Person: Hernando Grande, Antonio
Universidad Complutense de Madrid
Faculty / Institute
Física de Materiales
Física de la Materia Condensada
Now showing 1 - 10 of 80
Publicationsp magnetism in clusters of gold thiolates(IOP Publishing, 2012-01-31) Ayuela, A.; Crespo del Arco, Patricia; Garcia, M.A.; Hernando Grande, Antonio; Echenique, P. M.Using first-principles calculations, we consider the bond between thiolate and small Au clusters, with particular emphasis on the resulting magnetic moment. The moment of pure gold clusters is 1 mu(B) for clusters with an odd number of Au atoms and zero for those with an even number. The addition of the thiolate, having an odd number of electrons itself, shifts the phase of the odd-even oscillations so that particles with an even number of Au atoms now have unit moment. Surprisingly, gold thiolate exhibits a dramatic and non-intuitive distribution of charge and spin moment. Our results show that the S-Au bond is such that sulfur does not get charge and an electron is transferred to the Au cluster. This extra electron is mainly sp in character and resides in an electronic shell below the Au surface. The calculations suggest that any thiolate-induced magnetism occurs in the gold nanoparticle and not the thiolate, and can be controlled by modifying the thiolate coverage. PublicationSurface plasmon resonance and magnetism of thiol-capped gold nanoparticles(Iop Publishing Ltd, 2008-04-30) Guerrero, E.; Muñoz Márquez, M. A.; García, M. A.; Crespo del Arco, Patricia; Fernandez Pinel, E.; Hernando Grande, Antonio; Fernandez, A.Surface plasmon resonance measurements and magnetic characterization studies have been carried out for two types of thiol-capped gold nanoparticles (NPs) with similar diameters between 2.0 and 2.5 nm and different organic molecules linked to the sulfur atom: dodecanethiol and tiopronin. In addition, Au NPs capped with tetraoctyl ammonium bromide have also been included in the investigation since such capping molecules weakly interact with the gold surface atoms and, therefore, this system can be used as a model for naked gold NPs; such particles presented a bimodal size distribution with diameters around 1.5 and 5 nm. The plasmon resonance is non-existent for tiopronin-capped NPs, whereas a trace of such a feature is observed for NPs covered with dodecanethiol molecules and a bulk-like feature is measured for NPs capped with tetralkyl ammonium salts. These differences would indicate that the modification of the surface electronic structure of the Au NPs depends on the geometry and self-assembling capabilities of the capping molecules and on the electric charge transferred between Au and S atoms. Regarding the magnetization, dodecanethiol-capped NPs have a ferromagnetic-like behaviour, while the NPs capped with tiopronin exhibit a paramagnetic behaviour and tetralkyl ammonium-protected NPs are diamagnetic across the studied temperature range; straight chains with a well-defined symmetry axis can induce orbital momentum on surface electrons close to the binding atoms. The orbital momentum not only contributes to the magnetization but also to the local anisotropy, giving rise to permanent magnetism. Due to the domain structure of the adsorbed molecules, orbital momentum is not induced for tiopronin-capped NPs and the charge transfer only induces a paramagnetic spin component. PublicationDirect measurements of the correlation between reentrant ferromagnetism and lattice expansion in FeCuZr alloys(American Physical Society, 2010-07-14) Martinez, A.; Spottorno Giner, Jorge; Figueroa, A.; Bartolome, F.; Garcia, L. M.; Prestipino, C.; Hernando Grande, Antonio; Crespo del Arco, PatriciaAmorphous metastable alloy of nominal composition [Fe_(0.5)Cu_(0.5)]_(87)Zr_(13) has been synthesized by high-energy ball milling. The alloy exhibit a ferromagnetic behavior with a Curie Temperature of T_(C)=255 K, as determined from low-field measurements whereas no transition to a paramagnetic state is observed under high- enough applied magnetic fields. The evolution of hysteresis loops with temperature as well as thermoremanence measurements indicate an anomalous magnetic behavior characterized by a spontaneous increase in the magnetization values as well as by a magnetic hardening when the temperature is increased above T_(C). These effects are strongly correlated with a dilation of the Fe-Fe nearest-neighbor