A. M. Pereira

Nanoporous alumina as templates for multifunctional applications

Due to its manufacturing and size tailoring ease, porous anodic alumina (PAA) templates are an elegant physical-chemical nanopatterning approach and an emergent alternative to more sophisticated and expensive methods currently used in nanofabrication. In this review, we will describe the ground work on the fabrication methods of PAA membranes and PAA-based nanostructures. We will present the specificities of the electrochemical growth processes of multifunctional nanomaterials with diversified shapes (e.g., nanowires and nanotubes), and the fabrication techniques used to grow ordered nanohole arrays. We will then focus on the fabrication, properties and applications of magnetic nanostructures grown on PAA and illustrate their dependence on internal (diameter, interpore distance, length, composition) and external (temperature and applied magnetic field intensity and direction) parameters. Finally, the most outstanding experimental findings on PAA-grown nanostructures and their trends for technological appl...

Designing Novel Hybrid Materials by One-Pot Co-condensation: From Hydrophobic Mesoporous Silica Nanoparticles to Superamphiphobic Cotton Textiles

This work reports the synthesis and characterization of mesoporous silica nanoparticles (MSNs) functionalized with tridecafluorooctyltriethoxysilane (F13) and their in situ incorporation onto cotton textiles. The hybrid MSNs and the functional textiles were prepared by a one-pot co-condensation methodology between tetraethylorthosilicate (TEOS) and F13, with hexadecyltrimethylammonium chloride (CTAC) as the template and triethanolamine as the base. The influence of the F13 to TEOS molar ratio (1:10, 1:5 and 1:3) on the nanoparticle morphology, porosity, degree of functionalization, and hydro/oleophobic properties is discussed. The hybrid nanosilicas presented high colloidal stability and were spherical and monodispersed with average particle size of ∼45 nm. They also showed high surface areas, large pore volumes, and a wormhole-type mesoporous structure. The increase in the organosilane proportion during the co-condensation process led to a more radially branched wormhole-like mesoporosity, a decrease in the surface area, pore volume, and amount of surface silanol groups, and an enrichment of the surface with fluorocarbon moieties. These changes imparted hydrophobic and oleophobic properties to the materials, especially to that containing the highest F13 loading. Cotton textiles were coated with the F13-MSNs through an efficient and less time-consuming route. The combination between surface roughness and mesoporosity imparted by the MSNs, and the low surface energy provided by the organosilane resulted in superhydrophobic functional textiles. Moreover, the textile with the highest loading of fluorocarbon groups was superamphiphobic.

On the Curie temperature dependency of the magnetocaloric effect

We investigate the magnetocaloric effect dependency on the most important microscopic parameters of ferromagnetic materials, such as the Curie temperature (TC), the spin value (J), and the magnetic field change (ΔH). Second- and first-order phase transition systems are considered, using the Bean-Rodbell model [C. P. Bean and D. S. Rodbell, Phys. Rev. 126, 104 (1962)] of magnetovolume interactions on the Weiss mean-field model [P. Weiss, J. Phys. Theory Appl. 6, 661 (1907)]. The magnetocaloric effect simulations show a surprising TC−2/3 linear dependence of the maximum entropy change (ΔSmmax), which is observed for all simulated systems. An approximate state equation establishing the dependence of ΔSmmax on TC, ΔH, J, and the magnetic atoms density (N) is presented. The dependence of maximum magnetic entropy change on TC−2/3 is validated by a wide set of experimental results of second- and first-order phase transition materials that are promising for magnetic refrigeration applications at room temperature.

Room temperature magnetocaloric effect and refrigerant capacitance in La0.7Sr0.3MnO3 nanotube arrays

High aspect ratio La0.7Sr0.3MnO3 nanotube (NT) arrays have been synthesized using nitrates based sol-gel precursor by nanoporous anodized aluminum oxide template assisted method. Their phase purity and microstructures were analyzed by X-ray diffraction, scanning electron microscopy, and energy-dispersive x-ray spectroscopy. Magnetocaloric effect (MCE) of as prepared NTs was investigated by means of field dependence magnetization measurements. Significant magnetic entropy change, −△SM = 1.6 J/kg K, and the refrigerant capacitance, RC = 69 J/kg, were achieved near the transition temperature at 315 K for 5 T. For comparison, a bulk sample was also prepared using the same precursor solution which gives a value of −△SM = 4.2 J/kg K and a RC = 165 J/kg. Though the bulk sample exhibits higher △SM value, the NTs present an expanded temperature dependence of −△SM curves that spread over a broad temperature range and assured to be appropriate for active magnetic refrigeration. The diminutive MCE observed in mangani...

