B. Hernando

Giant-magnetoimpedance-based sensitive element as a model for biosensors

We study the magnetoimpedance effect, using a Co67Fe4Mo1.5Si16.5B11 amorphous, ribbon-based sensitive element, in the presence of a commercial Ferrofluid® liquid thin layer covering the ribbon surface. The magnetoimpedance response is clearly dependent on the presence of the magnetic ferroliquid, the value of the applied magnetic field, and the parameters of the driving current. The magnetoimpedance-based prototype is proposed as a biosensor with high sensitivity to the fringe field produced by magnetic nanoparticles. A special advantage of this sensor is its high stability to chemical aggressive media; hence, it can be used for in situ measurements during fabrication of biomaterials with a high level of affinity and specificity with nanoparticles employed as bimolecular labels.

Martensitic phase transformation in rapidly solidified Mn50Ni40In10 alloy ribbons

Heusler alloy Mn50Ni40In10 was produced as preferentially textured ribbon flakes by melt spinning, finding the existence of martensitic-austenic transformation with both phases exhibiting ferromagnetic ordering. A microcrystalline three-layered microstructure of ordered columnar grains grown perpendicularly to ribbon plane was formed between two thin layers of smaller grains. The characteristic temperatures of the martensitic transformation were MS=213K, Mf=173K, AS=222K, and Af=243K. Austenite phase shows a cubic L21 structure (a=0.6013(3)nm at 298K and a Curie point of 311K), transforming into a modulated fourteen-layer modulation monoclinic martensite.

Magnetocaloric effect in melt spun Ni50.3Mn35.5Sn14.4 ribbons

We determined the magnetic entropy change and refrigerant capacity of melt spun Ni50.3Mn35.5Sn14.4 ribbons around both the structural and the magnetic transitions for a field of 20kOe. The maximum entropy changes at the structural and magnetic transitions were of 4.1 and −1.1Jkg−1K−1. Ribbons studied show a larger refrigerant capacity around the magnetic transition (46Jkg−1) than around the structural transition (26Jkg−1), suggesting that the temperature range at the magnetic transition is more adequate for a refrigerant cycle than that at the structural transition.

Thermal and magnetic field-induced martensite-austenite transition in Ni50.3Mn35.3Sn14.4 ribbons

Thermal and field-induced martensite-austenite transition was studied in melt spun Ni50.3Mn35.3Sn14.4 ribbons. Its distinct highly ordered columnarlike microstructure normal to ribbon plane allows the direct observation of critical fields at which field-induced and highly hysteretic reverse transformation starts (H=17kOe at 240K), and easy magnetization direction for austenite and martensite phases with respect to the rolling direction. Single phase L21 bcc austenite with TC of 313K transforms into a 7M orthorhombic martensite with thermal hysteresis of 21K and transformation temperatures of MS=226K, Mf=218K, AS=237K, and Af=244K.

Magnetocaloric effect in preferentially textured Mn50Ni40In10 melt spun ribbons

Inverse and direct magnetocaloric properties were evaluated in preferentially textured Mn50Ni40In10 ribbons applying the magnetic field H∥ along the ribbon length and perpendicular H⊥ to the ribbon plane (ΔH=30 kOe). Maximum magnetic entropy change, hysteretic losses, and refrigerant capacity were not significantly affected by crystallographic texture. Refrigeration capacity around structural transition is strongly reduced by the large hysteretic losses associated to the metamagnetic field-induced reverse martensitic transformation and narrower working temperature range making the interval around the magnetic transition more efficient for a refrigerant cycle (RCstruct=71 J kg−1 versus RCstructeff≈60 J kg−1, and RCmagn=89–86 J kg−1, for H∥ and H⊥, respectively).

Magnetocaloric effect in ribbon samples of Heusler alloys Ni–Mn–M (M=In,Sn)

Direct measurements of the magnetocaloric effect in samples of rapidly quenched ribbons of Mn50Ni40In10 and Ni50Mn37Sn13 Heusler alloys with potential applications in magnetic refrigeration technology are carried out. The measurements were made by a precise method based on the measurement of the oscillation amplitude of the temperature in the sample while is subjected to a modulated magnetic field. In the studied compositions both direct and inverse magnetocaloric effects associated with magnetic (paramagnet-ferromagnet-antiferromagnet) and structural (austenite-martensite) phase transitions are found. Additional inverse magnetocaloric effects of small value are observed around the ferromagnetic transitions.