distances, as determined from extended x-ray absorption fine structure (EXAFS) studies. EXAFS results indicate an almost negligible thermal expansion at temperatures below T_(C) while normal thermal expansion takes place at higher temperatures. Such expansion seems to promote a reinforcement of the ferromagnetic interactions among Fe-Fe atoms that would account for the observed spontaneous increase in the magnetization as well as for the evolution of the coercive field. PublicationFormation of a magnetite/hematite epitaxial bilayer generated with low energy ion bombardment(American Institute of Physics, 2017-02-27) Ruiz Gómez, Sandra; Serrano, A.; Carabias,, I.; Garcıa, M. A.; Hernando Grande, Antonio; Mascaraque Susunaga, Arantzazu; Pérez García, Lucas; González Barrio, Miguel Ángel; Rodríguez de la Fuente, ÓscarWe have used a low-energy ion bombardment to fabricate an epitaxial single-crystalline magnetite/hematite bilayer grown on Au(111). This non-conventional fabrication method involves the transformation of the upper layers of a single-crystalline hematite thin film to single-crystalline magnetite, a process driven by the preferential sputtering of oxygen atoms and favoured by the good structural matching of both phases. We show the reversibility of the transformation between hematite and magnetite, always keeping the epitaxial and single- crystalline character of the films. The magnetic characterization of the bilayer grown using this method shows that the magnetic response is mainly determined by the magnetite thin film, exhibiting a high coercivity. Published by AIP Publishing. PublicationEffect of preparation methods on magnetic properties of stoichiometric zinc ferrite(Elsevier Science, 2020-12-30) Cobos, M.A.; Presa Muñoz del Toro, Patricia de la; Llorente, I.; García Escorial, A.; Hernando Grande, Antonio; Jiménez, J.A.Synthesis and processing methods can have a significant influence on the physical properties of spinel zinc ferrite since it can produce microstructures characterized by different microstructural parameter such as the arrangement of the ions, lattice parameter, crystallite size and lattice strains (or microstrain). In this work, it has been evaluated the magnetic properties of samples prepared by ceramic synthesis, sol-gel and a third one provided by a ceramic powder supplier. Later, the synthesized samples have been ball-milled for 50 h and compared with a sample prepared by mechanochemical synthesis during 150 h. X-ray powder diffraction (XRD) analysis, performed in order to understand the relation between microstructure and magnetic properties, showed that these four materials are characterized by different values of the microstructural parameters (lattice constant, inversion degree and crystallite size). It was concluded that cation inversion is the most important parameters that can be effective in the deviation of the magnetic properties of zinc ferrite from the properties of the bulk form. Finally, the magnetization curves for the samples milled for 50 h were very similar to the sample prepared by mechanochemical, as expected from the similar microstructural parameters obtained from their XRD patterns. The inversion degree for these samples varied from 0.56 to 0.61 with saturation magnetization at 5 K around 79.0 emu/g. Although structural and chemical properties of this material can be selected by a proper choice of the preparation method, it is also possible to obtain the same microestructural and magnetic properties by a thermomechanic process, regardless of the synthesis method used. PublicationMany-impurity scattering on the surface of a topological insulator(Nature publishing group, 2021-03-11) Hernando Grande, Antonio; Baba, Yuriko Caterina; Díaz García, Elena; Domínguez-Adame Acosta, FranciscoWe theoretically address the impact of a random distribution of non-magnetic impurities on the electron states formed at the surface of a topological insulator. The interaction of electrons with the impurities is accounted for by a separable pseudo-potential method that allows us to obtain closed expressions for the density of states. Spectral properties of surface states are assessed by means of the Green's function averaged over disorder realisations. For comparison purposes, the configurationally averaged Green's function is calculated by means of two different self-consistent methods, namely the self-consistent