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Dive into the research topics where Alfonso Ibarra is active.

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Featured researches published by Alfonso Ibarra.


arXiv: Materials Science | 2010

Magnetic properties and energy absorption of CoFe2O4 nanoparticles for magnetic hyperthermia

T. E. Torres; A. G. Roca; Maria del Puerto Morales; Alfonso Ibarra; C. Marquina; M. R. Ibarra; Gerardo F. Goya

We have studied the magnetic and power absorption properties of three samples of CoFe2O4 nanoparticles with sizes from 5 to 12 nm prepared by thermal decomposition of Fe (acac)3 and Co(acac)2 at high temperatures. The blocking temperatures TB estimated from magnetization M(T) curves spanned the range 180 ≤ T B ≤ 320 K, reflecting the large magnetocrystalline anisotropy of these nanoparticles. Accordingly, high coercive fields HC ≈ 1.4 - 1.7 T were observed at low temperatures. Specific Power Absorption (SPA) experiments carried out in ac magnetic fields indicated that, besides particle volume, the effective magnetic anisotropy is a key parameter determining the absorption efficiency. SPA values as high as 98 W/g were obtained for nanoparticles with average size of ≈ 12 nm.


Journal of Materials Chemistry C | 2014

Tailored design of CoxMn1−xFe2O4 nanoferrites: a new route for dual control of size and magnetic properties

Carlos Fernandes; Clara Pereira; M. P. Fernández-García; André M. Pereira; Alexandra Guedes; Rodrigo Fernández-Pacheco; Alfonso Ibarra; M. Ricardo Ibarra; J. P. Araújo; Cristina Freire

This work reports the tailored design of novel mixed ferrite nanoparticles, CoxMn1−xFe2O4 (x = 0, 0.3, 0.7, and 1), through an optimized one-pot aqueous coprecipitation process. The influence of the substitution between Mn(II) and Co(II) and of the alkaline agent, isopropanolamine (MIPA) or NaOH, on the morphological, chemical and magnetic properties of the nanomaterials was investigated. The joint action between chemical substitution and type of alkaline agent allowed a precise tuning of the particle size, magnetic properties and inversion degree of the spinel structure. A wide range of particle dimensions (from 3 to 30 nm) and saturation magnetization (from 57 to 71 emu g−1) was achieved. The increase of Co(II) content from x = 0 to x = 1 led to a systematic decrease of the particle size, regardless of the base type, which could be modelled by an exponential decay function. For each Co:Mn composition, the use of MIPA instead of the traditional NaOH promoted a three times reduction of the particle size and simultaneously switched the magnetic state of the CoxMn1−xFe2O4 nanomaterials from ferromagnetic to superparamagnetic. These results constitute a step forward in the challenging quest for high-performance magnetic nanoprobes by an eco-friendly synthesis route.


Chemistry of Materials | 2015

Gas Slug Microfluidics: A Unique Tool for Ultrafast, Highly Controlled Growth of Iron Oxide Nanostructures.

Ane Larrea; Victor Sebastian; Alfonso Ibarra; Manuel Arruebo; Jesus Santamaria

The use of nanomaterials in real life applications is often hampered by our inability to produce them in large quantities while preserving their desired properties in terms of size, shape, and crystalline phase. Here we present a novel continuous method to synthesize nanostructures with an unprecedented degree of control regarding their properties. In particular, the excellent properties of microreactors for chemical synthesis are enhanced by the introduction of gas slugs of tailored composition. Slug dynamics accelerate mixing, reduce processing times (from hours in batch processes to minutes or even seconds), and, depending on the gas atmosphere used, allows one to accurately control the crystalline phase and shape of the resulting nanostructures. Inert (N2), oxidizing (O2), or reducing (CO, H2) gases were used, leading to different morphologies and crystalline structures in a high yield, highly reproducible fabrication process.