Magnetoimpedance effect in amorphous and nanocrystalline ribbons

The magnetoimpedance effect in several Co-rich amorphous ribbons is overviewed. Results are classified in the following sections: influence of anisotropies induced by stress annealing, dependence on applied stress, its dependence on stress or stress-impedance, and the appearance of hysteresis. The influence of nanocrystallization of given Fe-rich ribbons is also analyzed.

An effective method to probe local magnetostatic properties in a nanometric FePd antidot array

A simple method to quantitatively characterize the local magnetic behaviour of a patterned nanostructure, like a ferromagnetic thin film of antidot arrays, is proposed. The first-order reversal curve (FORC) analysis, coupled with simulations using physically meaningful hysterons, allows us to obtain a quantitative and physically related description of the interaction field and each magnetization reversal process. The hysterons system is built from previously known hypotheses on the magnetic behaviour of the sample. This method was successfully applied to a highly hexagonal ordered FePd antidot array with nanometric dimensions. We achieved a complete characterization of the two different magnetization reversal mechanisms in function of the in-plane applied field angle. For a narrow range of high fields, the magnetization initiates rotating reversibly around the pores, while at lower fields, domain walls are nucleated and propagated. This in-plane magnetization reversal mechanism, partly reversible and partly irreversible, is the only angularly dependent one. While going away from the easy axis, its reversible proportion increases, as well as its switching field distribution. Finally, the results indicate that the high surface roughness between adjacent holes of the antidot thin film induces a parallel interaction field. The proposed method demonstrates its ability also to be applied to characterizing patterned nanostructures with rather complex magnetization reversal processes.

Correlation between structure, magnetic properties and MI effect during the nanocrystallisation process of FINEMET type alloys

FeNbCuSiB metallic glasses show excellent soft magnetic properties in nanocrystalline state such as high saturation induction and permeability, low magnetostriction, coercive field and anisotropy, which make these materials very suitable for use in magnetic devices or sensors based on magnetoimpedance (MI) effect. The main aim of this paper is to emphasise the great importance of structure characterisation when a complete understanding of the magnetic behaviour is pursued. In this way, we present an exhaustive study of the correlation between the magnetic properties and the structural changes occurring along the crystallisation process, focusing our interest on the first stages of the crystallisation, where magnetic parameters, such as the magnetic permeability or the Curie temperature of the amorphous matrix, together with magnetic domain structure undergo more sensitive changes. Several experimental results obtained by means of X-ray and neutron diffraction, differential thermal analysis, thermomagnetization, Mossbauer spectroscopy, hysteresis loops and MI measurements or surface domain structure observation are presented and discussed.

Changes in Morphology and Ionic Transport Induced by ALD SiO2 Coating of Nanoporous Alumina Membranes

Nanoporous anodic alumina membranes (NPAMs) were produced by the two-step anodization method in sulphuric, oxalic and phosphoric acidic electrolytes displaying a hexagonally ordered spatial arrangement of pores with well controlled nanopore size distribution and low porosity. Some selected NPAMs were further modified by conformal coating their surface and inner pore walls with a thin layer of SiO2 by means of atomic layer deposition (ALD), which reduces both the pore radii and porosity but it also seems to affect to the electric fixed charge on the membranes surface. A comparative study about the influence of silica modification of NPAMs surfaces on the ionic transport through the nanoporous membranes has been performed by measuring membrane potentials and electrochemical impedance spectroscopy with NaCl solutions. According to these results, a direct correlation between the membrane effective fixed charge and the NaCl diffusion coefficient can be established. The coating with a SiO2 thin layer causes a reduction of 75% in the positive effective fixed charge of the NPAMs independently of their pore radii and the increase in counterion transport (cation transport number and diffusion coefficient) even through constrained nanopores, which can be of interest in several applications (microfluidics, drug delivery, nanofilter devices, etc.). Moreover, slight changes in the membrane/solution interface due to the SiO2 cover layer are also indicated.