the synthesis parameters on the crystal structure and the magnetic behavior of these solid solutions. The precursors, Fe2O3, Y2O3 and Gd2O3, mixed in a stoichiometric ratio to obta...

Multiferroic systems formed by a mixing of a ferroelectric phase and a ferrimagnetic phase are receiving significant attention because of their wide possibilities for tailoring properties. In this work, the magnetic properties of the cobalt ferrite-barium titanate system were investigated on samples prepared by an original combination of synthesis...

Yttrium iron garnet, Y3Fe5O12 (YIG) powders were synthesized by mechanochemical processing (MCP) from different iron sources (FeO, Fe2O3 and Fe3O4) mixed with Y2O3, followed by a heat treatment. The aim of this work is to demon- strate that MCP followed by annealing at very low temperatures (as compared with the classic solid state reaction) can in...

Amorphous wires of composition Fe64Co8B19.2Si4.8M4(M = Ta, Y) and 140 μm of diameter were obtained by rotating-water-bath-melt-spinning technique. Saturation magnetization (M s) and coercivity values (H c), at room temperature, were in the range of 1 T and 31 A/m, respectively, for the Ta samples, and M s = 1.32 T and H c = 109 A/m, respectively, f...

Ferrite nanoparticles of composition Zn0.5Ni0.5Fe2O4 were synthesized by forced hydrolysis in polyol from the corresponding zinc, nickel and iron acetates. By varying the preparation conditions, different aggregation states were obtained, ranging from isolated nanoparticles with average diameter of 5 nm, to clusters of some 20 nm, formed as well by...

The low field microwave absorption (LFMA at 9.5 GHz) properties of clusters of Zn0.5Ni0.5Fe2O4 ferrite nanoparticles (NPs) consolidated by spark plasma sintering (PSP at temperatures in the 350–500 °C range) are analyzed. LFMA measurements were carried out at 77 and 300 K. The clusters, formed by ∼6 nm NPs, were first obtained by the forced hydroly...

We report on the synthesis, consolidation, magnetic properties and electrical conductivity of cobalt ferrite-barium titanate nanostructured ceramics. The precursor oxides were separately synthesized as nanoparticles by the polyol method, namely forced hydrolysis of metallic salts in a polyol solvent, mixed and consolidated by spark plasma sintering...

Yttrium iron garnet, Y3Fe5O12 (YIG) powders were synthesized by mechanochemical processing (MCP) from Fe2O3 and Y2O3, followed by an annealing. The aim of this work was to demonstrate that MCP followed by annealing at relative low temperatures can induce the formation of nanostructured YIG. The effect of synthesis process on the final magnetic prop...

The applications of ferrimagnetic oxides, or ferrites, in the last 10 years are reviewed, including thin films and nanoparticles. The general features of the three basic crystal systems and their magnetic structures are briefly discussed, followed by the most interesting applications in electronic circuits as inductors, in high-frequency systems, i...

Bulk metallic glasses of nominal composition Nd60Fe30Al10 were prepared by copper mold casting in the form of rods of 3 mm in diameter and 50 mm in length. Preparation conditions were varied to assess the effects of the injection distance and the injection pressure of the melt. In order to determine their microstructure, disk-shaped samples were ob...

Ferrite nanoparticles of composition Zn0.5Ni0.5Fe2O4 with average diameter of 5-8 nm were prepared by forced hydrolysis in polyol from the corresponding iron, nickel and zinc acetates, in different states of aggregation, by playing with the preparation parameters. High-resolution transmission electron microscopy showed that for the aggregated state...

Ferrite nanoparticles of composition Zn0.5Ni0.5Fe2O4 were prepared by forced hydrolysis in a polyol (polyol process) from the corresponding iron, nickel, and zinc acetates. Synthesis conditions allowed for obtaining polycrystalline epitaxial clusters of about 22 nm in size with an average crystal size of about 5 nm. These powders were subsequently...

We present a detailed investigation of the role of the disorder degree on magnetic properties for melt spun and die cast intermetallic Nd-Fe-Al-based alloys. For melt spun alloys, the roll speed was varied between 30 m/s and 5 m/s, which resulted in a clear dependence of coercivity on increasing roll speed, showing a steep decrease (from 250 kA/m t...

Impedance spectroscopy (IS) is a powerful characterization methodology for a wide range of electric materials such as ceramics, ferroelectrics, ion conductors, piezoelectrics, etc. In this paper, an extension of IS to the case of ferromagnetic materials is presented. In particular, the resolution of the most common magnetization processes (domain w...

Spinel ferrites of composition Ni0.5Zn0.5Fe2O4 were prepared by coprecipitation. Different grain size and grain size distributions were obtained by various heat treatments. Their electrical properties were investigated by impedance spectroscopy in the temperature range 25-200°C and in the frequency range of 5Hz-13MHz. A decrease in resistivity was...

The thermal variations in electrical conductivity of polycrystalline Mn-Zn ferrites was investigated by impedance spectroscopy. Two well defined semicircles were observed in complex impedance plots, which were associated with grain (or bulk) and grain boundary impedance response, for the high and low frequency ranges, respectively. From these resul...

