J.I. Pérez-Landazábal

Effect of a SiO2 coating on the magnetic properties of Fe3O4 nanoparticles

In this work the effect of a SiO2 coating on the magnetic properties of Fe3O4 nanoparticles obtained by the sol-gel method is analyzed. Two sets of samples were prepared: Fe3O4 nanoparticles and Fe3O4@SiO2 core-shell composites. The samples display the characteristic spinel structure associated with the magnetite Fe3O4 phase, with the majority of grain sizes around 5-10 nm. At room temperature the nanoparticles show the characteristic superparamagnetic behavior with mean blocking temperatures around 160 and 120 K for Fe3O4 and Fe3O4@SiO2, respectively. The main effect of the SiO2 coating is reflected in the temperature dependence of the high field magnetization (μ(0)H = 6 T), i.e. deviations from the Bloch law at low temperatures (T < 20 K). Such deviations, enhanced by the introduction of the SiO2 coating, are associated with the occurrence of surface spin disordered effects. The induction heating effects (magnetic hyperthermia) are analyzed under the application of an AC magnetic field. Maximum specific absorption rate (SAR) values around 1.5 W g(-1) were achieved for the Fe3O4 nanoparticles. A significant decrease (around 26%) is found in the SAR values of the SiO2 coated nanocomposite. The different heating response is analyzed in terms of the decrease of the effective nanoparticle magnetization in the Fe3O4@SiO2 core-shell composites at room temperature.

Entropy change linked to the magnetic field induced martensitic transformation in a Ni–Mn–In–Co shape memory alloy

Experimental results on the temperature dependence of the entropy change induced by magnetic field in a Ni–Mn–In–Co ferromagnetic shape memory alloy have been analyzed. Different behaviors of the entropy change ΔS versus temperature have been observed, depending on the value of polarizing magnetic field. In addition, the magnetocaloric effect shows over certain temperature range, a limit value corresponding to the transformation entropy ΔStr. To explain the experimental results, a model, which takes into account the value of the martensitic transformation temperature shift and the transformation temperature range, has been proposed. The model allows to estimate the entropy change as a function of temperature and applied magnetic field from a few experimental data and therefore a first estimation of the refrigerant capacity of the system can be done.

Magnetocaloric effect linked to the martensitic transformation in sputter-deposited Ni–Mn–Ga thin films

In this letter, the analysis of the magnetocaloric effect (MCE) in a thin film of Ni–Mn–Ga alloy sputter deposited on alumina substrate is reported. This film has 0.4 μm thickness and exhibits merged martensitic and ferromagnetic transitions. The temperature dependence of the dc magnetization under different constant applied magnetic fields has been measured. The calculated MCE effect under applied magnetic fields up to 60 kOe shows the feasibility of these materials to be implemented in refrigeration system for functional microsystems. In addition, the shift of the martensitic transformation temperature, as a function of the applied magnetic field, has been determined.

Effect of atomic order on the martensitic and magnetic transformations in Ni-Mn-Ga ferromagnetic shape memory alloys

The influence of long-range L2(1) atomic order on the martensitic and magnetic transformations of Ni-Mn-Ga shape memory alloys has been investigated. In order to correlate the structural and magnetic transformation temperatures with the atomic order, calorimetric, magnetic and neutron diffraction measurements have been performed on polycrystalline and single-crystalline alloys subjected to different thermal treatments. It is found that both transformation temperatures increase with increasing atomic order, showing exactly the same linear dependence on the degree of L2(1) atomic order. A quantitative correlation between atomic order and transformation temperatures has been established, from which the effect of atomic order on the relative stability between the structural phases has been quantified. On the other hand, the kinetics of the post-quench ordering process taking place in these alloys has been studied. It is shown that the activation energy of the ordering process agrees quite well with the activation energy of the Mn self-diffusion process.

Magnetocaloric effect in Ni–Fe–Ga shape memory alloys

The magnetic entropy change in three different polycrystalline Ni53+xFe20−xGa27 (x=0.5,1,2) alloys was analyzed as a function of temperature under different applied magnetic fields. The temperature dependence of the ac magnetic susceptibility (χ) and the magnetization of the alloys have been used to characterize the different structural and magnetic transformations. In spite of the different magnetic states, the alloys show comparable magnetic entropy values. For x⩽1 the martensitic transformation takes place in the ferromagnetic state for measuring temperatures below room temperature, whereas the alloy with x=2 displays the martensitic transformation above room temperature between two paramagnetic phases. Maximum values of the magnetic entropy change are correlated with the martensitic transformation, irrespective of the particular magnetic state (ferromagnetic or paramagnetic) during the transformation.

