Dario Bueno-Baques

Enhanced magnetoelectric effect in core-shell particulate composites

A method for the preparation of magnetostrictive-piezoelectric particulate composites with enhanced magnetoelectric effect was developed. The composites were synthesized in situ forming a shell of barium titanate around nanoparticles of cobalt ferrite, varying the composition of the cobalt ferrite magnetostrictive phase from 20to60wt.%. Cobalt ferrite nanoparticles were obtained by coprecipitation and then added to the precursor gel of barium titanate, allowing the in situ formation of the composite and thereby restricting the contact of the ferrite particles during sintering. The samples were sintered at a temperature ranging from 1100to1250°C for 12h, followed by a plating step to be electrically poled. Additional samples were prepared by conventional mechanical milling for comparison, starting from cobalt ferrite prepared either by coprecipitation or the sol-gel technique and commercial barium titanate. Samples of same compositions prepared by different methods and sintered under the same conditions sh...

Highly textured Sr, Nb co-doped BiFeO3 thin films grown on SrRuO3/Si substrates by rf- sputtering

In this study, (011)-highly oriented Sr, Nb co-doped BiFeO3 (BFO) thin films were successfully grown on SrRuO3/Si substrates by rf-magnetron sputtering. The presence of parasite magnetic phases was ruled out based on the high resolution x-ray diffraction data. BFO films exhibited a columnar-like grain growth with rms surface roughness values of ≈5.3 nm and average grain sizes of ≈65–70 nm for samples with different thicknesses. Remanent polarization values (2Pr) of 54 μC cm−2 at room temperature were found for the BFO films with a ferroelectric behavior characteristic of an asymmetric device structure. Analysis of the leakage mechanisms for this structure in negative bias suggests Schottky injection and a dominant Poole-Frenkel trap-limited conduction at room temperature. Oxygen vacancies and Fe3+/Fe2+ trap centers are consistent with the surface chemical bonding states analysis from x-ray photoelectron spectroscopy data. The (011)-BFO/SrRuO3/Si film structure exhibits a strong magnetic interaction at the...

The magnetoelectric effect in pulsed magnetic fields

A measurement method for studying the magnetoelectric effect in pulsed magnetic fields was developed. Besides magnetization, the system allows the measurement of magnetostriction (using strain gauges) and the magnetoelectric effect. For measuring the magnetoelectric effect a special sample holder with fixed contacts and compensation coils was developed to ensure the reproducibility of the measurements. The magnetoelectric effect can be measured by two different methods: (i) During the application of a magnetic field pulse when the voltage appearing on the sample is directly recorded and (ii) As a function of a high frequency (f=10kHz) ac field superimposed to the main field pulse. In this way an ac voltage appears in the sample which is proportional to the charge change ΔQ=Q0sin(wt+Φ). This voltage is measured using a fast analog lock-in amplifier. A detailed description of the total system is provided. The magnetoelectric effect on well known composites of cobalt ferrite and barium titanate was measured ...

Preparation of Electrospun Barium Titanate – Polyvinylidene Fluoride Piezoelectric Membranes

Piezoelectric fibrous membranes of barium titanate (BTO) – polyvinylidene fluoride (PVDF) nanocomposites were studied. BTO nanoparticles of about 40 nm were synthesized by the sol-gel method and mixed in PVDF–dimethyl formamide solutions at 0, 0.1, 1 and 3 wt.% relative to the polymer weight. The suspensions were electrospun using a horizontal set-up with an applied voltage of 15 kV. The samples were heat treated for 24 hours at 100°C in air to increase the crystallinity of the polymer. The heat treatment induced a phase transformation from α to β phase in the pure polymer sample, while the nanocomposite membranes did not undergo such phase transformation. It was found that the addition of nanoparticles affected not only the morphology and diameter of the fibers, but also the content of beta phase of the polymer. In order to pursue the crystallization of β phase, additional samples were prepared by surface modification of the BTO nanoparticles and the addition of tetraisopentyl ammonium chloride. The best results were obtained with the last additive, which lead to the crystallization of only β phase and a homogeneous fibrous morphology. All these aspects were strongly correlated and consequently, governed the ferroelectric behavior of the samples.

Magnetic Properties of Spinel Cobalt–Manganese Ferrites

Cobalt ferrite (CoFe2O4) and manganese substituted cobalt ferrite (Co0.25Mn0.75Fe2O4) were synthesized by coprecipitation and sol-gel methods. X-ray diffraction revealed that manganese ions slightly enlarged the crystal lattice with respect to pure cobalt ferrite. Magnetization, linear magnetostriction, and piezomagnetic coefficient values tended to decrease with the ion substitution. Linear magnetostriction dropped from a value of -111.5 ppm for cobalt ferrite to values of -22.6 and -30.6 ppm for the substituted samples obtained from coprecipitation and sol-gel, respectively. The sample prepared by sol-gel technique resulted in a higher magnetization, magnetostriction, and piezomagnetic coefficient than that synthesized by coprecipitation method.

Development of a new module for the measurement of the magneto-electric direct and converse effects based on an alternating current susceptometer

A new module for the measurement of magneto-electric properties was developed as an add-on for a magnetic AC susceptibility option of a Physical Properties Measurement System (PPMS). The module is capable of recording direct dynamic and static converse magneto-electric effect, i.e., the change in electric polarization due to the application of a small AC magnetic field with a DC magnetic field bias, or the change in the magnetic moment induced by an applied electric field. The versatile module setup supports both measurements in a sequential order without the need of removing or repositioning the sample. Furthermore, AC and DC magnetic susceptibilities can be recorded while performing direct and inverse magneto-electric measurements, respectively, which adds outstanding capabilities to the existing instrument while saving time and resources. Measurements are fully automated and integrated in the PPMS Multivu software platform. Magneto-electric behavior of a BaTiO3/CoFe2O4 and BaTiO3/NiFe2O4 magneto-electric composites, and a Pb(Fe(0.5)Nb(0.5))O3 single phase compound were recorded as test measurements.

Synthesis and Characterization of Magnetic Polyurethane Nanocomposite Foams

New magnetic polyurethane nanocomposite foams have been synthesized by the one-shot method. The opened-cell soft foams contain a dispersion of uniformly sized nanocrystalline iron oxide prepared ex situ by the high temperature thermal decomposition of iron acetylacetonate in phenyl ether in the presence of oleic acid. The magnetic particles were dispersed in polyol by sonication prior to the reaction of the latter with isocyanate to produce the magnetic foams. A 7 wt% loading of iron oxide yielded a soft, flexible foam with a room temperature magnetization of 3.5 emu/g at one Tesla. Higher loadings of iron oxide are possible without destruction of the soft open-cell polyurethane structure. Physicochemical characterization of the foams will be presented along with magnetic and mechanical properties. Potential applications include smart materials such as magnetic shape memory foams, adhesive-free metal to foam seals, inductively coupled thermal foams and applications for conformable foams having a magnetic component such as healthcare products.