Papers by Author: José M. Barandiarán

Abstract: The characteristics of neutron diffraction applied to the study of Ferromagnetic Shape Memory Alloys are revised. Main studies refer to crystal structure, preferential site occupancy, variant reorientation and magnetic moment distribution in the alloys

Abstract: Structure and magnetic properties of nanoscale [Gd/Ti]n multilayers prepared by rf-sputtering are studied. It is found that the decrease in the Gd layer thickness LGd leads to beginning of the structure transformation in Gd layers from the fine-crystalline to amorphous state when LGd becomes less than 2 nm. The Curie temperature TC decreases as a function of the Gd layer thickness in the same way as in early studied epitaxially grown Gd films, i.e. in case for which the finite-size effect plays most important role. A deviation of the TC(LGd) behaviour at very low LGd from the fit according to the finite-size law is probably caused by the island-like structure of the Gd layers.

Abstract: FeNi films were deposited by DC magnetron sputtering at different Ar pressures. The structure and magnetic properties of the FeNi films are affected by the Ar pressure. Ferromagnetic resonance (FMR) measurements were done at a frequency of about 8.85 GHz. Both the value of resonance field and resonance line width show strong dependence on the Ar pressure: the lowest value of the resonance field and the narrowest resonance width correspond to the smallest argon pressure. Increase of the Ar pressure causes the films to have a significant perpendicular anisotropy with the easy axis pointing out of the plane. The magnetic properties and FMR were also studied for the [FeNi(170 nm)/Ti]n/FeNi(170 nm) (n = 1, 2, 5) structures prepared at the smallest Ar pressure. The FMR studies showed that the obtained multilayers are very robust: the value of the resonance field and resonance line width of the [FeNi/Ti]n/FeNi multilayers are very close to the corresponding values for the FeNi films.

Abstract: Ferromagnetic shape memory alloy-based actuators offer distintive features that make them advantageous competitors to traditional electromechanical devices. The production of force and motion without contact is one of the most important features. However, the largely non-linear and hysteretic nature of the response of such materials makes them of little use apart from on-off or continuous actuation. In this work we present the results obtained in a laboratory prototype of linear position FSMA actuator, where the active element is a 12 mm long Ni-Mn-Ga single crystal. The crystal expands a maximum of 12 micrometers and in control experiments, is commanded to expand and contract alternatively to reach positions at 5 µm and 8 µm. It shows that the commanded position could be controlled within 20 nm.

Abstract: A Mn-enriched Ni46.4Mn32.8Sn20.8 Heusler alloy has been prepared by the induction melting and casting method. The samples used were annealed at 900°C during 72 h with a subsequent water quench. The magnetostructural transformation (MST) in the vicinity of Tm=230K and Ta=250K has been found, and characterized by DSC, magnetization and resistivity measurements. Changes in the crystal structure due to the MST were observed using neutron diffraction at different temperatures. The crystal structure of martensitic phase shows a good fits to the 4-layered orthorhombic martensite with a Pmma space group. The linear shift of transformation temperatures was found in the entire high magnetic field range up 14T to be dTm/dB0 = -1.50 K/T and dTa/dB0 = -1.26 K/T, whereas in the low field range we found an increase of both characteristic temperatures. The entropy changes at the MST were evaluated using the Clausius-Clapeyron relationship. The transformation-induced magnetoresistance has been measured.

Abstract: Magnetic, magnetoresistive and structural properties of Tb/Ti and Tb/Si nanoscale multilayers prepared by alternative deposition of Tb layers and Ti or Si spacers are comparatively studied. It was concluded that spin disorder scattering is responsible for the negative longitudinal magnetoresistance observed in multilayers of both types.

Abstract: A series of Ni51.4Mn28.3Ga20.3 films sputter-deposited on Si(100) wafer (with 500 nm thick buffer layer of SiNx) and annealed at 800 oC for 1h. are investigated with respect to their transformation behavior and magnetic properties. The film thickness, d, varies from 0.1 to 5.0 μm. Resistivity measurements reveal martensitic transformation above room temperature for all the films except for 0.1μm-thick film which is transforming at much lower temperature. The magnetic characteristics of martensitic films such as susceptibility and anisotropy field extracted from the inplane and out-of-plane magnetization curves show film thickness dependence likewise Curie temperature obtained from the resistivity curves. The surface topography and micromagnetic structure are studied by scanning probe microscopy. A stripe magnetic domain pattern featuring a large out-of-plane magnetization component is found in the films. The domain width, δ, depends on the film thickness, d, as δ ~ d .

Abstract: Composites of Ni–Mn–Ga particles in a polyurethane matrix can be made by mixing the particles with the polymer, and allowing them to cure under a magnetic field to texture the composites. These composites show large hysteresis and mechanical losses, when subjected to a cyclic stress, that were far larger than the matrix polymer ones. The additional losses are attributed to the motion of twin boundaries in the filler particles and provide a way for obtaining mechanical energy absorption in a wide frequency range. By means of X-ray and neutron diffraction we present evidence that confirms that twins are present in the particles and that they do move when mechanically loading the composite