Nanotechnology | 2009

Single-step chemical synthesis of ferrite hollow nanospheres

Enio Lima; J. M. Vargas; R. D. Zysler; H.R. Rechenberg; Renato Cohen; Jordi Arbiol; Gerardo F. Goya; Alfonso Ibarra; M. Ricardo Ibarra

We report a single-step chemical synthesis of iron oxide hollow nanospheres with 9.3 nm in diameter. The sample presents a narrow particle diameter distribution and chemical homogeneity. The hollow nature of the particles is confirmed by HRTEM and HAADF STEM analysis. Electron and x-ray diffraction show that the outer material component is constituted by 2 nm ferrite crystals. Mössbauer data provide further evidence for the formation of iron oxide with high structural disorder, magnetically ordered at 4.2 K and superparamagnetism at room temperature. An unusual magnetic behavior under an applied field is reported, which can be explained by the large fraction of atoms existing at both inner and outer surfaces.


Angewandte Chemie | 2016

A Nanoarchitecture Based on Silver and Copper Oxide with an Exceptional Response in the Chlorine-Promoted Epoxidation of Ethylene

Adrian Ramirez; Jose L. Hueso; Hugo Suarez; Reyes Mallada; Alfonso Ibarra; Silvia Irusta; Jesus Santamaria

The selective oxidation of ethylene to ethylene epoxide is highly challenging as a result of competing reaction pathways leading to the deep oxidation of both ethylene and ethylene oxide. Herein we present a novel catalyst based on silver and copper oxide with an excellent response in the selective oxidation pathway towards ethylene epoxide. The catalyst is composed of different silver nanostructures dispersed on a tubular copper oxide matrix. This type of hybrid nanoarchitecture seems to facilitate the accommodation of chlorine promoters, leading to high yields at low reaction temperatures. The stability after the addition of chlorine promoters implies a substantial improvement over the industrial practice: a single pretreatment step at ambient pressure suffices in contrast with the common practice of continuously feeding organochlorinated precursors during the reaction.


Materials Research Express | 2015

Exchange bias in ferrite hollow nanoparticles originated by complex internal magnetic structure

Emilio De Biasi; Enio Lima; J. M. Vargas; R. D. Zysler; Jordi Arbiol; Alfonso Ibarra; Gerardo F. Goya; M. Ricardo Ibarra

Iron-oxide hollow nanospheres (HNS) may present unusual magnetic behavior as a consequence of their unique morphology. Here, we report the unusual magnetic behavior of HNS that are 9 nm in diameter. The magnetic properties of HNS originate in their complex magnetic structure, as evidenced by Mossbauer spectroscopy and magnetization measurements. We observe a bias in the hysteresis when measured at very low temperature in the field cooling protocol (10 kOe). In addition, dc (static) and ac (dynamic) magnetization measurements against temperature and applied field reveal a frustrated order of the system below 10 K. High-resolution transmission electron microscopy (HRTEM) studies reveal that the HNS are composed of small crystalline clusters of about 2 nm in diameter, which behave as individual magnetic entities. Micromagnetic simulations (using conjugate gradient in order to minimize the total energy of the system) reproduce the experimentally observed magnetic behavior. The model considers the hollow particles as constituted by small ordered clusters embedded in an antiferromagnetic environment (spins localized outside the clusters). In addition, the surface spins (in both inner and outer surfaces of the HNS) are affected by a local surface anisotropy. The strong effective magnetic anisotropy field of the clusters induces the bias observed when the system is cooled in the presence of a magnetic external field. This effect propagates through the exchange interaction into the entire particle.


Nano Letters | 2018

Vertical Growth of Superconducting Crystalline Hollow Nanowires by He+ Focused Ion Beam Induced Deposition

