Papers by Raul Valenzuela
A position sensor based on magnetoimpedance
Journal of Applied Physics, 1996
A magnetic-field sensor based on the giant magnetoimpedance phenomenon is presented. It is shown ... more A magnetic-field sensor based on the giant magnetoimpedance phenomenon is presented. It is shown that a low, negative magnetostriction CoFeBSi amorphous wire can be used to detect the presence or passage of moving pieces or vehicles, simply by pasting a small permanent magnet on the vehicles/pieces. The detection is observed as a decrease in the ac voltage on the wire’s ends. A system of such devices can be used to monitor and control a number of industrial processes.
Magnetic ceramics: Preface
Magnetic Transitions Studied by Electrically Based Methods in Mn-Zn Ferrite
MRS Proceedings, 1997
ABSTRACTThe thermal variations in electrical conductivity of polycrystalline Mn-Zn ferrites was i... more ABSTRACTThe 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 results, the characteristic relaxation frequencies were obtained for each process. The grain boundary frequency exhibited a monotonous behavior as a function of temperature, while the bulk frequency showed a maximum at 132 °C. By a magnetic method, it was verified that this temperature corresponds to the Curie transition. These results show therefore that a magnetic phase transition can be studied by means of electrical methds.
Magnetoimpedance: a Choice Property for Smart Materials
MRS Proceedings, 1999
A phenomenon known as Giant Magnetoimpedance (GMI) is attracting interest both for its technologi... more 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 finally, an interpretation of the physical basis underpinning GMI is proposed
Le Journal de Physique Colloques, 1977
Rbum6.-Les spectres d'absorption Mossbauer du ferrite de nickel et de ferrites Ni-Zn nonstoechiom... more Rbum6.-Les spectres d'absorption Mossbauer du ferrite de nickel et de ferrites Ni-Zn nonstoechiometriques, dans les compositions (0,3 Ni0-0,7 Zn0)1-~(Fe20,)1+~, x = 0 et x = 0,02, ont ete obtenus A la temperature ambiante et a 77 K. Pour le ferrite de Ni, les champs hyperfins associes aux ions ferriques dans les sites octaedriques et tetraedriques ont CtC rCsolus, pour les deux temperatures. Les intensites relatives de ces spectres presentent un comportement particulier en fonction de la temperature. Dans le cas des ferrites Ni-Zn non-stoechiomCtriques, les champs hyperfins n'ont pas CtC rksolus. Cependant, ces spectres presentent des caracteristiques differentes, ce qui montre que, mCme pour des petits &arts par rapport a la composition stoechiomktrique, les changements structuraux sont importants.
Materials Science and Engineering: A, 1991
The crystallization kinetics of amorphous ribbons in the system F%0 .~VrBj2Sis was studied by dif... more The crystallization kinetics of amorphous ribbons in the system F%0 .~VrBj2Sis was studied by differential scanning calorimetry, for x = 0.5-15. Except for x= 8, two crystallization peaks were observed for the whole composition range. The existence of a single crystallization peak for x = 8 indicates the eutectic composition. Effective activation energies for the two crystallization peaks showed a similar constant value of about 3.6 eV for x < 8. The effective activation energy values are different for x > 8, exhibiting a minimum for the first peak and a maximum for the second at x = 12.5. Evidence of surface crystallization and ageing was observed for the whole composition range.
Journal of Non-Crystalline Solids, 2008
Microwave absorption measurements on a NiFe/Au/NiFe multilayer film were carried out at X-band (8... more 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 longitudinal orientation, the film axis makes angles between 0°a nd 90°with the DC magnetic field (H dc). In the transverse orientation, the film axis is always perpendicular to H dc , marking angles between 0°to 90°with the axis of the induced transverse anisotropy. For the longitudinal orientation, FMR spectra suggested a compound absorption mode that can be interpreted as the combination of two different magnetic phases. Additionally, these measurements showed an increase in the resonance field as a function of the angle, which can be explained in terms of a contribution of shape anisotropy field (SAF). For this same orientation, the LFA spectra exhibited a compound antisymmetric shape around zero field with double peaks, which we associated with each one of the magnetic phases. The separation of these peaks increased as a function of the angle between the DC field and the multilayer film axis, suggesting also a contribution from SAF. In the transverse orientation, we observed an additional contribution of induced transverse anisotropy field (ITAF) in FMR measurements. The LFA measurements exhibited differences with the longitudinal orientation which are also associated with ITAF.
Characterization of Joule-heated Co-rich amorphous alloys under applied tensile stress by the inductance spectroscopy method
Journal of Magnetism and Magnetic Materials, 1999
The inductance response of stress-annealed (1.55 A for 5 min, with tensile stress in the 300–615 ... more The inductance response of stress-annealed (1.55 A for 5 min, with tensile stress in the 300–615 MPa range) amorphous ribbons of Vitrovac® 6025 was measured by applying an AC magnetic field (5 Hz—13 MHz) in the longitudinal and transverse axis. Results can be interpreted in terms of a combination of axial domains isolated and surrounded by a fine system of
Domain wall pinning, bulging, and displacement in circumferential domains in CoFeBSi amorphous wires
Journal of Applied Physics, 2000
A detailed study of the complex inductance response of as-cast CoFeBSi wires as a function of fre... more A detailed study of the complex inductance response of as-cast CoFeBSi wires as a function of frequency (100 Hz-13 MHz range) and circular field amplitude [0.1-17 A/m room mean square (rms) on the surface of the wire] in the giant magnetoimpedance arrangement is presented. The analysis of experimental results show evidence of magnetization processes associated with circumferential domain walls, such as domain wall pinning, bulging, and displacement. The unpinning field was measured as 0.2 A/m (rms) at 5 kHz. It is shown that the analysis of spectroscopic plots of real and imaginary inductance leads to characterization of the magnetization process involved.
