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Macrophages are known to depict two major phenotypes: classically activated macrophages (M1), associated with high production of pro-inflammatory cytokines, and alternatively activated macrophages (M2), which present an anti-inflammatory function. A precise control over M1-M2 polarization is a promising strategy in therapeutics to modulate both tissue regeneration and tumor progression processes. However, this is not a simple task as macrophages behave differently depending on the microenvironment. In agreement with this, non-consistent data have been reported regarding macrophages response to magnetic iron oxide nanoparticles (MNPs). To investigate the impact of both tissue microenvironment and MNPs properties on the obtained macrophage responses, single-core (SC) and multi core (MC) citrate coated MNPs, are synthesized and, afterwards, loaded with a macrophage polarization trigger, IL-4. The developed MNPs are then tested in macrophages subjected to different stimuli. We demonstrate that macrophages treated with low concentrations of MNPs behave differently depending on the polarization stage independently of the concentration of iron. Moreover, we find out that MNPs size and morphology determines the effect of the IL-4 loaded MNPs on M1 macrophages, since IL-4 loaded SC MNPs favor the polarization of M1 macrophages towards M2 phenotype, while IL-4 loaded MC MNPs further stimulate the secretion of pro-inflammatory cytokines

Molybdenum-99 (99Mo) is used to produce technetium-99 m (99mTc), one of the most used medical radioisotopes in the world. 99Mo is currently created by fission of uranium-235 (235U) in a small number of research nuclear fission reactors. Several problems related to the use of 235U as primary source is causing a global 99Mo generation crisis. Therefore, we are facing the need of finding new viable and safe production alternatives. Neutron activation of molybdenum is a promising candidate in which the ideal target for 99Mo production are small monodisperse molybdenum nanoparticles (Mo NPs). However, the reported procedures for the obtention of Mo NPs present either the lack of simplicity of the employed synthetic methods or the inappropriate size and monodispersity of the synthesized Mo NPs. Here, we report a simple synthetic procedure for the obtention of small-sized monodisperse metallic Mo NPs. In the studied conditions, thermal decomposition of Mo(CO)6 produced oleylamine coated Mo NPs. The oleylamine layer was included with the purpose of avoiding oxidation. Although an oxide patina was still formed on the surface of the Mo NPs, the core remains in a metallic phase after several weeks from its production, as it was confirmed by X-ray photoelectron spectroscopy analysis (XPS). The described synthetic procedure provides a simple method for the obtention of Mo NPs with optimum characteristics as non-fission method for 99Mo production

En la presente tesis se ha investigado el papel de las interacciones de sistemas nanoestructurados magnéticos y su influencia tanto en aplicaciones tecnológicas como en biomédicas. Por un lado, se investigó la interacción de canje, mediante el efecto exchange bias, en sistemas de nanopartículas huecas. En segundo lugar, se diseñó un sistema modelo basado en nanopartículas magnéticas que permitió obtener la constante de anisotropía libre de interacciones dipolares, clave en procesos de hipertermia magnética. Por último, la variación en los parámetros de diseño y métodos sintéticos permitió explorar la aplicación de las nanopartículas magnéticas como agentes ópticos y agentes terapéuticos en hipertermia magnética.

Magnetic nanoparticles (MNPs) with different magnetic, physical, and chemical properties can be used to develop magnetic tags, offering a unique traceability capability that can then be used to improve products’ safety. In this work, synthesized MNPs were incorporated into a cellulose acetate (CA) matrix and characterized in their pristine form and when incorporated in the films. Results show that MNPs are different in composition, morphology, size, and magnetic response. Afterwards, iron carbide (Fe5C2), hexagonal-shaped (Fe3O4 @OA) and multicore magnetite (m-Fe3O4) nanoparticles (NPs) were embedded within the CA matrix showing and good distribution in the films’ matrix. Nevertheless, magnetic films with 0.1 wt% of Fe5C2 NPs (FM1), and m-Fe3O4 NPs (FM4) offered a more uniform dispersion of the NPs over the CA matrix. Incorporating MNPs does not affect the films' mechanical properties and water sensitivity. Nevertheless, the optical properties of the films were affected with high impact for the films with m-Fe3O4 NPs. Particularly for magnetic tagging, the CA films displayed similar magnetic responses to their respective bulk NPs, showing their potential as anti-counterfeiting tags for food packaging applications