Born approximation (SCBA) and the coherent potential approximation (CPA). The latter is often regarded as the best single-site theory for the study of the spectral properties of disordered systems. However, although a large number of works employ the SCBA for the analysis of many-impurity scattering on the surface of a topological insulator, CPA studies of the same problem are scarce in the literature. In this work, we find that the SCBA overestimates the impact of the random distribution of impurities on the spectral properties of surface states compared to the CPA predictions. The difference is more pronounced when increasing the magnitude of the disorder. PublicationSpin imbalance of charge carriers induced by an electric current(Elsevier, 2020-02-15) Hernando Grande, Antonio; Gálvez Alonso, Fernando; García, Miguel A.; Guinea, FranciscoWe analyze the contribution of the inhomogeneous magnetic field induced by an electrical current to the spin Hall effect in metals. The Zeeman coupling between the field and the electron spin leads to a spin dependent force, and to spin accumulation at the edges. We compare the effect of this relativistic correction to the electron dynamics to the features induced by the spin-orbit interaction. The effect of current induced magnetic fields on the spin Hall effect can be comparable to the extrinsic contribution from the spin-orbit interaction, although it does not require the presence of heavy elements with a strong spin-orbit interaction. PublicationStress-induced large Curie temperature enhancement in Fe_(64)Ni_(36) Invar alloy(American Physical Society, 2009-08) Hernando Grande, AntonioWe have succeeded in increasing up to 150 K the Curie temperature in the Fe_(64)Ni_(36) invar alloy by means of a severe mechanical treatment followed by a heating up to 1073 K. The invar behavior is still present as revealed by the combination of magnetic measurements with neutron and x-ray techniques under extreme conditions, such as high temperature and high pressure. The proposed explanation is based in a selective induced microstrain around the Fe atoms, which causes a slight increase in the Fe-Fe interatomic distances, thus reinforcing ferromagnetic interactions due to the strong magnetoelastic coupling in these invar compounds. PublicationExtended x-ray-absorption fine-structure studies of heat-treated fcc-Fe_50Cu_50 powders processed via high-energy ball milling(American Institute of Physics, 1994-11-15) Crespo del Arco, Patricia; Hernando Grande, Antonio; Garcia Escorial, A.; Kemner, K.M.; Harris, V. J.The local structure and chemistry of a ferromagnetic fcc-Fe_50Cu_50 solid solution obtained through high-energy ball milling were measured before and after heat-treatment-induced decomposition using extended x-ray-absorption fine-structure measurements. The decomposition is first evident with the phase separation of a-Fe after a heat treatment at 523 K. Analysis of the residual fee component revealed that the Fe atoms were predominantly surrounded by other Fe atoms, suggesting that the Fe has coalesced within the fee structure. The Fe atoms within the fee phase likely exist in low-spin clusters which provide an explanation for the reduced values of low-temperature magnetization previously measured in annealed samples [P. Crespo et aZ., Phys. Rev. B 48, 7134 (1993)]. PublicationBias free magnetomechanical coupling on magnetic microwires for sensing applications(American Institute of Physics, 2013-09-30) Herrero Gómez, Carlos; Marín Palacios, Pilar; Hernando Grande, AntonioIn the present paper, we report a systematic study of the magnetoelastic resonance of amorphous magnetic microwires of composition Fe73Si11B13Nb3. The study was performed for samples annealed at different temperatures. It was observed that such microwires present the key feature of performing magnetoelastic resonance in the absence of applied field. This fact, in addition to their small size, gives the microwires unique advantages over the widespreaded ribbons, currently in use as magnetoelastic sensors. Beyond the study of the resonance, magnetic properties of the samples were studied by means of Vibrating Sample Magnetometer (VSM) measurement in order to find an explanation to their bias-free resonance property. Finally, we show two possible applications of microwire based magnetoelastic sensors, a fluid density sensor and a mass-loading sensor.