R. Córdoba; Alfonso Ibarra; D. Mailly; José María de Teresa

Novel physical properties appear when the size of a superconductor is reduced to the nanoscale, in the range of its superconducting coherence length (ξ0). Such nanosuperconductors are being investigated for potential applications in nanoelectronics and quantum computing. The design of three-dimensional nanosuperconductors allows one to conceive novel schemes for such applications. Here, we report for the first time the use of a He+ focused-ion-beam-microscope in combination with the W(CO)6 precursor to grow three-dimensional superconducting hollow nanowires as small as 32 nm in diameter and with an aspect ratio (length/diameter) of as much as 200. Such extreme resolution is achieved by using a small He+ beam spot of 1 nm for the growth of the nanowires. As shown by transmission electron microscopy, they display grains of large size fitting with face-centered cubic WC1-x phase. The nanowires, which are grown vertically to the substrate, are felled on the substrate by means of a nanomanipulator for their electrical characterization. They become superconducting at 6.4 K and show large critical magnetic field and critical current density resulting from their quasi-one-dimensional superconducting character. These results pave the way for future nanoelectronic devices based on three-dimensional nanosuperconductors.


Nano Letters | 2018

Air-Stable Anisotropic Monocrystalline Nickel Nanowires Characterized Using Electron Holography

Glenna L. Drisko; Christophe Gatel; Pier-Francesco Fazzini; Alfonso Ibarra; Stefanos Mourdikoudis; Vincent Bley; Katia Fajerwerg; Pierre Fau; Myrtil L. Kahn

Nickel is capable of discharging electric and magnetic shocks in aerospace materials thanks to its conductivity and magnetism. Nickel nanowires are especially desirable for such an application as electronic percolation can be achieved without significantly increasing the weight of the composite material. In this work, single-crystal nickel nanowires possessing a homogeneous magnetic field are produced via a metal-organic precursor decomposition synthesis in solution. The nickel wires are 20 nm in width and 1-2 μm in length. The high anisotropy is attained through a combination of preferential crystal growth in the ⟨100⟩ direction and surfactant templating using hexadecylamine and stearic acid. The organic template ligands protect the nickel from oxidation, even after months of exposure to ambient conditions. These materials were studied using electron holography to characterize their magnetic properties. These thin nanowires display homogeneous ferromagnetism with a magnetic saturation (517 ± 80 emu cm-3), which is nearly equivalent to that of bulk nickel (557 emu cm-3). Nickel nanowires were incorporated into carbon composite test pieces and were shown to dramatically improve the electric discharge properties of the composite material.


Journal of Alloys and Compounds | 2018

Crystal structure and local ordering in epitaxial Fe100−xGax/MgO(001) films

M. Ciria; Maria Grazia Proietti; Edna C. Corredor; David Coffey; Adrián Begué; César de la Fuente; J. I. Arnaudas; Alfonso Ibarra

Abstract In this work we present a study of the structural properties of Fe100−x Gax (x  T s below 400 °C we obtain bcc films while, for x ≈ 24 and Ts ≥ 400oC the nucleation of the fcc phase is observed. For both systems a Ga anticlustering or local range ordering phenomenon appears. The Ga/Fe composition in the first and second coordination shells of the bcc films is different from that expected for a random Ga distribution and is close to a D03 phase, leading to a minimization of the number Ga-Ga pairs. On the other side, a long-range D03 phase is not observed indicating that atomic ordering only occurs at a local scale. Overall, the epitaxial growth procedure presented in this work, first, avoids the formation of a long range ordered D03 phase, which is known to be detrimental for magnetostrictive properties, and second, demonstrates the possibility of growing fcc films at temperatures much lower than those required to obtain bulk fcc samples.


Archive | 2008

Morphological characterization by HRTEM and STEM of Fe3O4 hollow nano-spheres

Alfonso Ibarra; Gerardo F. Goya; Jordi Arbiol; Enio Lima; H.R. Rechenberg; J. M. Vargas; R. D. Zysler; M. R. Ibarra

Morphology, surface and finite size effects in magnetic nanoparticles have been the subject of growing interest in recent years from both experimental and theoretical point of view [1]. The magnetic properties are strongly associated with the morphological and structural homogeneity of the nanoparticles [2]. Interparticle interactions also play an important role in the magnetic behaviour of an ensemble of nanoparticles, which differs from that of non-interacting systems [3].

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Enio Lima

National Scientific and Technical Research Council

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Jordi Arbiol

Spanish National Research Council

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R. D. Zysler

National Scientific and Technical Research Council

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