The presence of organic micropollutants in water bodies represents a threat to living organisms and ecosystems due to their toxicological effects and recalcitrance in conventional wastewater treatments. In this context, the application of heterogeneous photo-Fenton based on magnetite nanoparticles supported on mesoporous silica (SBA15) is proposed to carry out the non-specific degradation of the model compounds ibuprofen, carbamazepine, hormones, bisphenol A and the dye ProcionRed®. The operating conditions (i.e., pH, catalyst load and hydrogen peroxide concentration) were optimized by Response Surface Methodology (RSM). The paramagnetic properties of the nanocatalysts allowed their repeated use in sequential batch operations with catalyst losses below 1%. The feasibility of the process was demonstrated as removal rates above 90% after twelve accomplished after twelve consecutive cycles. In addition, the contributions of different reactive oxygen species, mainly •OH, were analyzed together with the formation of by-products, achieving total mineralization values of 15% on average

El cáncer se caracteriza por errores en nuestros genes que se van a cumulando de forma progresiva. A pesar de estos errores mantiene siempre 2 procesos básicos: crecer y dividirse; por tanto mantiene una biosíntesis proteica constante y pronunciada. La vía de señalización E2F-pRb parece estar alterada en la mayoría de tumores humanos, siendo esta una de las principales vías que regula el ciclo celular, y por tanto la división. Una terapia génica dirigida, empleando esta vía de señalización E2F-pRB podría constituir una alternativa de tratamiento atractiva y especifica. En base a todo esto postulamos que usando plásmidos modificados, subrogados a la expresión del factor E2F podría producirse un producto el cual sería capaz de interrumpir el proceso de síntesis proteica, en células tumorales provocando que estas murieran por apoptosis. Asimismo usando un sistema de administración basado en nanopartículas, se podría favorecer la transferencia del plásmido al interior de la célula.

Recientemente, la nanomedicina ha surgido de la interfaz entre la nanotecnología y la biotecnología, dando lugar a la aparición de importantes avances en el sector biomédico. Esta nueva vertiente de la biomedicina, requiere el desarrollo de materiales que incorporen nanoestructuras multifuncionales que integren en una única estructura materiales con propiedades específicas con actividad teragnóstica, como son las nanopartículas magnéticas, para diversas aplicaciones biomédicas in vitro o in vivo. En el presente trabajo, se ha llevado a cabo la síntesis de nanoestructuras magnéticas por diferentes rutas sintéticas optimizadas y su posterior evaluación en aplicaciones biomédicas de actual interés tales como la detección de biomarcadores de enfermedades neurodegenerativas, terapias celulares o el diagnóstico por imagen multimodal.

The present thesis is focused on the development of biocompatible nanostructured materials with magnetic properties, combining biopolymers of algae orgin (e.g., alginate), plant origin (e.g., pectin,cellulose, pectin) and animal origin (e.g., chitosan). The main idea was to design magnetic materials with different morphology (beads, microfibers, films and capsules) and study their biocompatability,reusability and magnetic properties for posterior use in future and environmental applications.

In this work, ZnO nanoparticles (NPs) were successfully synthesized and coated with oleic acid (OA). The mean diameter of these NPs (ZnO-OA) was around 11.5 nm, their core was characterized by XRD and their coating by FTIR and Raman. Homogeneous dispersions at different concentrations (0.10, 0.25, 0.50, 0.75 and 1.00 wt%) of ZnO-OA in polyalphaolefin 40 (PAO40) oil were thermophysically and tribologically characterized. Both density and viscosity values increased with the concentration of NPs, reaching relative increments of 0.5% and 4.0%, respectively, for the 1 wt% nanodispersion. Tribological tests were performed at 353.15 K using an Anton Paar MCR 302 rheometer equipped with a tribological ball-on three-pins configuration testing module. Regarding the tribological behavior, the optimal concentration was 0.25 wt% of ZnO-OA (25% of reduction in the friction coefficient and 82% wear reduction in terms of cross sectional area, respect to those obtained with the neat base oil). The rolling mechanism owing to the spherical shape of the nanoadditives, transforming sliding friction into rolling friction, and the mending effect could explain the better tribological performance of nanolubricants with respect to that of neat PAO40. In addition, the presence of PAO40, ZnO-OA NPs and iron oxides was evidenced from confocal Raman microscopy on the worn surfaces obtained from tribological test with PAO40 + 0.25 wt% ZnO-OA dispersion

