Tunable Magnetism in Nanoporous CuNi Alloys by Reversible Voltage-Driven Element-Selective Redox Processes

Voltage-driven manipulation of magnetism in electrodeposited 200 nm thick nanoporous single-phase solid solution Cu₂₀ Ni₈₀ (at%) alloy films (with sub 10 nm pore size) is accomplished by controlled reduction-oxidation (i.e., redox) processes in a protic solvent, namely 1 m NaOH aqueous solution. Owing to the selectivity of the electrochemical processes, the oxidation of the CuNi film mainly occurs on the Cu counterpart of the solid solution, resulting in a Ni-enriched alloy. As a consequence, the magnetic moment at saturation significantly increases (up to 33% enhancement with respect to the as-prepared sample), while only slight changes in coercivity are observed. Conversely, the reduction process brings Cu back to its metallic state and, remarkably, it becomes alloyed to Ni again. The reported phenomenon is fully reversible, thus allowing for the precise adjustment of the magnetic properties of this system through the sign and amplitude of the applied voltage.

Micelle-Assisted Electrodeposition of Mesoporous Fe-Pt Smooth Thin Films and their Electrocatalytic Activity towards the Hydrogen Evolution Reaction

Mesoporous Fe-Pt thin films are obtained by micelle-assisted electrodeposition onto metallic substrates with dissimilar activity (namely, gold, copper, and aluminum seed layers evaporated on Si/Ti) under constant applied potential (E=-1.1 V vs. Ag/AgCl) and deposition time (600 s). The amphiphilic triblock copolymer Pluronic P-123 is used as a soft template to guide the formation of mesopores. The occurrence of pores (ca. 7 nm in diameter) with narrow size distribution, distributed evenly over the surface, is observed in all cases. Despite the applied conditions being the same, the roughness and the amount of Fe incorporated in the films are influenced by the nature of the substrate. In particular, ultra-smooth films containing a larger amount of Fe (21 wt %) are obtained when deposition takes place on the Au surface. X-ray diffraction analyses reveal that Pt and Fe are alloyed to a certain extent, although some iron oxides/hydroxides also unavoidably form. The resulting films have been tested as electrocatalysts in the hydrogen evolution reaction (HER) in alkaline media. The mesoporous Fe-rich Fe-Pt films on Au show excellent HER activity and cyclability.

Clustering analysis strategies for electron energy loss spectroscopy (EELS)

In this work, the use of cluster analysis algorithms, widely applied in the field of big data, is proposed to explore and analyze electron energy loss spectroscopy (EELS) data sets. Three different data clustering approaches have been tested both with simulated and experimental data from Fe₃O₄/Mn₃O₄ core/shell nanoparticles. The first method consists on applying data clustering directly to the acquired spectra. A second approach is to analyze spectral variance with principal component analysis (PCA) within a given data cluster. Lastly, data clustering on PCA score maps is discussed. The advantages and requirements of each approach are studied. Results demonstrate how clustering is able to recover compositional and oxidation state information from EELS data with minimal user input, giving great prospects for its usage in EEL spectroscopy.

Micelle-assisted electrodeposition of highly mesoporous Fe-Pt nodular films with soft magnetic and electrocatalytic properties

Mesoporous Fe-Pt nodular films (with a regular spatial arrangement of sub-15 nm pores) are grown onto evaporated Au, Cu and Al conductive layers by micelle-assisted electrodeposition from metal chloride salts in the presence of Pluronic P123 tri-block copolymer dissolved in the aqueous electrolytic bath. This synthetic approach constitutes a simple, one-step, versatile procedure to grow multifunctional mesoporous layers appealing for diverse applications that take advantage of materials with an ultra-high surface area-to-volume ratio. The films exhibit tuneable composition with relative Fe/Pt weight ratios, disregarding oxygen, varying from 4/96 to 52/48. All the mesoporous alloys show a soft magnetic behaviour with tuneable saturation magnetization and coercivity values (the latter ranging from ca. 5 Oe to 40 Oe). In addition, the Au/Fe-Pt deposits (even the ones with higher Fe content) exhibit good performance towards hydrogen evolution reaction in both alkaline and acidic media due to the inherent mesoporosity, with excellent stability after running 50 cycles. The interest of alloying Fe with Pt is thus two-fold: (i) to confer magnetic properties to the mesoporous alloys and (ii) to reduce the amount of the costly noble metal in the electrocatalyst in an environmentally sustainable manner.