Remanence of the interparticle interactions and its influence on the microwave absorption in Co-ferrite
Journal of Magnetism and Magnetic Materials, 2008
Cobalt ferrite nanoparticles were obtained by the sol–gel method at several annealing temperature... more 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 room temperature showed the presence of a partial
Low field magnetoimpedance in the GHz range
Sensors and Actuators A: Physical, 2008
In this paper we investigate different aspects of the high-frequency magnetoimpedance at low fiel... more 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 respond to the same physical causes. In the
Journal of Magnetism and Magnetic Materials, 2007
Recent measurements of the magnetoimpedance (at a fixed frequency of 1 MHz) of cobalt-rich wires ... more Recent measurements of the magnetoimpedance (at a fixed frequency of 1 MHz) of cobalt-rich wires subjected to torsion stress show an asymmetry as a function of torsion angle stemming from residual anisotropies induced during wire fabrication. We interpret these measurements with a simple model based on the competition between a circumferential magnetic anisotropy and another one induced by torsional and residual stress. This allows extraction of the physical parameters of the wire and explains the positive and negative torsion cases. The agreement between theoretical and experimental results provides a firm support for the model describing the behaviour of the anisotropy field versus static magnetic field for all torsion angles.
Journal of Magnetism and Magnetic Materials, 1995
Journal of Magnetism and Magnetic Materials, 2004
A novel broad-band measurement method of the MI in thin films and ribbons is presented. It is bas... more A novel broad-band measurement method of the MI in thin films and ribbons is presented. It is based on the automated measurement of the reflection coefficient of a cell loaded with the sample. Illustrative results obtained with a permalloy multilayer thin film are presented and discussed.
Nanostructured Materials - Fabrication to Applications, 2017
Ferrites are a well-known class of ferrimagnetic materials. In the form of nanoparticles (NPs), t... more 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 properties. For applications in electronic devices, however, a high-density, consolidated body, with very fine grains is needed, in order to retain the nanoscale properties. To our knowledge, spark plasma sintering (SPS) is the only method permitting a full densification with final grain size in the nanometer range. In this review, we examine the SPS method as applied to ferrites and, in particular, the effects of SPS parameters on the final nanostructures obtained. Due to their technological impact, we also discuss the SPS fabrication of hybrid multiferroic nanostructures composed of a ferrite and a ferroelectric phase.
Giant Exchange-Bias in Polyol-Made CoFe2 O4 -CoO Core-Shell Like Nanoparticles
Particle & Particle Systems Characterization, 2018
Nanostructured tetragonal barium titanate produced by the polyol and spark plasma sintering (SPS) route
Applied Physics A, 2017
There is a great interest to synthesize ferroelectric ceramics both with fine grain size and sign... more 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 5 min. The stabilization of highly dense (density of 90%), nanostructured (grains about 50 nm) tetragonal barium titanate ceramic was achieved. The produced ceramic exhibited ferroelectric behavior and a dielectric permittivity of 3600 at 1 kHz and room temperature.
Ultrafine grained high density manganese zinc ferrite produced using polyol process assisted by Spark Plasma Sintering
Journal of Magnetism and Magnetic Materials, 2015
ABSTRACT We report the synthesis of Mn–Zn ferrite (MZFO) nanoparticles (NPs) by the polyol proces... more ABSTRACT We report the synthesis of Mn–Zn ferrite (MZFO) nanoparticles (NPs) by the polyol process and their consolidation by Spark Plasma Sintering (SPS) technique at relatively low temperature and short time, namely 500 °C for 10 min. NPs were obtained as perfectly epitaxied aggregated nanoclusters forming a kind of spherical pseudo-single-crystals of about 40 nm in size. The results on NPs consolidation by SPS underlined the importance of this clustering on the grain growth mechanism. Grain growth proceeds by coalescing nanocrystalline aggregates into single grain of almost the same average size, thus leading to a high density ceramic. Due to magnetic exchange interactions between grains, the produced ceramic does not exhibit thermal relaxation whereas their precursor polyol-made NPs are superparamagnetic.
On the microstructural and magnetic properties of fine-grained CoFe2O4 ceramics produced by combining polyol process and spark plasma sintering
Journal of Magnetism and Magnetic Materials, 2014
Abstract Starting from polyol-made CoFe2O4 ferrite nanoparticles of different aggregation states,... more Abstract Starting from polyol-made CoFe2O4 ferrite nanoparticles of different aggregation states, high-density fine-grained ceramics were produced using Spark Plasma Sintering technique at 600 and 500 °C, under vacuum and applying a uniaxial pressure of more than 80 MPa. The grain growth of thus produced ceramics appears to be proportional to the temperature plateau and inversely proportional to the aggregation state of the initial powders. Average grain sizes ranging between about 50 and 350 nm were obtained. In all the cases, magnetic measurements evidenced a ferrimagnetic behavior at room temperature with non-zero coercivity, while their starting powders exhibited superparamagnetic features.
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Papers by Raul Valenzuela