Showing papers by "M. R. Ibarra published in 2013"

Observation of the spin Seebeck effect in epitaxial Fe3O 4 thin films

Abstract: We report the experimental observation of the spin Seebeck effect in magnetite thin films. The signal observed at temperatures above the Verwey transition is a contribution from both the anomalous Nernst (ANE) and spin Seebeck (SSE) effects. The contribution from the ANE of the Fe3O4 layer to the SSE is found to be negligible due to the resistivity difference between Fe3O4 and Pt layers. Below the Verwey transition, the SSE is free from the ANE of the ferromagnetic layer and it is also found to dominate over the ANE due to magnetic proximity effect on the Pt layer.

Magnetic field-induced dissipation-free state in superconducting nanostructures

Abstract: The motion of magnetic vortices induced in type-II superconductors by a magnetic field degrades their ability to conduct electricity with zero resistance. Cordoba et al. demonstrate a means to immobilize these vortices, reversing their deleterious effect as the applied magnetic field is increased.

Induced cell toxicity originates dendritic cell death following magnetic hyperthermia treatment

Abstract: Magnetic hyperthermia (MH) is based on the use of magnetic nanoparticles (MNPs) to selectively increase the temperature of MNP-loaded target tissues when applying an alternating magnetic field (AMF) in the range of radiofrequency. To date, all MH research has focused on heat generation in an attempt to elucidate the mechanisms for the death of MNP-loaded cells submitted to AMF. However, recent in vitro studies have demonstrated the feasibility of inducing dramatic cell death without increasing the macroscopic temperature during AMF exposure. Here, we show that the cell death observed following AMF exposure, specifically that of MNP-loaded dendritic cells (DCs) in culture, was caused by the release of toxic agents into the cell culture supernatants and not due to a macroscopic temperature increase. We performed MH in vitro experiments to demonstrate that the supernatant of the cell culture following AMF exposure was highly toxic when added to control unloaded DCs, as this treatment led to nearly 100% cell death. Therefore, our results demonstrate that heat is not the only agent responsible for triggering cell death following MH treatment. This finding offers new perspectives for the use of DCs as the proverbial Trojan horse to vectorise MNPs to the target tumour area and these results further support the use of DCs as therapeutic agents against cancer when submitted to AMF. Furthermore, this discovery may help in understanding the mechanism of cell death mediated by exposure to AMF.

Size dependence of the magnetic relaxation and specific power absorption in iron oxide nanoparticles

Abstract: In this study, magnetic and power absorp- tion properties of a series of iron oxide nanoparticles with average sizes hdi ranging from 3 to 23 nm were reported. The nanoparticles were prepared by thermal decomposition of Iron(III) acetylacetonate in organic media. From the careful characterization of the mag- netic and physicochemical properties of these samples, the specific power absorption (SPA) values experi- mentally found were numerically reproduced, as well as their dependence with particle size, using a simple model of Brownian and Neel relaxation at room temperature. SPA experiments in ac magnetic fields (H0 = 13 kA/m and f = 250 kHz) indicated that the magnetic and rheological properties played a crucial role determining the heating efficiency at different conditions. A maximum SPA value of 344 W/g was obtained for a sample containing nanoparticles with hdi = 12 nm and dispersion r = 0.25. The observed SPA dependence with particle diameter and their magnetic parameters indicated that, for the size range and experimental conditions of f and H studied in this study, both Neel and Brown relaxation mechanisms are important to the heat generation observed.

Optimized cobalt nanowires for domain wall manipulation imaged by in situ Lorentz microscopy

Abstract: Direct observation of domain wall (DW) nucleation and propagation in focused electron beam induced deposited Co nanowires as a function of their dimensions was carried out by Lorentz microscopy (LTEM) upon in situ application of magnetic field. Optimal dimensions favoring the unambiguous DW nucleation/propagation required for applications were found in 500-nm-wide and 13-nm-thick Co nanowires, with a maximum nucleation field and the largest gap between nucleation and propagation fields. The internal DW structures were resolved using the transport-of-intensity equation formalism in LTEM images and showed that the optimal nanowire dimensions correspond to the crossover between the nucleation of transverse and vortex walls.