Biodegradable FeMnSi Sputter-Coated Macroporous Polypropylene Membranes for the Sustained Release of Drugs

Pure Fe and FeMnSi thin films were sputtered on macroporous polypropylene (PP) membranes with the aim to obtain biocompatible, biodegradable and, eventually, magnetically-steerable platforms. Room-temperature ferromagnetic response was observed in both Fe- and FeMnSi-coated membranes. Good cell viability was observed in both cases by means of cytotoxicity studies, though the FeMnSi-coated membranes showed higher biodegradability than the Fe-coated ones. Various strategies to functionalize the porous platforms with transferrin-Alexa Fluor 488 (Tf-AF488) molecules were tested to determine an optimal balance between the functionalization yield and the cargo release. The distribution of Tf-AF488 within the FeMnSi-coated PP membranes, as well as its release and uptake by cells, was studied by confocal laser scanning microscopy. A homogeneous distribution of the drug within the membrane skeleton and its sustained release was achieved after three consecutive impregnations followed by the addition of a layer made of gelatin and maltodextrin, which prevented exceedingly fast release. The here-prepared organic-inorganic macroporous membranes could find applications as fixed or magnetically-steerable drug delivery platforms.

Nanoporous Fe-Based Alloy Prepared by Selective Dissolution: An Effective Fenton Catalyst for Water Remediation

A fully nanoporous Fe-rich alloy, prepared by selective dissolution of melt-spun Fe_43.5Cu_56.5 ribbons, exhibits outstanding properties as a heterogeneous Fenton catalyst toward the degradation of methyl orange (MO) in aqueous solution. In addition, the ferromagnetic characteristics of this material enable its wireless manipulation toward specific locations within polluted wastewater. The influence of selective dissolution on the microstructure, sample morphology (surface and cross-section), elemental composition, and magnetic properties of the resulting nanoporous alloy is investigated. The dealloying procedure enhances the saturation magnetization and drastically increases the catalytic performance (i.e., the time required for full degradation of MO from the medium is reduced by a factor of approximately 2 by subjecting the Fe_43.5Cu_56.5 ribbons to prior dealloying). Remarkably, the effectiveness of this nanoporous material surpasses the results obtained by the homogeneous Fenton reaction using an equivalent concentration of Fe cations leached into the media from the nanoporous alloy. The different factors that promote the high catalytic activity are discussed. The outstanding catalytic activity, together with the simplicity of the synthetic procedure, makes this material very appealing for water remediation using advanced Fenton processes.

Effect of Surface Modifications of Ti40Zr10Cu38Pd12 Bulk Metallic Glass and Ti-6Al-4V Alloy on Human Osteoblasts In Vitro Biocompatibility

The use of biocompatible materials, including bulk metallic glasses (BMGs), for tissue regeneration and transplantation is increasing. The good mechanical and corrosion properties of Ti₄₀Zr₁₀Cu₃₈Pd₁₂ BMG and its previously described biocompatibility makes it a potential candidate for medical applications. However, it is known that surface properties like topography might play an important role in regulating cell adhesion, proliferation and differentiation. Thus, in the present study, Ti₄₀Zr₁₀Cu₃₈Pd₁₂ BMG and Ti6-Al-4V alloy were surface-modified electrochemically (nanomesh) or physically (microscratched) to investigate the effect of material topography on human osteoblasts cells (Saos-2) adhesion, proliferation and differentiation. For comparative purposes, the effect of mirror-like polished surfaces was also studied. Electrochemical treatments led to a highly interconnected hierarchical porous structure rich in oxides, which have been described to improve corrosion resistance, whereas microscratched surfaces showed a groove pattern with parallel trenches. Cell viability was higher than 96% for the three topographies tested and for both alloy compositions. In all cases, cells were able to adhere, proliferate and differentiate on the alloys, hence indicating that surface topography plays a minor role on these processes, although a clear cell orientation was observed on microscratched surfaces. Overall, our results provide further evidence that Ti₄₀Zr₁₀Cu₃₈Pd₁₂ BMG is an excellent candidate, in the present two topographies, for bone repair purposes.

Room-temperature synthesis of three-dimensional porous ZnO@CuNi hybrid magnetic layers with photoluminescent and photocatalytic properties

A facile synthetic approach to prepare porous ZnO@CuNi hybrid films is presented. Initially, magnetic CuNi porous layers (consisting of phase separated CuNi alloys) are successfully grown by electrodeposition at different current densities using H2 bubbles as a dynamic template to generate the porosity. The porous CuNi alloys serve as parent scaffolds to be subsequently filled with a solution containing ZnO nanoparticles previously synthesized by sol-gel. The dispersed nanoparticles are deposited dropwise onto the CuNi frameworks and the solvent is left to evaporate while the nanoparticles impregnate the interior of the pores, rendering ZnO-coated CuNi 3D porous structures. No thermal annealing is required to obtain the porous films. The synthesized hybrid porous layers exhibit an interesting combination of tunable ferromagnetic and photoluminescent properties. In addition, the aqueous photocatalytic activity of the composite is studied under UV-visible light irradiation for the degradation of Rhodamine B. The proposed method represents a fast and inexpensive approach towards the implementation of devices based on metal-semiconductor porous systems, avoiding the use of post-synthesis heat treatment steps which could cause deleterious oxidation of the metallic counterpart, as well as collapse of the porous structure and loss of the ferromagnetic properties.