The urgency for the availability of new antibacterial/disinfectant agents has become a worldwide priority. At the same time, along with the extensive use of other metal nanoparticles (NPs), the investigation of magnetic NPs (MNPs) in antibacterial studies has turned out to be an increasingly attractive research field. In this context, we present the preparation and characterization of superparamagnetic iron oxide NPs, electrodecorated with antimicrobial copper NPs, able to modulate the release of bioactive species not only by the NP’s stabilizer, but also through the application of a suitable magnetic field. Antimicrobial synergistic CuNPs stabilized by benzalkonium chloride have been used in the current study. We demonstrate the successful preparation of Cu@Fe3O4 MNPs composites through morphological and spectroscopic results. Additionally, an extensive magnetic characterization is reported, along with hyperthermia-induced copper ionic release. On the basis of our results, we propose a new generation of antimicrobial magnetic nanomaterials, whose bioactivity can be also tuned by the application of a magnetic field

Today, the presence of recalcitrant pollutants in wastewater, such as pharmaceuticals or other organic compounds, is one of the main obstacles to the widespread implementation of water reuse. In this context, the development of innovative processes for their removal becomes necessary to guarantee effluent quality. This work presents the potentiality of magnetic nanoparticles immobilized on SBA-15 mesoporous silica as Fenton and photo-Fenton catalysts under visible light irradiation. The influence of the characteristics of the compounds and nanoparticles on the removal yield was investigated. Once the key aspects of the reaction mechanism were analyzed, to evaluate the feasibility of this process, an azo dye (Orange II) and an antibiotic (sulfamethoxazole) were selected as main target compounds. The concentration of Orange II decreased below the detection limit after two hours of reaction, with mineralization values of 60%. In addition, repeated sequential experiments revealed the recoverability and stability of the nanoparticles in a small-scale reactor. The benchmarking of the obtained results showed a significant improvement of the process using visible light in terms of kinetic performance, comparing the results to the Fenton process conducted at dark. Reusability, yield and easy separation of the catalyst are its main advantages for the industrial application of this process

In recent years, the application of magnetic nanoparticles as alternative catalysts to conventional Fenton processes has been investigated for the removal of emerging pollutants in wastewater. While this type of catalyst reduces the release of iron hydroxides with the treated effluent, it also presents certain disadvantages, such as slower reaction kinetics associated with the availability of iron and mass transfer limitations. To overcome these drawbacks, the functionalization of the nanocatalyst surface through the addition of coatings such as polyacrylic acid (PAA) and their immobilization on a mesoporous silica matrix (SBA15) can be factors that improve the dispersion and stability of the nanoparticles. Under these premises, the performance of the nanoparticle coating and nanoparticle-mesoporous matrix binomials in the degradation of dyes as examples of recalcitrant compounds were evaluated. Based on the outcomes of dye degradation by the different functionalized nanocatalysts and nanocomposites, the nanoparticles embedded in a mesoporous matrix were applied for the removal of estrogens (E1, E2, EE2), accomplishing high removal percentages (above 90%) after the optimization of the operational variables. With the feasibility of their recovery in mind, the nanostructured materials represented a significant advantage as their magnetic character allows their separation for reuse in different successive sequential batch cycles

The main task of this work is to study the tribological performance of nanolubricants formed by trimethylolpropane trioleate (TMPTO) base oil with magnetic nanoparticles coated with oleic acid: Fe3O4 of two sizes 6.3 nm and 10 nm, and Nd alloy compound of 19 nm. Coated nanoparticles (NPs) were synthesized via chemical co-precipitation or thermal decomposition by adsorption with oleic acid in the same step. Three nanodispersions of TMPTO of 0.015 wt% of each NP were prepared, which were stable for at least 11 months. Two different types of tribological tests were carried out: pure sliding conditions and rolling conditions (5% slide to roll ratio). With the aim of analyzing the wear by means of the wear scar diameter (WSD), the wear track depth and the volume of the wear track produced after the first type of the tribological tests, a 3D optical profiler was used. The best tribological performance was found for the Nd alloy compound nanodispersion, with reductions of 29% and 67% in friction and WSD, respectively, in comparison with TMPTO. On the other hand, rolling conditions tests were utilized to study friction and film thickness of nanolubricants, determining that Fe3O4 (6.3 nm) nanolubricant reduces friction in comparison to TMPTO

