P. Gorria

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.

Relative cooling power enhancement in magneto-caloric nanostructured Pr2Fe17

The magneto-caloric effect (MCE) of arc-melted bulk and 10 h ball-milled nanostructured Pr2Fe17 powders has been investigated. The maximum value for the magnetic entropy change, |�S M|, in the milled alloy is 4.5 J kg −1 K −1 for µ0H = 5 T, at around room temperature. The full width at half maximum, δTFWHM ,o f|�S M|(T ) for the nanostructured powders is about 60% greater than that of the starting bulk alloy, thus giving rise to large relative cooling power values of 573 J kg −1 (4.5 J cm −3 ) for µ0H = 5 T estimated from the product of |�S M| max × δTFWHM. These results have been compared with those of well-known magnetic materials that exhibit a large or giant MCE effect. The potential for using these low-cost iron based nanostructured Pr2Fe17 powders in magnetic refrigeration at room temperature is also discussed. (Some figures in this article are in colour only in the electronic version)

Influence of Fe in giant magnetoresistance ratio and magnetic properties of La0.7Ca0.3Mn1−xFexO3 perovskite type compounds

In this work polycrystalline perovskite with composition La0.7Ca0.3Fe0.05Mn0.95O3 has been produced by standard ceramic method. A considerable change occurs in magnetic, transport and magnetoresistance properties with respect to the classic composition without Fe. The introduction of a different metal, Fe, in Mn-O layer causes a decrease of about 50 K in the value of TC. In the same way, a decrease of about 10–15% in the average magnetic moment measured at 1 T is also observed. On the other hand, the introduction of Fe does not cause any appreciable change in the value of the lattice parameter. This new compound presents 60% of giant magnetoresistance ratio at 200 K. Magnetization vs. temperature measurements in zero field cooling and field cooling show clear differences at applied fields below 80 kA m−1. Such a behavior, also observed in La0.7Ca0.3MnO3 sample, is not recognizable as a simple ferromagnetic one. A qualitative discussion of the effect in the magnetic and transport properties of these compou...

Enhanced refrigerant capacity and magnetic entropy flattening using a two-amorphous FeZrB(Cu) composite

The temperature dependence of the isothermal magnetic entropy change, ΔSM, and the magnetic field dependence of the refrigerant capacity, RC, have been investigated in a composite system xA + (1 − x)B, based on Fe87Zr6B6Cu1 (A) and Fe90Zr8B2 (B) amorphous ribbons. Under a magnetic field change of 2 T, the maximum improvement of the full-width at half maximum of ΔSM(T) curve (47% and 29%) and the RC (18% and 23%), in comparison with those of the individual alloys (A and B), is observed for x ≈ 0.5. Moreover, a flattening over 80 K in the ΔSM(T) curve around room temperature range is observed, which is a key feature for an Ericsson magnetic refrigeration cycle.

Structural and magnetic changes in FeNbCuSiB amorphous alloys during the crystallization process

Calorimetric and magnetic measurements, x-ray powder diffraction and Mossbauer spectroscopy have been used to study the magnetic and structural changes occurring after each of the two steps of crystallization that take place in FeNbCuSiB-type alloys. Two samples with different boron and silicon concentrations, (x=6, 9), have been studied. They give a somewhat different composition of the crystalline phases appearing after crystallization processes. The most noticeable phenomenon is the observed increase of about 50 K in the Curie temperature of the FeSi crystalline phase between the end of the first crystallization process and the end of the second one, although the composition of this phase remains unchanged. This result is discussed in terms of crystal boundary effects. Also, the Curie temperature of the remaining amorphous phase, in the crystallized samples, is greater than the expected one, due to the coupling with magnetic phases with higher Curie points and inhomogeneities in such a phase.

Resistivity changes of some amorphous alloys undergoing nanocrystallization

Abstract The electrical resistivity of amorphous alloys with compositions: Fe73.5Nb3Cu1Si13.5B9, Fe86Zr7Cu1B6 and Co80Nb8B12 has been studied in the temperature range from 300 to 1100 K, where crystallization occurs. The products of crystallization and the grain size have been studied by X-ray diffraction. In a first step, all the alloys crystallize with small grains of a few nanometers in diameter (nanocrystalline state), and the resistivity behavior at this process accounts for the difference between the amorphous and nanocrystalline phases. The nanocrystalline phases are: α-Fe-Si, α-Fe and fcc Co for the three compounds studied respectively. A second process, at which grain growth and precipitation of intermetallic compounds and borides takes place, has been found for all the alloys. The resistivity is sensitive, not only to the total transformed sample amount, but to the topological distribution of the crystalline phases, and therefore shows a more complex behavior than other well established techniques, as differential scanning calorimetry. This supplementary information given by the resistivity is also discussed.

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.

Magnetic behavior of Fe‐Nb and Fe‐Zr alloys nanocrystallized by means of flash annealing

Recently nanocrystalline alloys obtained from Fe‐rich amorphous metallic glasses have shown excellent soft magnetic properties for technical applications. They include 4d elements like Nb or Zr for obtaining nanometer‐size crystals after heat treatments of about 1 h at temperatures around 500 °C. Flash annealing, using an electrical current flowing along the sample, is a useful method for performing heat treatments in short periods of time without need of controlled atmosphere. Grain sizes in the range 8–20 nm were determined from x‐ray diffraction in crystallized alloys prepared by flash annealing technique. These are grain sizes even smaller than the ones obtained in furnace treated samples. Magnetic softening of the alloys is observed after the heating. Magnetization work, permeability, coercitive force, etc., are very similar after applying both types of treatments.

Nanocrystalline Nd2Fe17 synthesized by high-energy ball milling: crystal structure, microstructure and magnetic properties.

Nanocrystalline Nd(2)Fe(17) powders have been obtained by means of high-energy ball milling from nearly single-phase bulk alloys produced by arc melting and high temperature homogenization annealing. The rhombohedral Th(2)Zn(17)-type crystal structure of the bulk alloy remains unaltered after the milling process, with almost unchanged values for the cell parameters. However, the severe mechanical processing induces drastic microstructural changes. A decrease of the mean crystalline size down to around 10 nm is observed, giving rise to a considerable augmentation of the disordered inter-grain boundaries. This modification of the microstructure affects the magnetic behaviour of the milled powders, although the magnetic structure remains collinear ferromagnetic. While a unique ferro-to-paramagnetic transition temperature, T(C) = 339 ± 2 K, is observed in the bulk alloy, the nanocrystalline samples exhibit a more likely distribution of T(C) values. The latter seems to be responsible for the significant broadening of the temperature range in which magneto-caloric effect is observed, and the lowering of the maximum value of the magnetic entropy change.

Magneto-impedance effect in nanostructured soft ferromagnetic alloys

Nanostructured Fe-based alloys have softer magnetic properties, such as larger saturation polarizations and magnetic permeabilities, smaller anisotropies and coercive fields and vanishing magnetostrictions, than their precursor alloys in the amorphous state. The softest magnetic properties are obtained for the smallest nanocrystalline grain sizes (between 10 and 20 nm), and these nanostructured materials are very suitable for use as high-frequency electronic components in magnetic devices or magnetic sensors based on the magnetoimpedance (MI) effect. In this work we study the correlation between the structural, electrical and magnetic properties together with the MI effect response in some heat-treated Finemet type and FeZrB ribbons. We show that the maximum MI ratio of around 130% and a sensitivity to the applied magnetic field of 0.07% (A m−1)−1 is obtained in the heat-treated samples that show an optimum nanocrystalline state and exhibit softer magnetic properties.