We present differential scanning calorimetry (DSC) and in situ Mössbauer spectroscopy results for Metglas ribbons, to which different heat treatments were made. The Curie temperature of the amorphous phase is determined and the evolution of the magnetic field of this phase is studied as a function of temperature.

A phenomenon known as Giant Magnetoimpedance (GMI) is attracting interest both for its technological applications in magnetic field sensors and for its physical basis. In this paper, a review of GMI is presented: first, a brief description of the main characteristics of GMI is discussed, an overview of the main applications is presented, and finall...

Impedance spectroscopy has been widely used to analyze polarization phenomena in dielectric, ferroelectric and solid electrolyte materials. In this paper, we review its extension to ferromagnetic materials, and discuss its application to the case of irreversible domain wall displacements which lead to ferromagnetic hysteresis phenomena.

An investigation of the frequency behavior of polycrystalline ferrites is presented. It is shown that the low frequency dispersion (f < 10 MHz) of permeability is associated with the bulging of pinned domain walls, and has a mixed resonance-relaxation character, closer to the latter. It is also shown that there is a linear relationship between the...

Microstructure and magnetic properties for an as-quenched, single rod (3mm diameter, 50mm length) of Nd60Fe30Al10 alloy obtained by copper mold casting technique, were characterized on three different zones along the rod length selected at the bottom, middle and upper part. The bottom part was in direct contact with an additional copper plate, whil...

The thermomagnetic behaviour (within the temperature range 553-300 K) for the bulk composite Nd60Fe30Al10 alloy is described in terms of a transition from paramagnetic to superferromagnetic state at T = 553 K, followed by a ferromagnetic ordering for T < 473 K. For the superferromagnetic regime, the alloy thermomagnetic response was associated to a...

A study of low-field non-resonant microwave absorption (LFA) at 9.8 GHz, on as-cast amorphous Co-rich CoFeBSi microwires under different measuring geometries is presented. Results confirm that LFA is associated with the magnetization processes from the unmagnetized state (HDC = 0) to the saturated condition, in many aspects similar to Giant Magneto...

Metallic microwires with ferromagnetic nature and amorphous structure have been prepared by rapid solidification and drawing. Their alloy composition and thickness, 1 to 30 micron diameter, can be tailored to exhibit outstanding properties. Particularly, non-magnetostrictive CoFeSiB alloy microwires exhibit giant-magnetoimpedance effect while FeSiB...

Microwave absorption measurements on a NiFe/Au/NiFe multilayer film were carried out at X-band (8.8–9.8 GHz). The angular dependence of microwave absorption, both in ferromagnetic resonance (FMR) and low-field microwave absorption (LFA), is investigated in two orientations. In both cases the film plane is orientated parallel to the AC field. In the...

Melt-spun Co66Fe4B12Si13Nb4Cu soft magnetic ribbons were devitrified at low annealing temperatures (623 K), for times 5–20 min. Microwave power absorption measurements at 9.4 GHz (X-band) were carried out in two geometries. In geometry 1, the ribbon’s plane was oriented parallel to AC magnetic field. For the orientation 2, the ribbon’s plane was no...

An electron paramagnetic resonance (EPR) study of ferroelectromagnet Pb(Fe1/2Nb1/2)O3 (PFN) powders is presented. The EPR spectra show a single broad line in the 300–500 K temperature range, attributable to Fe3+ ions. The onset of the ferro paraelectric transition was determined from the temperature dependence of three main parameters deduced from...

In this paper we investigate different aspects of the high-frequency magnetoimpedance at low fields. In the first place, we will study this effect from a phenomenological point of view, comparing it with the low-frequency microwave absorption. The results, discussed in terms of an analytical description of the experiments, prove that both effects r...

A review of the dynamic properties of nanostructured ferromagnetic materials at microwave frequencies (1-40 GHz) is presented. Since some confusion has recently appeared between giant magnetoimpedance (GMI) and ferromagnetic resonance (FMR), a detailed analysis is made in order to establish their differences. A brief review of a novel microwave abs...

We report on the effects of temperature and dc magnetic field on the microwave power absorption measurements at X-band (8.8–9.8 GHz), in powder samples of YMnO3 (YM). Two techniques are used: magnetically modulated microwave absorption spectroscopy (MAMMAS) and low-field microwave absorption spectroscopy (LFMAS). The measurements were performed in...

We report electron paramagnetic resonance (EPR) and magnetically modulated microwave absorption spectroscopy (MAMMAS) studies at X-band (8.8–9.6 GHz) on powdered SrMnO3 in the 90–400 K temperature range. EPR spectrum shows one broad single-line at room temperature, which is observed only above 280.5 K, being compatible with an antiferromagnetic ord...

A low-field non-resonant microwave absorption has recently been observed in a variety of magnetically ordered materials at low DC fields (−1000 Oe ⩽HDC⩽+1000 Oe), which is known as low-field microwave absorption (LFA). It has been shown that LFA is essentially similar to giant magnetoimpedance (GMI), and clearly different from ferromagnetic resonan...