Electrochemically synthesized amorphous and crystalline nanowires: dissimilar nanomechanical behavior in comparison with homologous flat films

The effects of constrained sample dimensions on the mechanical behavior of crystalline materials have been extensively investigated. However, there is no clear understanding of these effects in nano-sized amorphous samples. Herein, nanoindentation together with finite element simulations are used to compare the properties of crystalline and glassy CoNi(Re)P electrodeposited nanowires (ϕ ≈ 100 nm) with films (3 μm thick) of analogous composition and structure. The results reveal that amorphous nanowires exhibit a larger hardness, lower Young's modulus and higher plasticity index than glassy films. Conversely, the very large hardness and higher Young's modulus of crystalline nanowires are accompanied by a decrease in plasticity with respect to the homologous crystalline films. Remarkably, proper interpretation of the mechanical properties of the nanowires requires taking the curved geometry of the indented surface and sink-in effects into account. These findings are of high relevance for optimizing the performance of new, mechanically-robust, nanoscale materials for increasingly complex miniaturized devices.

Nanocasting synthesis of mesoporous SnO2 with a tunable ferromagnetic response through Ni loading

Ni-doped SnO₂ mesoporous powders exhibit ferromagnetism at low and room temperatures, which can be tuned by adjusting the Ni loading.

Novel Fe–Mn–Si–Pd alloys: insights into mechanical, magnetic, corrosion resistance and biocompatibility performances

Cell viability and release ion concentration of two newly developed Fe-based alloys.

Origin of the large dispersion of magnetic properties in nanostructured oxides: Fe(x)O/Fe3O4 nanoparticles as a case study

The intimate relationship between stoichiometry and physicochemical properties in transition-metal oxides makes them appealing as tunable materials. These features become exacerbated when dealing with nanostructures. However, due to the complexity of nanoscale materials, establishing a distinct relationship between structure-morphology and functionalities is often complicated. In this regard, in the FexO/Fe3O4 system a largely unexplained broad dispersion of magnetic properties has been observed. Here we show, thanks to a comprehensive multi-technique approach, a clear correlation between the magneto-structural properties in large (45 nm) and small (9 nm) FexO/Fe3O4 core/shell nanoparticles that can explain the spread of magnetic behaviors. The results reveal that while the FexO core in the large nanoparticles is antiferromagnetic and has bulk-like stoichiometry and unit-cell parameters, the FexO core in the small particles is highly non-stoichiometric and strained, displaying no significant antiferromagnetism. These results highlight the importance of ample characterization to fully understand the properties of nanostructured metal oxides.

New binuclear copper(ii) coordination polymer based on mixed pyrazolic and oxalate ligands: structural characterization and mechanical properties

A new inorganic–organic coordination polymer based on a copper(ii) binuclear complex coordinated with pyrazole (L1), 1-(hydroxymethyl)pyrazole) (L2) and oxalate (Ox) ligands has been unexpectedly obtained.

Nanostructured Ti-Zr-Pd-Si-(Nb) bulk metallic composites: Novel biocompatible materials with superior mechanical strength and elastic recovery

The microstructure, mechanical behaviour, and biocompatibility (cell culture, morphology, and cell adhesion) of nanostructured Ti45 Zr15 Pd35- x Si5 Nbx with x = 0, 5 (at. %) alloys, synthesized by arc melting and subsequent Cu mould suction casting, in the form of rods with 3 mm in diameter, are investigated. Both Ti-Zr-Pd-Si-(Nb) materials show a multi-phase (composite-like) microstructure. The main phase is cubic β-Ti phase (Im3m) but hexagonal α-Ti (P63/mmc), cubic TiPd (Pm3m), cubic PdZr (Fm3m), and hexagonal (Ti, Zr)5 Si3 (P63/mmc) phases are also present. Nanoindentation experiments show that the Ti45 Zr15 Pd30 Si5 Nb5 sample exhibits lower Young's modulus than Ti45 Zr15 Pd35 Si5 . Conversely, Ti45 Zr15 Pd35 Si5 is mechanically harder. Actually, both alloys exhibit larger values of hardness when compared with commercial Ti-40Nb, (HTi-Zr-Pd-Si ≈ 14 GPa, HTi-Zr-Pd-Si-Nb ≈ 10 GPa and HTi-40Nb ≈ 2.7 GPa). Concerning the biological behaviour, preliminary results of cell viability performed on several Ti-Zr-Pd-Si-(Nb) discs indicate that the number of live cells is superior to 94% in both cases. The studied Ti-Zr-Pd-Si-(Nb) bulk metallic system is thus interesting for biomedical applications because of the outstanding mechanical properties (relatively low Young's modulus combined with large hardness), together with the excellent biocompatibility.