Magnetotunable left-handed FeSiB ferromagnetic microwires.

The magnetotunable left-handed characteristics of Fe(77.5)Si(12.5)B(10) glass-coated ferromagnetic microwires are analyzed in array and single microwire configuration, employing a rectangular waveguide working in X band. While the negative permeability is ascribed to the natural ferromagnetic resonance (NFMR) of the highly and positive magnetostrictive microwire, the negative permittivity features of the medium are attributed to the interaction of the microwires with the metallic rectangular waveguide. The dependence of the NFMR frequency on the applied external magnetic field enables the design of magnetotunable left-handed systems with wide-frequency band.

Sol-gel NiFe2O4 nanoparticles: Effect of the silica coating

NiFe2O4 and NiFe2O4-SiO2 nanoparticles were synthesized by a sol-gel method using citric acid as fuel, giving rise its combustion to the crystallization of the spinel phase. Different synthesis conditions were analyzed with the aim of obtaining stoichiometric NiFe2O4 nanoparticles. The spinel structure in the calcined nanoparticles (400 °C, 2 h) was evaluated by x-ray diffraction. Their nanometer size (mean diameters around 10–15 nm) was confirmed through electron microscopy (field emission scanning electron microscopy and transmission electron microscopy). Rietveld refinement indicates the existence of a small percentage of NiO and Fe3O4 phases and a certain degree of structural disorder. The main effect of the silica coating is to enhance the disorder effects and prevent the crystalline growth after post-annealing treatments. Due to the small particle size, the nanoparticles display characteristic superparamagnetic behaviour and surface effects associated to a spin-glass like state: i.e., reduction in t...

Magnetic field induced martensitic transformation linked to the arrested austenite in a Ni-Mn-In-Co shape memory alloy

The so-called metamagnetic shape memory alloys transform from a ferromagnetic austenite into a weak magnetic martensitic phase, thus the application of a magnetic field, stabilizing the high magnetization phase, can induce the reverse martensitic transformation. Moreover, the martensitic transformation itself becomes arrested as its temperature range is lowered by the application of high enough magnetic fields. In this work the effect of the magnetic field on a Ni-Mn-In-Co metamagnetic shape memory has been studied by SQUID magnetometry. The arrest of the transformation produced by the field results in metastable states, whose evolution when the field is removed or reduced, follows logarithmic time dependence. The observed behavior is interpreted in terms of the magnetic contribution to the total entropy change associated with the magnetostructural transformation.

Theoretical Modeling and Experimental Verification of the Scattering From a Ferromagnetic Microwire

This paper presents a theoretical modeling of the scattering of ferromagnetic microwires in free space and inside a rectangular waveguide, providing both an analytical solution and a physical interpretation of the problem. Special attention is devoted to the impact of the microwire radius and its magnetic properties. Theoretical results have been experimentally verified measuring the reflection, absorption, and transmission coefficients of a ferromagnetic microwire inside a rectangular waveguide.

Structural and magnetic properties of Cr-doped Ni?Mn?In metamagnetic shape memory alloys

The effect of the partial substitution of Mn by Cr on the structural and magnetic properties of Ni–Mn–In metamagnetic shape memory alloys is investigated. It is found that a Cr-rich second phase appears for quite low Cr concentrations, pointing out a very low solubility of Cr in Ni–Mn–In. Nevertheless, the martensitic transformation (MT) temperature of the doped alloys can be related to the variation in the electron concentration in the matrix phase, just as it occurs in the ternary Ni–Mn–In system. The effect of magnetic field on the structural transformation has been evaluated on both a ternary and a quaternary alloy. It is shown that the presence of the second phase reduces the magnetically induced shift of the MT and the associated magnetocaloric effect, thus limiting the potential applicability of Ni–Mn–In alloys. The obtained results prevent the addition of high amounts of Cr to Ni–Mn–In.