Modification of domain-wall propagation in Co nanowires via Ga+ irradiation

Abstract: The propagation of domain walls in polycrystalline Co nanowires grown by focused-electron-beam-induced deposition is explored. We have found that Ga+ irradiation via focused ion beam is a suitable method to modify the propagation field of domain walls in magnetic conduits. Magneto-optical Kerr effect measurements show that global Ga+ irradiation of the nanowires increases the domain-wall propagation field. Additionally, we have observed by means of scanning transmission X-ray microscopy that it is possible to produce substantial domain-wall pinning via local Ga+ irradiation of a narrow region of the nanowire. In both cases, Ga+ doses of the order of 1016 ions/cm2 are required to produce such effects. These results pave the way for the controlled manipulation of domain walls in Co nanowires via Ga+ irradiation.

Critical magnetic behavior of magnetocaloric materials with the Gd5Si4-type structure

Abstract: An extensive investigation on the magnetism of Gd5(SixGe1−x)4 compounds which stabilize in the Gd5Si4-type structure is here presented, focusing on the Gd5Si4 and Gd5Si2Ge2 compositions Through Arrott plot analysis, the temperature dependencies of the spontaneous magnetizations are retrieved and the magnetic interaction coefficients (Jexch) are estimated The obtained Jexch value for Gd5Si4 is 15% higher than that of Gd5Si2Ge2 Such enhancement cannot be attributed to a magneto-volume coupling only, being mainly associated with the magnetic polarization of the (Si,Ge)-(Si,Ge) interslab connections The critical exponents were estimated (δ=31 and 29, γ=101 and 106, β=048 and 056, and n=067 and 063 for Gd5Si4 and Gd5Si2Ge2, respectively) demonstrating that both compositions belong to the same universal class This conclusion is reinforced by the construction of a universal logarithmic scaling plot of the M(H) isothermal curves Hence, we conclude that the magnetism of the Gd5Si4-type structure comp

Improvement of domain wall conduit properties in cobalt nanowires by global gallium irradiation.

Abstract: Applications based on the movement of domain walls (DWs) in magnetic nanowires (NWs) require a good DW conduit behavior, i.e. a significant difference between DW nucleation and propagation fields. In this work, we have systematically studied how this property evolves in cobalt NWs grown by focused electron beam induced deposition (FEBID) as a function of global gallium irradiation, for irradiation doses up to 1.24 × 1017 ions cm−2. Whereas for high doses the DW conduit is lost, below 6.42 × 1015 ions cm−2 the difference between the two fields increases with irradiation, becoming up to ~9 times larger than for non-irradiated wires, due to a strong increase in the nucleation field, while the propagation field remains approximately constant. This behavior stems from two effects. The first effect is a decrease in the magnetic volume of the parasitic halo around the NW, typically present in FEBID nanostructures, leading to the disappearance of weak nucleation centers. The second effect is the formation of a 20 nm outer shell with Co crystals about twice the size of those forming the NW core, causing a net increase of the local magnetocrystalline anisotropy. The results presented here are important for the potential use of magnetic NWs grown by FEBID in DW-based devices, and might also be of interest for magnetic NWs fabricated by other techniques.

Conductance steps in electromigrated Bi nanoconstrictions

Abstract: Bismuth nanostructures of initial lateral size of about 150 nm were successfully electromigrated at room temperature under high vacuum conditions through the application of voltage ramps and accurate control of their conductance. The imaging of the nanogap formation was followed by scanning electron microscopy. An appropriate design of the initial Bi nanostructures has made the electromigration process of semimetallic Bi feasible. Beyond the intrinsic interest in the generation of Bi structures with size tailored at the nanoscale, remarkable features have been observed in the time-dependent conductance curves of the Bi nanoconstrictions. In particular, sub-quantum conductance plateaus can be detected before the rupture of the constriction. An alternative procedure to study the transport through Bi nanoconstrictions has been explored using a focused-Ga-ion etching process with simultaneous control of the conductance. This second approach confirms the transport behavior observed in electromigrated Bi nanoconstrictions.