Electrodeposition of magnetic, superhydrophobic, non-stick, two-phase Cu-Ni foam films and their enhanced performance for hydrogen evolution reaction in alkaline water media

Two-phase Cu-Ni magnetic metallic foams (MMFs) with tunable composition have been prepared by electrodeposition taking advantage of hydrogen co-evolution as a source of porosity. It is observed that Ni tends to deposit inside the porous network defined by the Cu building blocks. Contact angle measurements reveal that the prepared porous films show a remarkable superhydrophobicity (contact angle values larger than 150°) and a non-sticking property to aqueous droplets. This behavior is predominately ascribed to the morphology of the films - hierarchical micro/nanoporosity, wall thickness, and spatial arrangement. The electrochemical activity and stability towards hydrogen evolution reaction of the Cu-Ni MMFs has been investigated by cyclic voltammetry in 1 M KOH at 298 K, and the optimal Ni content is found to be 15 at%. Furthermore, all the foam-like films exhibit ferromagnetic behaviour due to the presence of the Ni-rich phase, with coercivity values ranging from 114 Oe to 300 Oe. From the technological point of view, the Cu-Ni MMFs are promising candidates for magnetically-actuated micro/nano-electromechanical systems (MEMS/NEMS) and micro/nanorobotic platforms with a large surface-area to volume ratio or in magnetic sensors or separators.

Direct evidence for an interdiffused intermediate layer in bi-magnetic core-shell nanoparticles

Core-shell nanoparticles attract continuously growing interest due to their numerous applications, which are driven by the possibility of tuning their functionalities by adjusting structural and morphological parameters. However, despite the critical role interdiffused interfaces may have in the properties, these are usually only estimated in indirect ways. Here we directly evidence the existence of a 1.1 nm thick (Fe,Mn)3O4 interdiffused intermediate shell in nominally γ-Fe2O3-Mn3O4 core-shell nanoparticles using resonant inelastic X-ray scattering spectroscopy combined with magnetic circular dichroism (RIXS-MCD). This recently developed magneto-spectroscopic probe exploits the unique advantages of hard X-rays (i.e., chemical selectivity, bulk sensitivity, and low self-absorption at the K pre-edge) and can be advantageously combined with transmission electron microscopy and electron energy loss spectroscopy to quantitatively elucidate the buried internal structure of complex objects. The detailed information on the structure of the nanoparticles allows understanding the influence of the interface quality on the magnetic properties.

Facile in situ synthesis of BiOCl nanoplates stacked to highly porous TiO₂: a synergistic combination for environmental remediation

A novel nanocomposite material made of two-dimensional BiOCl nanoplates assembled into highly porous titania has been successfully prepared following a facile sol-gel reaction. Both the TiO2 (anatase) and BiOCl components are crystalline as demonstrated by X-ray diffraction and transmission electron microscopy analyses. TiO2 exhibits a highly porous network and possesses a small crystallite size, whereas BiOCl forms micrometer-sized plates with nanometer thicknesses. Aqueous photocatalytic activity tests with this novel material have been performed on photodegradation of Rhodamine B under ultraviolet-visible light irradiation. Interestingly, the attachment of the BiOCl nanoplates to the TiO2 network significantly enhances the photocatalytic activity of the material compared to that of pure TiO2 due to the formation of BiOCl/TiO2 heterojunctions. Thus, this pertinent synergistic combination of TiO2 and BiOCl proves to be a promising strategy for the large-scale production of a new generation of photocatalysts with excellent properties for the degradation of organic pollutants.