This study reports the ability of magnetic alginate activated carbon (MAAC) beads to remove Cd(II), Hg(II), and Ni(II) from water in a mono-metal and ternary system. The adsorption capacity of the MAAC beads was highest in the mono-metal system. The removal efficiency of such metal ions falls in the range of 20–80% and it followed the order Cd(II) > Ni(II) > Hg(II). The model that best fitted in the ternary system was the Freundlich isotherm, while in the mono-system it was the Langmuir isotherm. The maximum Cd(II), Hg(II), and Ni(II) adsorption capacities calculated from the Freundlich isotherm in the mono-metal system were 7.09, 5.08, and 4.82 (mg/g) (mg/L)1/n, respectively. Lower adsorption capacity was observed in the ternary system due to the competition of metal ions for available adsorption sites. Desorption and reusability experiments demonstrated the MAAC beads could be used for at least five consecutive adsorption/desorption cycles. These findings suggest the practical use of the MAAC beads as efficient adsorbent for the removal of heavy metals from wastewater

Early diagnosis of disease and follow-up of therapy is of vital importance for appropriate patient management since it allows rapid treatment, thereby reducing mortality and improving health and quality of life with lower expenditure for health care systems. New approaches include nanomedicine-based diagnosis combined with therapy. Nanoparticles (NPs), as contrast agents for in vivo diagnosis, have the advantage of combining several imaging agents that are visible using different modalities, thereby achieving high spatial resolution, high sensitivity, high specificity, morphological, and functional information. In this work, we present the development of aluminum hydroxide nanostructures embedded with polyacrylic acid (PAA) coated iron oxide superparamagnetic nanoparticles, Fe3O4@Al(OH)3, synthesized by a two-step co-precipitation and forced hydrolysis method, their physicochemical characterization and first biomedical studies as dual magnetic resonance imaging (MRI)/positron emission tomography (PET) contrast agents for cell imaging. The so-prepared NPs are size-controlled, with diameters below 250 nm, completely and homogeneously coated with an Al(OH)3 phase over the magnetite cores, superparamagnetic with high saturation magnetization value (Ms = 63 emu/g-Fe3O4), and porous at the surface with a chemical affinity for fluoride ion adsorption. The suitability as MRI and PET contrast agents was tested showing high transversal relaxivity (r2) (83.6 mM−1 s −1 ) and rapid uptake of 18F-labeled fluoride ions as a PET tracer. The loading stability with 18F-fluoride was tested in longitudinal experiments using water, buffer, and cell culture media. Even though the stability of the 18F-label varied, it remained stable under all conditions. A first in vivo experiment indicates the suitability of Fe3O4@Al(OH)3 nanoparticles as a dual contrast agent for sensitive short-term (PET) and high-resolution long-term imaging (MRI).

Neurological diseases (Alzheimer’s disease, Parkinson’s disease, and stroke) are becoming a major concern for health systems in developed countries due to the increment of ageing in the population, and many resources are devoted to the development of new therapies and contrast agents for selective imaging. However, the strong isolation of the brain by the brain blood barrier (BBB) prevents not only the crossing of pathogens, but also a large set of beneficial drugs. Therefore, an alternative strategy is arising based on the anchoring to vascular endothelial cells of nanoplatforms working as delivery reservoirs. In this work, novel injectable mesoporous nanorods, wrapped by a fluorescent magnetic nanoparticles envelope, are proposed as biocompatible reservoirs with an extremely high loading capacity, surface versatility, and optimal morphology for enhanced grafting to vessels during their diffusive flow. Wet chemistry techniques allow for the development of mesoporous silica nanostructures with tailored properties, such as a fluorescent response suitable for optical studies, superparamagnetic behavior for magnetic resonance imaging MRI contrast, and large range ordered porosity for controlled delivery. In this work, fluorescent magnetic mesoporous nanorods were physicochemical characterized and tested in preliminary biological in vitro and in vivo experiments, showing a transversal relaxivitiy of 324.68 mM−1 s −1 , intense fluorescence, large specific surface area (300 m2 g −1 ), and biocompatibility for endothelial cells’ uptake up to 100 µg (in a 80% confluent 1.9 cm2 culture well), with no liver and kidney disability. These magnetic fluorescent nanostructures allow for multimodal MRI/optical imaging, the allocation of therapeutic moieties, and targeting of tissues with specific damage.

