Tailoring of perpendicular magnetic anisotropy in Dy13Fe87 thin films with hexagonal antidot lattice nanostructure

In this article, the magnetic properties of hexagonally ordered antidot arrays made of Dy13Fe87 alloy are studied and compared with corresponding ones of continuous thin films with the same compositions and thicknesses, varying between 20 nm and 50 nm. Both samples, the continuous thin films and antidot arrays, were prepared by high vacuum e-beam evaporation of the alloy on the top-surface of glass and hexagonally self-ordered nanoporous alumina templates, which serve as substrates, respectively. By using a highly sensitive magneto-optical Kerr effect (MOKE) and vibrating sample magnetometer (VSM) measurements an interesting phenomenon has been observed, consisting in the easy magnetization axis transfer from a purely in-plane (INP) magnetic anisotropy to out-of-plane (OOP) magnetization. For the 30 nm film thickness we have measured the volume hysteresis loops by VSM with the easy magnetization axis lying along the OOP direction. Using magnetic force microscopy measurements (MFM), there is strong evidence to suggest that the formation of magnetic domains with OOP magnetization occurs in this sample. This phenomenon can be of high interest for the development of novel magnetic and magneto-optic perpendicular recording patterned media based on template-assisted deposition techniques

Influence of TiO2-Coating Layer on Nanoporous Alumina Membranes by ALD Technique

Geometrical, chemical, optical and ionic transport changes associated with ALD of TiO2-coating on the porous structure of two nanoporous alumina membranes (NPAMs), which were obtained by the two-step aluminum anodization method but with different pore size and porosity, are presented. Chemical and morphological changes were determined by analyzing XPS spectra and SEM images, showing practically total coverage of the NPAMs surface and leading to a reduction in the geometrical parameters of both samples, while SAED and high resolution TEM measurements allowed us to determine the crystalline structure and thickness of the TiO2-coating, with the latter confirmed by depth-profile XPS analysis. Spectroscopic ellipsometry measurements were also carried out in order to detect changes in characteristic optical parameters (refractive index, n, and extinction coefficient, k), due to the TiO2-coating of NPAMs. Considering the common application of NPAMs in solute/ion diffusion processes, the effect of the TiO2-coverage on electrochemical parameters was analyzed by measuring the concentration potential with a typical model electrolyte (KCl solutions), leading to an increase of the electropositive character for both kinds of samples

Tailoring of magnetocaloric effect in Ni45.5Mn43.0In11.5 metamagnetic shape memory alloy

We investigate the direct and inverse magnetocaloric effect in Ni45.5Mn43.0In11.5 Heusler alloy ribbons comparing the results obtained for the as-quenched sample with the ones after different annealing procedures. An enhancement and shift of the entropy maximum to near room temperature is observed in all annealed samples. A remarkable magnetocaloric effect is observed in samples with short-time treatment (10 minutes) and at the lowest annealing temperature. We show that the suppressing of uncompensated martensitic transition and thermal hysteresis are both influenced by the heat treatment. Also, an improvement on Curie’s temperature is observed and, at low magnetic field, it has been risen up to 310 K. Our results demonstrate that the martensitic transformation is highly sensitive to the applied magnetic field and also to the annealing treatment, which means that the magnetocaloric effect can be tuned showing different behaviors for each sample.Authors are thankful to Spanish MICINN for financial support: MAT2009-13108-C02-01-02 and MAT2010-20798-C05-04. L. Gonzalez also thanks MICINN for a FPI Grant and J. García FICYT for a “Severo Ochoa” Grant

Magnetoimpedancia en aleaciones amorfas de cofesib de baja magnetostricción

En esta tesis doctoral se realiza un estudio del fenómeno de magnetoimpedancia MI en algunas aleaciones con geometría en forma de cinta, ricas en CO de composición CoFeSi(Mo)B, y nanocristalinas ricas en Fe de composición FeCuNbSiB cuya característica común es que poseen una constante de magnetostricción muy pequeña, así como otras óptimas propiedades magnéticamente blandas para observar el efecto de MI. El efecto de mgnetoimpedancia gigante, es un fenómeno de origen electromagnético clásico relacionado con la vriación de impedancia que experimenta un conductor ferromagnético al aplicarle un campo magnético contínuo. Ello se debe a los cambios en la profundidad de penetración, de la corriente de exitación a alta frecuencia, al modificarse la permeabilidad magnética transversal del conductor ferromagnético, con la frecuencia de la corriente alterna y con el campo contínuo externo. Se estudia la influencia de la anisotropía inducida enlas aleacciones ricas en Co, mediante tratmientos térmicos adeduados y la influencia de la magnetostricción y tensiones mecáncias de traccións obre el efecto de MI, con el objetivo del posible desarrollo de sensores magnéticos de tensiones mecánicas. También se estudia la influencia de los procesos de nanocristalización de las aleaciones ricas en fe sobre el efecto de mi. Se ha desarrollado y puesto a punto un dispositivo experimental para la detección y medida del módulo de impedancia, así como de sus componentes real e imaginaria, que permite aplicar simultáneamente tensiones mecánicas de tracciónn e incluso torsiones mecáncias a las muestras, durante el proceso de medida. Los resultados experimentales obtenidos,han sido explicados de forma cualitativa mediante un sencillo modelo fenomenológico basado en la dinámica del paredes de dominios, que en el rango de bajas frecuencias (f-MHz), tiene en cuenta la inclusión de parámetros del material [...