Albert Fert i Peter Grünberg: Premis Nobel de Física 2007

El descobriment per part d'Albert Fert i Peter Grünberg (Premis Nobel Física 2007) de la magnetoresistència gegant ha obert les portes a l'espintrònica. En aquest article fem una breu revisió de les diferents contribucions experimentals i conceptuals que van fer possible aquell descobriment i n'expliquem els fonaments

Albert Fert

Albert Fert va néixer a Carcassona l'any 1938. Membre de l'Acadèmia Francesa de les Ciències, és professor de Física a la Universitat de París Sud des de l'any 1975. Entre els anys 1970 i 1975 va ser director de recerca al Laboratoire de Physique des Solides (Université Paris-Surd, Orsay) i, des del 1995 és director científic de la Unité Mixte de Physique CNRS/Thales (Palaiseau). L'activitat científica d'Albert Fert s'ha centrat en treballs experimentals i teòrics en el camp de la Física de la Matèria Condensada (metalls i aliatges, magnetisme, vidres d'espí, transport electrònic, nanostructures magnètiques i electrònica d'espí). Ha fet contribucions pioneres, fonamentalment relacionades amb el transport de càrrega depenent de l'espí que, finalment, el van portar a descobrir la GMR i a fer de l'espintrònica un nou camp de la ciència que pot rellevar l'electrònica actual. Fert va descobrir la GMR al mateix temps, encara que de manera independent, que el científic Peter Grunberg. Tots dos van rebre el Premi Nobel de Física 2007 per aquesta troballa, que es pot considerar la primera aplicació a gran escala de les noves nanotecnologies. Aquest efecte és la base de l'espintrònica, que explota la càrrega de l'electró i l'orientació de l'espí, o imant, per obtenir dispositius més petits i eficaços, i permet que puguin tenir noves funcionalitats. L'investigador francès ha publicat prop de 300 articles i ha col·laborat en 7 llibres. S'han fet més d'11.000 citacions dels seus articles. A més del Premi Nobel, ha rebut diversos guardons nacionals i internacionals, entre les quals cal remarcar el premi internacional James C. McGroddy Prize for New Materials de la Societat Americana de Física i el premi Europhysics de la Societat Europea de Física. Albert Fert col·labora estretament amb diferents centres de recerca i laboratoris del campus de la UAB, especialment amb l'ICMAB, que forma part del Parc de Recerca UAB. Ha estat vinculat també amb altres institucions científiques i universitàries catalanes, destacant la seva col·laboració activa en la creació de l'Institut Català de Nanotecnologia, centre del Parc de Recerca UAB, pel que fa a les línies de recerca que s'hi desenvolupenNomenament 21/05/2009. A proposta de Facultat de Ciències. Investidura 21/05/2009. Padrí: Josep Fontcubert

Growth and magnetic properties of multiferroic LaxBi1-xMnO3 thin films

A comparative study of LaxBi1-xMnO3 thin films grown on SrTiO3 substrates is reported. It is shown that these films grow epitaxially in a narrow pressure-temperature range. A detailed structural and compositional characterization of the films is performed within the growth window. The structure and the magnetization of this system are investigated. We find a clear correlation between the magnetization and the unit-cell volume that we ascribe to Bi deficiency and the resultant introduction of a mixed valence on the Mn ions. On these grounds, we show that the reduced magnetization of LaxBi1-xMnO3 thin films compared to the bulk can be explained quantitatively by a simple model, taking into account the deviation from nominal composition and the Goodenough-Kanamori-Anderson rules of magnetic interactions

Distinct magnetism in ultrathin epitaxial NiFe2O4 films on MgAl2O4 and SrTiO3single crystalline substrates

Spinel ferrites are being considered for advanced spintronic applications. Here, we report on the magnetic properties of ultrathin (3-37 nm) epitaxial films of NiFe2O4 (NFO) on MgAl2O4 (MAO) and SrTiO3 (STO) single crystalline substrates. It is found that NFO films on STO display superparamagnetic response down to 50 K, whereas films grown on MAO display ferrimagnetic response up to room temperature. Microstructural information indicates that this distinct response can be attributed to the different growth mechanisms of the spinel ferrite on the isostructural MAO substrate (two-dimensional growth) and the perovskite STO (Volmer-Weber three-dimensional growth). We discuss the reasons for this distinct behavior and its relevance for the integration of ferrites in epitaxial heterostructures for tunnel devices

Structural and functional characterization of (110)-oriented epitaxial La2/3Ca1/3MnO3 electrodes and SrTiO3 tunnel barriers

La2/3Ca1/3MnO3 (LCMO) films have been deposited on (110)-oriented SrTiO3 (STO) substrates. X-ray diffraction and high-resolution electron microscopy reveal that the (110) LCMO films are epitaxial and anisotropically in-plane strained, with higher relaxation along the [1¿10] direction than along the [001] direction; x-ray absorption spectroscopy data signaled the existence of a single intermediate Mn3+/4+ 3d-state at the film surface. Their magnetic properties are compared to those of (001) LCMO films grown simultaneously on (001) STO substrates It is found that (110) LCMO films present a higher Curie temperature (TC) and a weaker decay of magnetization when approaching TC than their (001) LCMO counterparts. These improved films have been subsequently covered by nanometric STO layers. Conducting atomic-force experiments have shown that STO layers, as thin as 0.8 nm, grown on top of the (110) LCMO electrode, display good insulating properties. We will show that the electric conductance across (110) STO layers, exponentially depending on the barrier thickness, is tunnel-like. The barrier height in STO (110) is found to be similar to that of STO (001). These results show that the (110) LCMO electrodes can be better electrodes than (001) LCMO for magnetic tunnel junctions, and that (110) STO are suitable insulating barriers

Kerr measurements on single-domain SrRuO3 thin films

We report on the magneto-optical measurements of an epitaxial SrRuO3 film grown on SrTiO3 (0 0 1), which previously was determined to be single domain orientated by x-ray diffraction and Raman spectroscopy techniques. Our experiments reveal a large Kerr rotation, which reaches a maximum value of about 0.5° at low temperature. By measuring magnetic hysteresis loops at different temperatures, we determined the temperature dependence of the Kerr rotation in the polar configuration. Values of the anisotropic magnetoresistance ~ 20% have been measured. These values are remarkably higher than those of other metallic oxides such as manganites. This striking difference can be attributed to the strong spin-orbit interaction of the Ru 4d ion in the SrRuO3 compound

Epitaxial growth of biferroic YMnO3(0001) on platinum electrodes

Epitaxial films of the biferroic YMnO3 (YMO) oxide have been grown on platinum-coated SrTiO3(1 1 1) and Al2O3(0 0 0 1) substrates. The platinum electrodes, (1 1 1) oriented, are templates for the epitaxy of the hexagonal phase of YMO with a (0 0 0 1) out-of-plane orientation, which is of interest as this is the polarization direction of YMO. X-ray diffractometry indicates the presence of two crystal domains, 60° rotated in-plane, in the Pt(1 1 1) layers which subsequently are transferred on the upperlaying YMO. Cross-section analysis by high-resolution transmission electron microscopy (HRTEM) of YMnO3/Pt/SrTiO3(1 1 1) shows high-quality epitaxy and sharp interfaces across the structure in the observed region. We present a detailed study of the epitaxial growth of the hexagonal YMO on the electrodes

El Nobel de física i el teu portàtil

El darrer premi Nobel de física s'ha concedit per treballs que han posat les bases per a la fabricació de discs durs cada vegada més petits. Quins són els avenços que ho han fet possible