This study presents an effective magnetic separation method for cadmium removal, based on the use of a novel nanostructured material as an adsorbent. This adsorbent involves the incorporation of magnetite nanoparticles (Fe3O4-NPs), synthesized by the reverse coprecipitation method, into sodium alginate and activated carbon to form spherical structures by crosslinking Ca2+ ions with the charged alginate chains, referred to as magnetic alginate activated carbon (MAAC) beads. The effect of the experimental parameters, such as pH, contacting time, adsorbent dosage, agitation type, and rotating speed were investigated and optimized for an efficient removal of Cd(II) ions at an initial concentration of 250 mg/L. The amount of adsorbed Cd(II) by MAAC beads increased at a pH of 6 with a removal efficiency over 90%. The maximum adsorption capacity reached was 70 mg/g of adsorbent at an initial Cd(II) concentration of 150 mg/L, whereas at 250 mg/L the adsorption capacity lowered until 60 mg/g. Sorption isotherms were calculated using Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich equations, and were better described by the Freundlich and Temkin models. These results proved the removal efficiency and the potential use under real environmental conditions of the MAAC beads, due to their easy recovery from contaminated aqueous solutions.

In this paper, novel magnetic silica nanocomposites were prepared by anchoring magnetite nanoparticles onto the outer surface of mesoporous SBA-15 silica; the magnetic nanoparticles were prepared by microemulsion and solvothermal methods, varying the synthesis conditions in order to control the final physicochemical, textural and magnetic properties. The morphology and mesostructure of the materials were characterized by X-ray diffraction (XRD), Fourier-Transform Infrared Spectroscopy (FTIR), N2 adsorption–desorption, and Transmission and Scanning Electron Microscopy (TEM and SEM). Magnetic silica nanocomposites feature a two-dimensional hexagonal arrangement constituted by a homogeneous pore channel system with diameters between 13 and 18 nm and a Brunauer–Emmett–Teller (BET) surface area higher than 260 m2 g−1. The different morphologies of the samples are given by the presence of diverse magnetic nanoparticle arrangements covalently linked onto the outer surface of the mesoporous silica rods. This confers on them a superparamagnetic behaviour with a magnetic response between 50–80 emu g−1, even though the weight percent of magnetite present in the samples does not exceed 21.7%. In addition, the magnetic nanocomposites exhibit magnetic hyperthermia with moderate Specific Absorption Rate (SAR) values

Mesenchymal stem cells (MSCs) are a promising clinical therapy for ischemic stroke. However, critical parameters, such as the most effective administration route, remain unclear. Intravenous (i.v.) and intraarterial (i.a.) delivery routes have yielded varied outcomes across studies, potentially due to the unknown MSCs distribution. We investigated whether MSCs reached the brain following i.a. or i.v. administration after transient cerebral ischemia in rats, and evaluated the therapeutic effects of both routes. MSCs were labeled with dextran-coated superparamagnetic nanoparticles for magnetic resonance imaging (MRI) cell tracking, transmission electron microscopy and immunohistological analysis. MSCs were found in the brain following i.a. but not i.v. administration. However, the i.a. route increased the risk of cerebral lesions and did not improve functional recovery. The i.v. delivery is safe but MCS do not reach the brain tissue, implying that treatment benefits observed for this route are not attributable to brain MCS engrafting after stroke.

Nanomedicine is a new field employing nanotechnology tools for biomedical applications. Nanomaterials feature sizes between 1 and 100 hundred nanometers, and therefore they fill the gap between single molecules and bulk materials. They open the possibility to access biological processes on the same size scale. Nanoparticles have shown great potential to achieve the goal of personalized medicine due to the possibilities to modify their surface with proteins, targeting molecules, or imaging probes. Particularly, iron oxide nanoparticles show promising properties for successful application in biomedicine due to their low-cost production, high biocompatibility, and great magnetic response. Strategies for the functionalization of inorganic cores have been developed over the past years, and efforts have been devoted to engineering nanoparticles with multifunctional ligands on the surface to enable active tumor targeting, detection over different imaging modalities, and stimulus-driven cargo release. During this Ph.D. thesis, three main aspects of iron oxide nanoparticles have been evaluated. First, the stability of nanoparticles has been addressed in biological fluids. Second, the uptake of nanoparticles by stem cells for cell tracking applications was studied with nanoparticles of different sizes and bearing different organic coatings. Third, new possibilities of chemical functionalization of naked iron oxide nanoparticles by orthogonal processes have been explored, in order to achieve high yields, simplicity and high reproducibility.