Mesoporous Titania Powders: The Role of Precursors, Ligand Addition and Calcination Rate on Their Morphology, Crystalline Structure and Photocatalytic Activity

We evaluate the influence of the use of different titania precursors, calcination rate, and ligand addition on the morphology, texture and phase content of synthesized mesoporous titania samples, parameters which, in turn, can play a key role in titania photocatalytic performances. The powders, obtained through the evaporation-induced self-assembly method, are characterized by means of ex situ X-Ray Powder Diffraction (XRPD) measurements, N₂ physisorption isotherms and transmission electron microscopy. The precursors are selected basing on two different approaches: the acid-base pair, using TiCl₄ and Ti(OBu)₄, and a more classic route with Ti(OⁱPr)₄ and HCl. For both precursors, different specimens were prepared by resorting to different calcination rates and with and without the addition of acetylacetone, that creates coordinated species with lower hydrolysis rates, and with different calcination rates. Each sample was employed as photoanode and tested in the water splitting reaction by recording I-V curves and comparing the results with commercial P25 powders. The complex data framework suggests that a narrow pore size distribution, due to the use of acetylacetone, plays a major role in the photoactivity, leading to a current density value higher than that of P25.

Robust antiferromagnetic coupling in hard-soft bi-magnetic core/shell nanoparticles

The growing miniaturization demand of magnetic devices is fuelling the recent interest in bi-magnetic nanoparticles as ultimate small components. One of the main goals has been to reproduce practical magnetic properties observed so far in layered systems. In this context, although useful effects such as exchange bias or spring magnets have been demonstrated in core/shell nanoparticles, other interesting key properties for devices remain elusive. Here we show a robust antiferromagnetic (AFM) coupling in core/shell nanoparticles which, in turn, leads to the foremost elucidation of positive exchange bias in bi-magnetic hard-soft systems and the remarkable regulation of the resonance field and amplitude. The AFM coupling in iron oxide-manganese oxide based, soft/hard and hard/soft, core/shell nanoparticles is demonstrated by magnetometry, ferromagnetic resonance and X-ray magnetic circular dichroism. Monte Carlo simulations prove the consistency of the AFM coupling. This unique coupling could give rise to more advanced applications of bi-magnetic core/shell nanoparticles.

Drastic influence of minor Fe or Co additions on the glass forming ability, martensitic transformations and mechanical properties of shape memory Zr-Cu-Al bulk metallic glass composites

The microstructure and mechanical properties of Zr48Cu48 - x Al4M x (M ≡ Fe or Co, x = 0, 0.5, 1 at.%) metallic glass (MG) composites are highly dependent on the amount of Fe or Co added as microalloying elements in the parent Zr48Cu48Al4 material. Addition of Fe and Co promotes the transformation from austenite to martensite during the course of nanoindentation or compression experiments, resulting in an enhancement of plasticity. However, the presence of Fe or Co also reduces the glass forming ability, ultimately causing a worsening of the mechanical properties. Owing to the interplay between these two effects, the compressive plasticity for alloys with x = 0.5 (5.5% in Zr48Cu47.5Al4Co0.5 and 6.2% in Zr48Cu47.5Al4Fe0.5) is considerably larger than for Zr48Cu48Al4 or the alloys with x = 1. Slight variations in the Young's modulus (around 5-10%) and significant changes in the yield stress (up to 25%) are also observed depending on the composition. The different microstructural factors that have an influence on the mechanical behavior of these composites are investigated in detail: (i) co-existence of amorphous and crystalline phases in the as-cast state, (ii) nature of the crystalline phases (austenite versus martensite content), and (iii) propensity for the austenite to undergo a mechanically-driven martensitic transformation during plastic deformation. Evidence for intragranular nanotwins likely generated in the course of the austenite-martensite transformation is provided by transmission electron microscopy. Our results reveal that fine-tuning of the composition of the Zr-Cu-Al-(Fe,Co) system is crucial in order to optimize the mechanical performance of these bulk MG composites, to make them suitable materials for structural applications.

Improvement to the Corrosion Resistance of Ti-Based Implants Using Hydrothermally Synthesized Nanostructured Anatase Coatings

The electrochemical behavior of polycrystalline TiO₂ anatase coatings prepared by a one-step hydrothermal synthesis on commercially pure (CP) Ti grade 2 and a Ti13Nb13Zr alloy for bone implants was investigated in Hank’s solution at 37.5 °C. The aim was to verify to what extent the in-situ-grown anatase improved the behavior of the substrate in comparison to the bare substrates. Tafel-plot extrapolations from the potentiodynamic curves revealed a substantial improvement in the corrosion potentials for the anatase coatings. Moreover, the coatings grown on titanium also exhibited lower corrosion-current densities, indicating a longer survival of the implant. The results were explained by considering the effects of crystal morphology, coating thickness and porosity. Evidence for the existing porosity was obtained from corrosion and nano-indentation tests. The overall results indicated that the hydrothermally prepared anatase coatings, with the appropriate morphology and surface properties, have attractive prospects for use in medical devices, since better corrosion protection of the implant can be expected.