Erratum: Role of the surface states in the magnetotransport properties of ultrathin bismuth films [Phys. Rev. B 82, 125326 (2010)]

Abstract: We found out that the Hall resistivity data (ρHall) of our original paper should be corrected due to an error in the wiring in the experimental set-up. The correction implies a change in the sign of the Hall resistivity data (−ρHall) presented in the original publication. This correction does not affect the interpretation of the experiments and our quantitative results remain valid, except that the third carrier band, required to account for the magnetic field evolution of the magnetoresistance and Hall resistivity, is a surface hole band (and not a surface electron band as it was reported in our original paper). This modification in the character of the surface carriers does not influence the conclusions made in the paper. In order to illustrate these points we show revised versions of Figs. 5, 6, and 8, respectively. Figures 5(a) and 5(b) shows the corrected ρHall isotherms from room temperature down to 2 K for the Bi films with thicknesses t = 100 nm and 10 nm, respectively. The remarkable change both in the magnitude and the field dependency of ρHall with film

Effects of pressure on the magnetic-structural and Griffiths-like transitions in Dy5Si3Ge

Abstract: Magnetization studies have been performed on a polycrystalline sample of Dy${}_{5}$Si${}_{3}$Ge as a function of an applied magnetic field (up to 50 kOe) and hydrostatic pressure (up to 10 kbar) in the 5--300 K temperature range. The anomalous behavior of the magnetic susceptibility indicates that a Griffiths-like phase exists at low magnetic fields and pressures up to 10 kbar. We present evidence that the high-temperature second-order ferromagnetic transition can be coupled with the low-temperature first-order crystallographic transformation into a single first-order magnetic-crystallographic transformation using a magnetic field and hydrostatic pressure as tuning parameters. The effect of pressure on the Griffiths-like phase is reported and analyzed in the framework of the complex competition between the interslab and intraslab magnetic interactions.

Nanoscale Electrical Contacts Grown by Focused Ion Beam (FIB)-Induced Deposition

Abstract: A detailed description of the use of the focused ion beam (FIB) to grow electrical contacts is presented. The combination of FIB with precursor compounds, which are delivered on the area of interest by means of gas-injection systems, allows the growth of electrical contacts with nanometric resolution on targeted places. The technique has been coined focused ion beam-induced deposition (FIBID). Pros and cons with respect to other existing techniques for contacting are discussed. The FIBID contacts reported in this chapter are based on the use of Pt and W precursors, which result in the growth of deposits with resistivities down to 100 μΩ cm without any post-treatment. A comparison of FIBID with focused electron beam-induced deposition, the sister technique that uses focused electrons instead of ions, is also presented. The steps to follow in order to be successful in the contacting process by means of FIBID are described. Examples of the contacting to individual nanowires and nanoparticles carried out in our laboratory are shown, together with the corresponding four-probe transport measurements. Below 5 K, W deposits are superconducting and can be therefore used for zero-resistance lead contacts, superconductor-based nanocontacts and probing of proximity effects, opening new perspectives as described here.

Direct patterning of cobalt nanostructures using focused electron beam-induced deposition

Abstract: The fabrication of ferromagnetic nanostructures is one of the key factors for theimmense development of emerging fields such as spintronics, nanomagnetism orbiomagnetism. In order to fabricate cobalt nanostructures, standard techniques such asevaporation or sputtering, together with optical or electron beam lithography, arenormally used.During the last years, we have investigated a different route, using focused electronbeam-induced-deposition (FEBID) and Co2(CO)8 as gas precursor. FEBID is anadvanced "direct writing" nanolithography technique, consisting on the local chemicalvapor deposition of a gas adsorbed on a substrate, induced by the interaction of the gasmolecules with a focused beam of electrons in the keV-range.In this chapter, we summarize some of the work performed in our group regardingthe growth of Co nanostructures using FEBID. We show the strong influence the beam current has on composition, microstructure, shape and magnetic properties of Co FEBIDdeposits. Also, some examples of the huge potential of FEBID to fabricate functional twoand three-dimensional nanostructures are presented.

Nanostructural Characterization of Biomagnetic Cobalt Ferrite–Alginate Nanospheres

Abstract: Funded by: European Regional and Social Development Funds. Grant Number: MAT2011-26851-C02-01; Aragon Autonomous Government (DGA); ERC. Grant Number: 239931-NANOPUZZLE.

Antibody Distribution on Bio-Functionalized Magnetic Nanoparticles Analyzed by Spatial-Resolved EELS

Abstract: Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis (US).