Circulating tumour cells (CTCs) play a key role in the metastasis process, as they are responsible for micrometastasis and are a valuable tool for monitoring patients in real-time. Moreover, efforts to develop new strategies for CTCs isolation and characterisation, and the translation of CTCs into clinical practice needs to overcome the limitation associated with the sole use of Epithelial Cell Adhesion Molecule (EpCAM) expression to purify this tumour cell subpopulation. CTCs are rare events in the blood of patients and are believed to represent the epithelial population from a primary tumour of epithelial origin, thus EpCAM immunoisolation is considered an appropriate strategy. The controversy stems from the impact that the more aggressive mesenchymal tumour phenotypes might have on the whole CTC population. In this work, we first characterised a panel of cell lines representative of tumour heterogeneity, confirming the existence of tumour cell subpopulations with restricted epithelial features and supporting the limitations of EpCAM-based technologies. We next developed customised polystyrene magnetic beads coated with antibodies to efficiently isolate the phenotypically different subpopulations of CTCs from the peripheral blood mononuclear cells (PBMCs) of patients with metastatic cancer. Besides EpCAM, we propose Epidermal Growth Factor Receptor (EGFR) as an additional isolation marker for efficient CTCs detection.

The suitability of magnetic nanoparticles (MNPs) to act as heat nano-sources by application of an alternating magnetic field has recently been studied due to their promising applications in biomedicine. The understanding of the magnetic relaxation mechanism in biocompatible nanoparticle systems is crucial in order to optimize the magnetic properties and maximize the specific absorption rate (SAR). With this aim, the SAR of magnetic dispersions containing superparamagnetic magnetite nanoparticles bio-coated with polyacrylic acid of an average particle size of ≈10 nm has been evaluated separately by changing colloidal parameters such as the MNP concentration and the viscosity of the solvent. A remarkable decrease of the SAR values with increasing particle concentration and solvent viscosity was found. These behaviours have been discussed on the basis of the magnetic relaxation mechanisms involved

Here, we present experimental and computational evidences to support that rocksalt cubic VO is a strongly correlated metal with non-Fermi-liquid thermodynamics and an unusually strong spin-lattice coupling. An unexpected change of sign of metallic thermopower with composition is tentatively ascribed to the presence of a pseudogap in the density of states. These properties are discussed as signatures of the proximity to a magnetic quantum phase transition. The results are summarized in an electronic phase diagram for the 3d monoxides, which resembles that of other strongly correlated systems. The structural and electronic simplicity of 3d monoxides makes them ideal candidates to progress in the understanding of highly correlated electron systems.

En este trabajo se examina la anisotropía magnética de la perovskita de cobalto Lao.7Sro.3Co03, material de alto interés tecnológico. Las medidas convencionales, hechas en la zona de alto campo, arrojan un valor de anisotropía magnética muy elevado, que contrasta con los bajos valores de campo coercitivo medidos en los ciclos de histéresis del material. De los datos de campo coercitivo se calcula la anisotropía magnética en la región de bajo campo, evidenciando una sorprendente reducción de esta magnitud, lo que demuestra su dependencia con el campo externo aplicado. El efecto es explicado en términos de una transición, inducida por campo magnético, del estado de espín de los iones de cobalto, desde una configuración de bajo espín a una de espín intermedio, lo que covierte al ión de cobalto en un ión Jahn-Teller, In this work we examine the magnetic anisotropy of the cobalt perovskite Lao.7Sro.3Co03, a material of high technological interest. Conventional measurements, done in the high-field regime, yield a very high value of magnetic anisotropy, which is in clear contrast with the low coercive field values observed in hysteresis loops. From such values, magnetic anisotropy in the low-field region is calculated, evidencing a surprising reduction of this magnitude, demonstrating its dependence on the external applied field. This anomalous eff ect is explained in terms of a magnetic field induced spin state transition of the cobalt ions, from a low-spin to an intermediate-spin configuration, which transforms them into Jahn-Teller ions