In vitro biocompatibility assessment of Ti40Cu38Zr10Pd12 bulk metallic glass

The use of biocompatible materials has attained an increasing importance for tissue regeneration and transplantation. The excellent mechanical and corrosion properties of Ti40Cu38Zr10Pd12 bulk metallic glass (BMG) turn it into a potential candidate for its use in orthopaedic implants. Before being considered as a biomaterial, some biological parameters must be taken into account. In this study,mouse preosteoblasts were cultured in the presence or absence of the alloy at different times (24 h, 7 and 21 days) and no differences in cell viability were detected.Moreover, cells were able to adhere to the alloy surface by establishing focal contacts, and displayed a flattened polygonal morphology. After 14 days in culture, differentiation into osteoblasts was observed. Besides, the amount of Cu ions released and their potential toxic effects were analyzed, showing that the amount of Cu released did not increase cell death. Finally, the low levels of inflammatory cytokines secreted by THP-1 differentiated macrophages exposed to the alloy suggest the absence of an immunogenic response to the alloy. In conclusion, in vitro studies indicate that the Ti40Cu38Zr10Pd12 BMG could be considered as a biomaterial to be used in orthopaedic implants.

3D hierarchically porous Cu-BiOCl nanocomposite films: one-step electrochemical synthesis, structural characterization and nanomechanical and photoluminescent properties

Three-dimensional (3D) hierarchically porous composite Cu-BiOCl films have been prepared by a facile one-step galvanostatic electrodeposition process from acidic electrolytic solutions containing Cu(ii) and Bi(iii) chloride salts and Triton X-100. The films show spherical, micron-sized pores that spread over the whole film thickness. In turn, the pore walls are made of randomly packed BiOCl nanoplates that are assembled leaving micro-nanopore voids beneath. It is believed that Cu grows within the interstitial spaces between the hydrogen bubbles produced from the reduction of H(+) ions. Then, the BiOCl sheets accommodate in the porous network defined by the Cu building blocks. The presence of Cu tends to enhance the mechanical stability of the composite material. The resulting porous Cu-BiOCl films exhibit homogeneous and stable-in-time photoluminescent response arising from the BiOCl component that spreads over the entire 3D porous structure, as demonstrated by confocal scanning laser microscopy. A broad-band emission covering the entire visible range, in the wavelength interval 450-750 nm, is obtained. The present work paves the way for the facile and controlled preparation of a new generation of photoluminescent membranes.

Novel Ti-Zr-Hf-Fe Nanostructured Alloy for Biomedical Applications

The synthesis and characterization of Ti40Zr20Hf20Fe20 (atom %) alloy, in the form of rods (f = 2 mm), prepared by arc-melting, and subsequent Cu mold suction casting, is presented. The microstructure, mechanical and corrosion properties, as well as in vitro biocompatibility of this alloy, are investigated. This material consists of a mixture of several nanocrystalline phases. It exhibits excellent mechanical behavior, dominated by high strength and relatively low Young's modulus, and also good corrosion resistance, as evidenced by the passive behavior in a wide potential window and the low corrosion current densities values. In terms of biocompatibility, this alloy is not cytotoxic and preosteoblast cells can easily adhere onto its surface and differentiate into osteoblasts.

Controlled 3D-coating of the pores of highly ordered mesoporous antiferromagnetic Co3O4 replicas with ferrimagnetic Fe(x)Co(3-x)O4 nanolayers

The controlled filling of the pores of highly ordered mesoporous antiferromagnetic Co3O4 replicas with ferrimagnetic FexCo3-xO4 nanolayers is presented as a proof-of-concept toward the integration of nanosized units in highly ordered, heterostructured 3D architectures. Antiferromagnetic (AFM) Co3O4 mesostructures are obtained as negative replicas of KIT-6 silica templates, which are subsequently coated with ferrimagnetic (FiM) FexCo3-xO4 nanolayers. The tuneable magnetic properties, with a large exchange bias and coercivity, arising from the FiM/AFM interface coupling, confirm the microstructure of this novel two-phase core-shell mesoporous material. The present work demonstrates that ordered functional mesoporous 3D-materials can be successfully infiltrated with other compounds exhibiting additional functionalities yielding highly tuneable, versatile, non-siliceous based nanocomposites.