Three-state memory combining resistive and magnetic switching using tunnel junctions

Magnetic fields generated by current lines are the standard way of switching between resistance (R) states in magnetic random access memories. A less common but technologically more interesting alternative to achieve R-switching is to use an electrical current crossing the tunnel barrier. Such current induced magnetization switching (CIMS) or current induced switching (CIS) effects were recently observed in thin magnetic tunnel junctions, and attributed to spin transfer (CIMS) or electromigration of atoms into the insulator (CIS). In this work, electromigration-driven resistance changes (resistive switching) are superimposed with thermally induced pinned layer reversal (magnetic switching), producing a reproducible, three-state memory device. The tunnel junctions under study show a tunnel magnetoresistance of 14% with a RA product of 50 �µ m 2 . The reversible electromigration-driven resistance changes amount to 7–8% of the full resistance change and more than 10 4 R-switching events can be current induced without significant damage to the tunnel junction. Typical critical current densities are of the order of 2 × 10 6 Ac m −2 .

Understanding the role played by Fe on the tuning of magnetocaloric effect in Tb5Si2Ge2

In this letter, it is shown that when replacing Ge by Fe in Tb5Si2Ge2 the structural transition still occurs and enhances the magnetocaloric effect (MCE) (up to 66%) with maximum of MCE at a critical Fe amount where the magnetic and structural transitions become fully coupled. It is observed that Fe concentration is able to mimic the effect of external pressure as it induces a complex microstructure, that tunes long range strain fields. This knowledge is crucial for the development of strategies toward materials with improved performance for efficient magnetic refrigeration applications.

Precise control of the filling stages in branched nanopores

The filling of hierarchical branched porous anodic alumina (PAA) templates with magnetic and metallic nanowires using pulsed electrodeposition is here addressed in detail. We show that the electrodeposition potential (and current) reveals clear anomalies when each generation of branched pores created by steady-state or non-steady-state anodisation is completely filled, thus signalising clearly the growth stages. This allowed us to infer that each stage of PAA filling corresponds to a new hierarchical level of the branched structure. We show that the anomalies in the electrodeposition potential (current) were related to the change in porosity, and a linear relation between the electrodeposition potential and the template porosity during metal filling was established. Such a linear relation can be used as a new tool to easily determine the local porosity of anodic metal oxides, independent of the corresponding pore architecture. This work allows one to accomplish a precise control of the nanowires during their growth, opening a path to fill the several hierarchical levels of multiple branched pores with different materials, aiming for the production of multicomponent nanostructures.

High refrigerant capacity of PrNi5−xCox magnetic compounds exploiting its spin reorientation and magnetic transition over a wide temperature zone

The ferromagnetically coupled cobalt ion is observed to create a magnetocrystalline anisotropy in the PrNi5−xCox structure above a critical composition of x = 2. The competition of the anisotropy energies between Co and Pr sublattices gives rise to a spin reorientation (SR) phenomenon in PrNi5−xCox compounds at a low temperature (~150 K) which is then followed by a magnetic transition at a higher temperature. Co-doping has a strong influence on the Curie temperature, changing it from ~60 K (x = 1.95) to ~537 K (x = 3). The magnetic entropy change is associated with SR as well as a magnetic transition, and correspondingly a large full width at half maximum (δTFWHM) is obtained for this series of compounds. For example, the PrNi2.85Co2.15 compound presents δTFWHM = 166 K at a 1 T field. This series therefore has an appreciable relative cooling power, which makes this material a suitable magnetic refrigerant over a large temperature span.

Unveiling the (De)coupling of magnetostructural transition nature in magnetocaloric R5Si2Ge2 (R 5Tb, Gd) materials

We present a detailed study on the magnetization under high magnetic fields of Tb5Si2Ge2 and Gd5Si2Ge2 compounds. From the Arrott plot construction (A. Arrott, Phys. Rev. 108, 1394 (1957)), we were able to estimate the TC of each structure (M and O(I)) experimentally and found that the TC of the O(I) phase can be directly obtained by extrapolating the TC curve of this phase in the respective phase diagram. Using a physical model based on free energy considerations, one explains the (de)coupling of the magnetic and structural transitions in R5(SixGe1−x)4 (R = Tb, Gd) compounds.

Magnetocaloric effect in La0.7Ca0.3MnO3 nanotube arrays with broad working temperature span

We have studied the magnetic entropy change of highly ordered La0.7Ca0.3MnO3 nanotube arrays synthesized by template assisted sol-gel method in temperatures ranging from 179 to 293 K and in magnetic fields up to 5 T. From the measurements of isothermal magnetization, we have calculated the maximum isothermal magnetic entropy change of −△SM = 1.9 J/kg K around the Curie temperature at 236 K for a field of 5 T. The nanotubes present lower magnetic entropy change compared with their bulk counterpart (−△SM = 4.8 J/kg K) which was prepared by the same sol-gel route. Such diminished magnetic entropy change observed in nanotubes is explained by the disordered magnetic states which are created on the surface sites of nanograins due to the larger surface to volume ratio. However, the nanotubes present an expanded magnetic transition that extends over a wide temperature range and suggest that such manganite nanotubes could be used for magnetic refrigeration with broad working temperature span.