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, with x=0.40. The magneto-transport properties of such materials are peculiar and interesting: they show diodic behavior and large relaxation effects — these latter being specially important in the Nd compound — they display magnetoresistive effects specially at the M–I transition temperatures, and they age with time. All these results are discussed on the basis of the inhomogeneous electronic structure of these doped cobalt perovskites and taking into account the influence of the lanthanide ion on their magnetic and electrical properties

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, with x=0.40. The magneto-transport properties of such materials are peculiar and interesting: they show diodic behavior and large relaxation effects — these latter being specially important in the Nd compound — they display magnetoresistive effects specially at the M–I transition temperatures, and they age with time. All these results are discussed on the basis of the inhomogeneous electronic structure of these doped cobalt perovskites and taking into account the influence of the lanthanide ion on their magnetic and electrical properties.

En este trabajo se examina la naturaleza de la transición de fase ferromagnética de materiales de manganeso con estructura de perovskita, de gran interés tecnológico por sus enormes variaciones de resistividad eléctrica ante la presencia de un campo magnético aplicado (magnetorresistencia colosal). Se encuentra que, en contra de la opinión generalizada, no todos estos materiales presentan un comportamiento cualitativamente similar. En concreto, el carácter de primer orden de la transición ferromagnética no es un factor común. Las implicaciones físicas de este resultado son discutidas, In this work, the nature of the ferromagnetic phase transition of manganese-based materials with perovskite structure, is examined. These are materials of great technological interest due to the enormous variations of their electrical resistivity under the presence of an applied magnetic field (colossal magnetoresistance). lt is found that, contrary to the assumed belíef, not all these materials present a qualitatively similar behaviour. Concretely, the first order character of the ferromagnetic transition is not a common factor. The physical implications of this result are discussed

Neste traballo preséntase unha vía alternativa e totalmente orixinal de síntese de nanopartículas para a preparación de fluidos magnéticos, con mellaras respecto ós métodos tradicionais. Para elo, empréganse mezclas ternarias dun disolvente orgánico, un tensioactivo e unha disolución acuosa en forma de microemulsión, para sintetizar nanopartículas magnéticas de óxidos de ferro. Estas nanopartículas, lavadas e cubertas con ácido oleico, son estabilizadas nun líquido portador, producindo un fluido magnético. Preséntanse medidas de difracción de raios-X, microscopía electrónica de transmisión e medidas de magnetización, tanto das partículas coma do líquido preparado, In this work, an alternative and completely original route for the synthesis of magnetic fluids is presented. This method presents improvements with respect to the traditional methods. Por this purpose, ternary mixtures of organic solvent, surfactant and aqueous solution forming a microemulsion are used for the synthesis of magnetic iron oxide particles. These particles, washed and covered with oleic acid, are stabilized in a carrier liquid, producing a magnetic fluid. X-ray diffraction, transmission electron microscopy and magnetic measurements of the particles and the prepared liquid are reported

El descubrimiento de los superconductores de alta temperatura crítica (SATC) convirtió a la superconductividad en "el" problema de la física del estado sólido, ya que el mecanismo BCS propuesto para la superconductividad conocida hasta la época dejaba de tener validez total. ¿Cuál es el mecanismo que lleva a la bosonización de los electrones? ¿En dónde radica el hecho diferencial de los nuevos superconductores respecto de los clásicos? Preguntas de este estilo han dado el pistoletazo de salida en la carrera hacia una teoría nueva. En ese camino la coexistencia de superconductividad y magnetismo ha sido siempre un problema de interés tanto para la búsqueda de nuevos superconductores como para la explicación del fenómeno. Problemas representativos han sido el estudio de impurezas en superconductores tradicionales, o el de compuestos en los cuales orden magnético y superconductividad se presentan localizados en diferentes zonas de la estructura cristalina, como en las fases de Chevrel, e incluso el de otros materiales en los que los mismos electrones que pertenecen a bandas con altas correlaciones electrónicas, como es el caso de los fermiones pesados. Los cupratos de tierra rara R2Cu04 , R= lantánido, son compuestos base de superconductores de alta temperatura crítica y constituyen un marco excelente para el estudio de estos aspectos al estar presentes todas esas situaciones.