Resolving material-specific structures within Fe₃O₄|γ-Mn₂O₃ core|shell nanoparticles using anomalous small-angle X-ray scattering

Here it is demonstrated that multiple-energy, anomalous small-angle X-ray scattering (ASAXS) provides significant enhancement in sensitivity to internal material boundaries of layered nanoparticles compared with the traditional modeling of a single scattering energy, even for cases in which high scattering contrast naturally exists. Specifically, the material-specific structure of monodispersed Fe₃O₄|γ-Mn₂O₃ core|shell nanoparticles is determined, and the contribution of each component to the total scattering profile is identified with unprecedented clarity. We show that Fe₃O₄|γ-Mn₂O₃ core|shell nanoparticles with a diameter of 8.2 ± 0.2 nm consist of a core with a composition near Fe₃O₄ surrounded by a (Mn(x)Fe(1-x))₃O₄ shell with a graded composition, ranging from x ≈ 0.40 at the inner shell toward x ≈ 0.46 at the surface. Evaluation of the scattering contribution arising from the interference between material-specific layers additionally reveals the presence of Fe₃O₄ cores without a coating shell. Finally, it is found that the material-specific scattering profile shapes and chemical compositions extracted by this method are independent of the original input chemical compositions used in the analysis, revealing multiple-energy ASAXS as a powerful tool for determining internal nanostructured morphology even if the exact composition of the individual layers is not known a priori.

Strongly exchange coupled inverse ferrimagnetic soft/hard, Mn(x)Fe(3-x)O4/Fe(x)Mn(3-x)O4, core/shell heterostructured nanoparticles

Inverted soft/hard, in contrast to conventional hard/soft, bi-magnetic core/shell nanoparticles of Mn(x)Fe(3-x)O(4)/Fe(x)Mn(3-x)O(4) with two different core sizes (7.5 and 11.5 nm) and fixed shell thickness (∼0.6 nm) have been synthesized. The structural characterization suggests that the particles have an interface with a graded composition. The magnetic characterization confirms the inverted soft/hard structure and evidences a strong exchange coupling between the core and the shell. Moreover, larger soft core sizes exhibit smaller coercivities and loop shifts, but larger blocking temperatures, as expected from spring-magnet or graded anisotropy structures. The results indicate that, similar to thin film systems, the magnetic properties of soft/hard core/shell nanoparticles can be fine tuned to match specific applications.

Structural and magnetic characterization of batch-fabricated nickel encapsulated multi-walled carbon nanotubes

We report on the growth and fabrication of Ni-filled multi-walled carbon nanotubes (Ni-MWNTs) with an average diameter of 115 nm and variable length of 400 nm-1 µm. The Ni-MWNTs were grown using template-assisted electrodeposition and low pressure chemical vapor deposition (LPCVD) techniques. Anodized alumina oxide (AAO) templates were fabricated on Si using a current controlled process. This was followed by the electrodeposition of Ni nanowires (NWs) using galvanostatic pulsed current (PC) electrodeposition. Ni NWs served as the catalyst to grow Ni-MWNTs in an atmosphere of H2/C2H2 at a temperature of 700 °C. Time dependent depositions were carried out to understand the diffusion and growth mechanism of Ni-MWNTs. Characterization was carried out using scanning electron microscopy (SEM), focused ion beam (FIB) milling, transmission electron microscopy (TEM), Raman spectroscopy and energy dispersive x-ray spectroscopy (EDX). TEM analysis revealed that the Ni nanowires possess a fcc structure. To understand the effects of the electrodeposition parameters, and also the effects of the high temperatures encountered during MWNT growth on the magnetic properties of the Ni-MWNTs, vibrating sample magnetometer (VSM) measurements were performed. The template-based fabrication method is repeatable, efficient, enables batch fabrication and provides good control on the dimensions of the Ni-MWNTs.

Grain boundary segregation and interdiffusion effects in nickel-copper alloys: an effective means to improve the thermal stability of nanocrystalline nickel

Nanocrystalline (nc) Ni films show pronounced grain growth and suffer from concomitant deterioration of their mechanical and magnetic properties after annealing at relatively low temperatures (T(ANN) ≥ 475 K). This constitutes a drawback for their applicability as coatings or in components of miniaturized devices. This work reveals that the thermal stability of nc Ni is significantly improved by controllably alloying Ni with Cu, by means of electrodeposition, to form a Ni(1-x)Cu(x) solid solution. To tune the composition of such nc alloys, Ni(1-x)Cu(x) films are deposited galvanostatically using an electrolytic bath containing Ni and Cu sulfate salts as electroactive species, saccharine as grain-refining agent, and applying current densities ranging from -10 to -40 mA cm(-2). The enhanced thermal stability is ascribed to segregation of a Cu-rich phase at the Ni(1-x)Cu(x) grain boundaries, which acts as a shielding layer against grain growth. As a result, high values of hardness (in excess of 6 GPa) remain in nc Ni(1-x)Cu(x) for x ≥ 0.3, even after annealing at T(ANN) ≥ 575 K. From a magnetic point of view, Ni(1-x)Cu(x) films possess lower coercivity values than pure nc Ni films, both in the as-prepared and annealed states, thus offering potential advantages for certain soft magnetic applications.

