Open Access
Giant direct and inverse magnetocaloric effect linked to the same forward martensitic transformation
(Springer Nature, 2017) Pérez de Landazábal Berganzo, José Ignacio; Recarte Callado, Vicente; Sánchez-Alarcos Gómez, Vicente; Beato López, Juan Jesús; Rodríguez Velamazán, José Alberto; Sánchez Marcos, J.; Gómez Polo, Cristina; Cesari, Eduard; Fisika; Institute for Advanced Materials and Mathematics - INAMAT2; Física
Metamagnetic shape memory alloys have aroused considerable attraction as potential magnetic refrigerants due to the large inverse magnetocaloric effect associated to the magnetic-field-induction of a reverse martensitic transformation (martensite to austenite). In some of these alloys, the austenite phase can be retained on cooling under high magnetic fields, being the retained phase metastable after field removing. Here, we report a giant direct magnetocaloric effect linked to the anomalous forward martensitic transformation (austenite to martensite) that the retained austenite undergoes on heating. Under moderate fields of 10 kOe, an estimated adiabatic temperature change of 9 K has been obtained, which is (in absolute value) almost twice that obtained in the conventional transformation under higher applied fields. The observation of a different sign on the temperature change associated to the same austenite to martensite transformation depending on whether it occurs on heating (retained) or on cooling is attributed to the predominance of the magnetic or the vibrational entropy terms, respectively.
Open Access
Thrust actuator with passive restoration force for wide gap magnetic bearings
(Elsevier, 2019) Royo Silvestre, Isaac; Beato López, Juan Jesús; Castellano Aldave, Jesús Carlos; Gómez Polo, Cristina; Zientziak; Institute for Advanced Materials and Mathematics - INAMAT2; Ciencias
Active thrust magnetic bearings provide an axial force to balance the moving parts of machines. However, most devices produce null or unbalancing passive forces. Furthermore, reported designs usually feature very small axial and radial gaps. This paper presents a thrust actuator for wide axial gaps that produces both passive and active restoring axial forces. It features a long biconical rotor and a stator housing a single winding and two permanent magnets. Simulations are done using finite-element-analysis (FEA) and compared to magnetic circuit analysis and experimental results from a prototype with a diameter of 48 mm and 20 mm axial displacement.
Open Access
Magnetic transition in nanocrystalline soft magnetic alloys analyzed via ac inductive techniques
(American Physical Society, 2004) Gómez Polo, Cristina; Pérez de Landazábal Berganzo, José Ignacio; Recarte Callado, Vicente; Vázquez, M.; Hernando, A.; Física; Fisika; Gobierno de Navarra / Nafarroako Gobernua
The magnetic transition in a FeSiBCuNb nanocrystalline alloy, associated with the decoupling of ferromagnetic crystallites around the Curie point of the residual amorphous matrix, is analyzed in this work through the temperature dependence of the ac axial magnetic permeability and impedance of the samples. The temperature dependence of both complex magnitudes presents a maximum in the irreversible contribution at a certain transition temperature. While for low values of the exciting ac magnetic field the transition temperature lies below the Curie temperature of the amorphous phase, a shift above this Curie point is observed increasing the amplitude of the applied ac magnetic field. The detected field dependence is interpreted taking into account the ac nature of the inductive characterization techniques and the actual temperature dependence of the coercivity of the samples.
Open Access
Tailoring the structural and magnetic properties of Co-Zn nanosized ferrites for hyperthermia applications
(Elsevier, 2018) Gómez Polo, Cristina; Recarte Callado, Vicente; Cervera Gabalda, Laura María; Beato López, Juan Jesús; López García, Javier; Rodríguez Velamazán, José Alberto; Ugarte Martínez, María Dolores; Mendonça, E. C.; Duque, J. G. S.; Zientziak; Estatistika, Informatika eta Matematika; Institute for Advanced Materials and Mathematics - INAMAT2; Ciencias; Estadística, Informática y Matemáticas; Gobierno de Navarra / Nafarroako Gobernua
A comparative study of the magnetic properties (magnetic moment, magnetocrystalline anisotropy) and hyperthermia response in Co-Zn spinel nanoparticles is presented. The CoxZn1-xFe2O4 nanoparticles (x = 1, 0.5, 0.4, 0.3, 0.2 and 0.1) were synthesized by co-precipitated method and the morphology and mean crystallite size (around 10 nm) of the nanoparticles were analysed by TEM Microscopy. Regarding the magnetic characterization (SQUID magnetometry), Co-Zn nanoparticles display at room temperature anhysteretic magnetization curves, characteristic of the superparamagnetic behavior. A decrease in the blocking temperature, T-B, with Zn content is experimentally detected that can be ascribed to the reduction in the mean nanoparticle size as x decreases. Furthermore, the reduction in the magnetocrystalline anisotropy with Zn inclusion is confirmed through the analysis of TB versus the mean volume of the nanoparticles and the law of approach to saturation. Maximum magnetization is achieved for x = 0.5 as a result of the cation distribution between octahedral and tetrahedral spinel sites, analysed by neutron diffraction studies. The occurrence of a canted spin arrangement (Yafet-Kittel angle) is introduced to properly fit the magnetic spinel structures. Finally, the heating capacity of these spinel ferrites is analyzed under ac magnetic field (magnetic hyperthermia). Maximum SAR (Specific Absorption Rate) values are achieved for x = 0.5 that should be correlated to the maximum magnetic moment of this composition.