Cobalt ferrite nanoparticles were obtained by the sol–gel method at several annealing temperatures: 400, 450, 500, 550 and 600°C. X-ray diffraction (XRD) and transmission electron microscopy (TEM) showed the formation of the spinel phase with a nanoparticle size in the 17–26nm range as function of the annealing temperature. The Mössbauer spectra at...

We report on the effects of temperature and DC magnetic field on the modulated non resonant microwave power absorption measurements at 8.8-9.8 GHz, in powder samples of iron niobate, FeNbO4 (FN), in the monoclinic phase (wolframite-structure). Two techniques recently implemented are used: magnetically modulated microwave absorption spectroscopy (MA...

As-spun ribbons of YxPr1−xCo5 alloys with x=0.25, 0.50, and 0.75, were produced by single-roller melt spinning at wheel speeds of 5, 15, and 40 ms−1. X-ray diffraction patterns and SEM micrographs show that samples are essentially single phase with the hexagonal CaCu5-type crystal structure and dendritic microstructure. Hysteresis loops were measur...

We have investigated the Verwey phase transition (VPT) by two novel non-resonant microwave absorption techniques: low-field absorption (LFA) and magnetically modulated microwave absorption spectroscopy (MAMMAS). Measurements were carried out on sintered polycrystalline samples of Fe3O4, in the 77–300 K temperature range. LFA refers to the microwave...

We report on the effects of temperature and DC magnetic field on microwave power absorption measurements at 8.8–9.8 GHz, in powder samples of the ferroelectromagnet Pb(Fe1/2Nb1/2)O3 (PFN). Two techniques recently implemented are used: magnetically modulated microwave absorption spectroscopy (MAMMAS) and low-field microwave absorption spectroscopy (...

We report electron paramagnetic resonance (EPR) and magnetically modulated microwave absorption spectroscopy (MAMMAS) studies on powdered BaMnO3 and Li2MnO3 in the 77–300K temperature range. The two oxides showed one single-line EPR spectra at room temperature. For Li2MnO3 the absorption line changed continuously with temperature, and showed a para...

Cobalt ferrite particles of average crystallite size of 11nm were obtained by a sol–-gel process at 400°C. The powders were annealed at temperatures of 500, 600, 700 and 800°C in air. Derivative microwave power absorption (dP/dH) measurements were carried out as a function of magnetic field (HDC) at X band (9.4GHz), in the field range −80–796kA/m f...

In this work we present Giant Magneto-Impedance measurements of a NiFe-Au multilayer film at frequencies up to 3 GHz, using RF techniques, complemented with a derivative of the microwave absorption spectrum at 9.4 GHz. The curves allow to identify two different type of peaks: ones at low fields, associated to transverse permeability and others at f...

Microwave absorption measurements were carried out on glass-uncoated amorphous microwires of 24, 12, and 7 μm in metallic core diameter. LFA spectra show a maximum and minimum on the magnetic field cycle sweep, indicating an anisotropy field of 1.4 Oe for 12 and 24 μm and 2.4 Oe for 7 μm microwire diameters. When comparing these results with those...

For many applications, the efficiency of giant magnetoimpedance (GMI) increases as the working frequency increases; the natural limit seems to be ferromagnetic resonance (FMR). However, MI is an essentially different phenomenon than FMR. The latter is a quantum-mechanical phenomenon which should satisfy the Larmor equation, while MI extends continu...

We present measurements of low-field power absorption (LFA) at 9.45 GHz on an amorphous ribbon of nominal composition (Fe0.06Co0.94)75Si15B10, at room temperature. These results show an anti-symmetrical shape around zero magnetic field that peaks at ±34 Oe. We also present measurements of magnetoimpedance (MI) at 60 MHz and 3 GHz. In order to make...

Microwave power absorption measurements at 9.4 GHz (X-band) were carried out on as-cast amorphous ribbons of nominal composition Co66Fe4B12Si13Nb4Cu, prepared by melt spinning. The angular dependence of low-field absorption (LFA) was investigated from h = 0 to 180 in two orientations. In both cases the ribbon axis was orientated parallel to the DC...

High frequency [1-500 MHz] measurements of the magnetoimpedance (MI) of glass-coated Co$_{69.4}$Fe$_{3.7}$B$_{15.9}$Si$_{11}$ microwires are carried out with various metal-to-wire diameter ratios. A twin-peak, anhysteretic behaviour is observed as a function of magnetic field. A maximum in the normalized impedance, $\Delta Z$/$Z$, appears at differ...

An electron paramagnetic resonance (EPR) study on the ferroelectromagnet Pb(Fe1/2Nb1/2)O3 (PFN) powder is presented. The EPR spectra show one single broad line in the 300–90K temperature range. The onset of the para-antiferromagnetic (AF) transition has been determined from the temperature dependence for three main parameters deduced from the EPR s...

Ferromagnetic resonance (FMR) measurements were carried out on soft magnetic amorphous ribbons of composition Co66Fe4B12Si13Nb4Cu prepared by melt spinning. In the as-cast sample, a simple FMR spectrum was apparent. For treatment times of 5–20 min a complex resonant absorption at lower fields was detected; deconvolution calculations were carried ou...