Out-of-plane magnetic patterning based on indentation-induced nanocrystallization of a metallic glass

Periodic arrays of micrometer-sized ferromagnetic structures with perpendicular magnetic anisotropy are prepared by nanoindentation at the surface of a Fe(67.7)B(20)Cr(12)Nb(0.3) glassy ribbon initially showing in-plane magnetic anisotropy. The indented regions exhibit enhanced coercivity and saturation magnetization with respect to the surrounding nondeformed matrix. These effects are due to a mechanically driven selective nanocrystallization of the metallic glass, induced by nanoindentation, even without the need for thermal annealing. In addition, while the amorphous matrix becomes paramagnetic above 325 K, the crystallized regions (consisting of alpha-Fe) remain ferromagnetic upon heating to high temperatures. The local change in the magnetic anisotropy direction is ascribed to a certain degree of crystallographic texture, together with the inverse magnetostriction effect caused by the compressive indentation stresses.

Magnetic proximity effect features in antiferromagnetic/ferrimagnetic core-shell nanoparticles

A study of "inverted" core-shell, MnO/gamma-Mn(2)O(3), nanoparticles is presented. Crystal and magnetic structures and characteristic sizes have been determined by neutron diffraction for the antiferromagnetic core (MnO) and the ferrimagnetic shell (gamma-Mn(2)O(3)). Remarkably, while the MnO core is found to have a T_{N} not far from its bulk value, the magnetic order of the gamma-Mn(2)O(3) shell is stable far above T_{C}, exhibiting two characteristic temperatures, at T approximately 40 K [T_{C}(gamma-Mn(2)O(3))] and at T approximately 120 K [ approximately T_{N}(MnO)]. Magnetization measurements are consistent with these results. The stabilization of the shell moment up to T_{N} of the core can be tentatively attributed to core-shell exchange interactions, hinting at a possible magnetic proximity effect.

Direct magnetic patterning due to the generation of ferromagnetism by selective ion irradiation of paramagnetic FeAl alloys

Sub-100-nm magnetic dots embedded in a non-magnetic matrix are controllably generated by selective ion irradiation of paramagnetic Fe(60)Al(40) (atomic %) alloys, taking advantage of the disorder-induced magnetism in this material. The process is demonstrated by sequential focused ion beam irradiation and by in-parallel broad-beam ion irradiation through lithographed masks. Due to the low fluences used, this method results in practically no alteration of the surface roughness. The dots exhibit a range of magnetic properties depending on the size and shape of the structures, with the smallest dots (<100 nm) having square hysteresis loops with coercivities in excess of micro(0)H(C) = 50 mT. Importantly, the patterning can be fully removed by annealing. The combination of properties induced by the direct magnetic patterning is appealing for a wide range of applications, such as patterned media, magnetic separators, or sensors.

Magnetic properties of Ni-NiO (ferromagnetic-antiferromagnetic) nanocomposites obtained from a partial mechanochemical reduction of NiO

The magnetic properties of ferromagnetic (FM)-antiferromagnetic (AFM), Ni-NiO, nanocomposites obtained from a reactive ball milling reduction of NiO in H2 atmosphere have been studied. The formation of ferromagnetic Ni from antiferromagnetic NiO can be accurately followed by the increase of the saturation magnetization. The microstructure of the nanocomposite, consisting of FM Ni nanoparticles embedded in an AFM NiO matrix leads to exchange bias effects, i.e., loop shifts and coercivity enhancement, after field cooling from above the Néel temperature of NiO.

Imprinting vortices into antiferromagnets

The effect of imprinting symmetric and displaced vortex structures into an antiferromagnetic material is investigated in micron-sized disks consisting of exchange coupled ferromagnetic-antiferromagnetic bilayers. The imprint of displaced vortices manifests itself by the occurrence of a new type of asymmetric hysteresis loops characterized by curved, reversible, central sections with nonzero remanent magnetization. Such an imprint is achieved by cooling the disks through the blocking temperature of the system in small fields. Micromagnetic simulations reveal that asymmetric vortexlike loops naturally result from the competition between the different energies involved in the system.

Magnetization reversal in submicron disks: exchange biased vortices

Submicron, circular, ferromagnetic-antiferromagnetic dots exhibit different magnetization reversal mechanisms depending on the direction of the magnetic applied field. Shifted, constricted hysteresis loops, typical for vortex formation, are observed for fields along the exchange bias direction. However, for fields applied close to perpendicular to the exchange bias direction, magnetization reversal occurs via coherent rotation. Magnetic force microscopy imaging together with micromagnetic simulations are used to further clarify the different magnetic switching behaviors.