Open Access
A comprehensive analysis of the absorption spectrum of conducting ferromagnetic wires
(IEEE, 2012) Liberal Olleta, Íñigo; Ederra Urzainqui, Íñigo; Gómez Polo, Cristina; Labrador Otamendi, Alberto; Pérez de Landazábal Berganzo, José Ignacio; Gonzalo García, Ramón; Ingeniería Eléctrica y Electrónica; Ingeniaritza Elektrikoa eta Elektronikoa; Física; Fisika
A detailed analysis of the absorption spectrum of conductive ferromagnetic wires is presented. The absorption spectrum is computed from the solution to the scattering problem, and circuit models are formulated to clarify the interplay between losses, skin-effect and wire geometry. Both infinitely-long wires and the axial resonances introduced by finite-length wires are considered. The theoretical results are validated experimentally through measurements within a metallic rectangular waveguide.
Open Access
Theoretical modeling and experimental verification of the scattering from a ferromagnetic microwire
(IEEE, 2011) Liberal Olleta, Íñigo; Ederra Urzainqui, Íñigo; Gómez Polo, Cristina; Labrador Otamendi, Alberto; Pérez de Landazábal Berganzo, José Ignacio; Gonzalo García, Ramón; Física; Fisika; Ingeniería Eléctrica y Electrónica; Ingeniaritza Elektrikoa eta Elektronikoa
This contribution presents a theoretical modelling of the scattering of ferromagnetic microwires in free-space and inside a rectangular waveguide, providing both an analytical solution and a physical interpretation of the problem. Special attention is devoted to the impact of the microwire radius and its magnetic properties. Theoretical results have been experimentally verified measuring the reflection, absorption and transmission coefficients of a ferromagnetic microwire inside a rectangular waveguide.
Open Access
A survey on the mathematical foundations of axiomatic entropy: representability and orderings
(MDPI, 2018) Campión Arrastia, María Jesús; Gómez Polo, Cristina; Induráin Eraso, Esteban; Raventós Pujol, Armajac; Estatistika, Informatika eta Matematika; Zientziak; Institute for Advanced Research in Business and Economics - INARBE; Institute for Advanced Materials and Mathematics - INAMAT2; Estadística, Informática y Matemáticas; Ciencias
Different abstract versions of entropy, encountered in science, are interpreted in the light of numerical representations of several ordered structures, as total-preorders, interval-orders and semiorders. Intransitivities, other aspects of entropy as competitive systems, additivity, etc., are also viewed in terms of representability of algebraic structures endowed with some compatible ordering. A particular attention is paid to the problem of the construction of an entropy function or their mathematical equivalents. Multidisciplinary comparisons to other similar frameworks are also discussed, pointing out the mathematical foundations.
Open Access
Giant stress-impedance (GSI) sensor for diameter evaluation incylindrical elements
(Elsevier, 2018-01-01) Beato López, Juan Jesús; Vargas Silva, Gustavo Adolfo; Pérez de Landazábal Berganzo, José Ignacio; Gómez Polo, Cristina; Ingeniería; Ingeniaritza; Ciencias; Zientziak; Institute for Advanced Materials and Mathematics - INAMAT2; Gobierno de Navarra / Nafarroako Gobernua
In this work, a magnetoelastic sensor to detect the micrometer diameter variations of cylindrical elements is analyzed. A nearly zero magnetostrictive amorphous ribbon with nominal composition (Co₀.₉₃Fe₀․₀₇)₇₅Si₁₂․₅B₁₂․₅ was selected as sensor nucleus. The sensor, based on Giant Stress-Impedance (GSI), is attached (glued) along the external perimeter of the cylindrical element. Changes in the cylindrical diameter, DM, induce effective tensile stresses, S, on the ribbon, giving rise to sensitive changes in the high frequency impedance, Z. The sensor response is analyzed in terms of the relationship between the induced strains and the diameter variations, where the effect of geometrical factors (cylinder diameter and sample length) is taken into account. The results indicate that although the maximum GSI ratio depends on the pre-induced bending stresses associated to the cylindrical configuration, the sample length plays the dominant role in the sensor sensitivity. The proposed device enables to monitor the micrometric diameter variation in cylindrical elements, with a maximum strain gauge factor (GF≈-80) for low induced strains.
Open Access
Micrometric non-contact position magnetoimpedance sensor
(Elsevier, 2018) Beato López, Juan Jesús; Royo Silvestre, Isaac; Gómez Polo, Cristina; Zientziak; Institute for Advanced Materials and Mathematics - INAMAT2; Ciencias; Gobierno de Navarra / Nafarroako Gobernua, VITICS, IIM14244.RI1
In this work a sensitive micrometric non-contact position sensor based on the Giant MagnetoImpedance effect (GMI) is analyzed. A nearly zero magnetostrictive CoFeSiBCr wire was employed as sensor nucleus. The sensing principle is based on the changes in the high frequency electric impedance, Z, of the soft magnetic element as a function of the relative position of a permanent magnet generating a non-uniform magnetic field along the wires axis. The sensor sensitivity is analyzed in terms of the magnetic field gradient and wire's length. The comparison between the sensing response of a single wire element and a long wire (12 cm in length) with different voltage contacts along its axis is performed. Higher micrometric sensitivities are achieved in wires with a certain critical length. A slight enhancement of the sensor sensitivity is found under the single wire configuration below the critical wire length. These results are interpreted as the contribution of the characteristic closure domain structure at the sample ends in these soft magnetic wires. Finally, the application of the sensor for the detection of the daily micrometric trunk shrinkage variations in a lemon tree is presented. The results indicate that this type of magnetic sensors can be easily implemented in the agricultural sector, providing a low cost and sensitive detection technique regarding water monitoring purposes.