Simple mechanism for a positive exchange bias

We argue that the interface coupling, responsible for the positive exchange bias (HE) observed in ferromagnetic/compensated antiferromagnetic (FM/AF) bilayers, favors an antiferromagnetic alignment. At low cooling field this coupling polarizes the AF spins close to the interface, which spin configuration persists after the sample is cooled below the Neel temperature. This pins the FM spins as in Bean's model and gives rise to a negative HE. When the cooling field increases, it eventually dominates and polarizes the AF spins in an opposite direction to the low field one. This results in a positive HE. The size of HE and the crossover cooling field are estimated. We explain why HE is mostly positive for an AF single crystal, and discuss the role of interface roughness on the magnitude of HE, and the quantum aspect of the interface coupling.Comment: 10 pages, 2 figures, to be published on May 1 issue of PR

Comportamiento dieléctrico de materiales compuestos reforzados con Ground Tire Rubber (GTR)

La masiva fabricación de neumáticos y la dificultad para su eliminación o almacenamiento constituye un grave problema medioambiental. En la actualidad, se utilizan diversos métodos para el reciclaje de neumáticos, como la trituración mecánica, en la que se separa el caucho vulcanizado del acero y las fibras, utilizándose este caucho en numerosas aplicaciones como pavimentos, aislantes, calzados, etc.Con el presente estudio se pretende dar una nueva salida a los neumáticos fuera de uso (GTR), demostrándose su utilidad como dieléctricos. Para ello, se ha combinado GTR con policloruro de vinilo (PVC), obteniendo un material compuesto de matriz polimérica reforzado con GTR. Se ha caracterizado el comportamiento dieléctrico paralas diversas mezclas del PVC con GTR (desde el 0% al 70% de GTR), en un rango de temperaturas que varía desde los 30ºC hasta los 130ºC, y con unas frecuencias desde 1·10-2 Hz, hasta 1·107 Hz. Los ensayos eléctricos han permitido el análisis de la permitividad, la resistividad, el factor de pérdidas dieléctricas y los módulos de relajación (Arrhenius). Se ha comprobado como todas estas propiedades aumentan a bajas temperaturas (30ºC). Asimismo, también se observa que la evolución de estos parámetros varía significativamente en función de la frecuencia y la temperatura

Time-Resolved Spin Torque Switching and Enhanced Damping in Py/Cu/Py Spin-Valve Nanopillars

We report time-resolved measurements of current-induced reversal of a free magnetic layer in Py/Cu/Py elliptical nanopillars at temperatures T = 4.2 K to 160 K. Comparison of the data to Landau-Lifshitz-Gilbert macrospin simulations of the free layer switching yields numerical values for the spin torque and the Gilbert damping parameters as functions of T. The damping is strongly T-dependent, which we attribute to the antiferromagnetic pinning behavior of a thin permalloy oxide layer around the perimeter of the free layer. This adventitious antiferromagnetic pinning layer can have a major impact on spin torque phenomena.Comment: 5 pages, 4 figure

Global late Quaternary megafauna extinctions linked to humans, not climate change

The late Quaternary megafauna extinction was a severe global-scale event. Two factors, climate change and modern humans, have received broad support as the primary drivers, but their absolute and relative importance remains controversial. To date, focus has been on the extinction chronology of individual or small groups of species, specific geographical regions or macroscale studies at very coarse geographical and taxonomic resolution, limiting the possibility of adequately testing the proposed hypotheses. We present, to our knowledge, the first global analysis of this extinction based on comprehensive country-level data on the geographical distribution of all large mammal species (more than or equal to 10 kg) that have gone globally or continentally extinct between the beginning of the Last Interglacial at 132 000 years BP and the late Holocene 1000 years BP, testing the relative roles played by glacial–interglacial climate change and humans. We show that the severity of extinction is strongly tied to hominin palaeobiogeography, with at most a weak, Eurasia-specific link to climate change. This first species-level macroscale analysis at relatively high geographical resolution provides strong support for modern humans as the primary driver of the worldwide megafauna losses during the late Quaternary

Pseudo Spin Valves Using a (112)-textured DO_22 MnGa Fixed Layer

We demonstrate pseudo spin valves with a (112)-textured DO_22 MnGa (MnGa) tilted magnetization fixed layer and an in-plane CoFe free layer. Single D0_22 MnGa films exhibit a small magnetoresistance (MR) typically observed in metals. In MnGa/Cu/ CoFe spin valves a transition from a negative (-0.08%) to positive (3.88%) MR is realized by introducing a thin spin polarizing CoFe insertion layer at the MnGa/Cu interface and tailoring the MnGa thickness. Finally, the exchange coupling between the MnGa and CoFe insertion layer is studied using a first-order reversal curve (FORC) technique.Comment: 5 pages, 4 figures, submitted to IEEE Magnetics Letter

Size effects in the magnetic behaviour of TbAl_2 milled alloys

The study of the magnetic properties depending upon mechanical milling of the ferromagnetic polycrystalline TbAl_2 material is reported. The Rietveld analysis of the X-ray diffraction data reveals a decrease of the grain size down to 14 nm and -0.15 % of variation of the lattice parameter, after 300 hours of milling time. Irreversibility in the zero field cooled - field cooled (ZFC-FC) DC-susceptibility and clear peaks in the AC susceptibility between 5 and 300 K show that the long-range ferromagnetic structure is inhibited in favour of a disordered spin arrangement below 45 K. This glassy behaviour is also deduced from the variation of the irreversibility transition with the field (H^{2/3}) and frequency. The magnetization process of the bulk TbAl_2 is governed by domain wall thermal activation processes. By contrast, in the milled samples, cluster-glass properties arise as a result of cooperative interactions due to the substitutional disorder. The interactions are also influenced by the nanograin structure of the milled alloys, showing a variation of coercivity with the grain size, below the crossover between the multi- and single-domain behaviours.Comment: 23 pages, 11 figures, to appear in J. Phys.: Condens. Ma

Asymmetric magnetization reversal in exchange-biased hysteresis loops

This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics.Polarized neutron reflectometry is used to probe the in-plane projection of the net-magnetization vector M of polycrystalline Fe films exchange coupled to twinned (110) MnF2 or FeF2 antiferromagnetic (AF) layers. The magnetization reversal mechanism depends upon the orientation of the cooling field with respect to the twinned microstructure of the AF, and whether the applied field is increased to (or decreased from) a positive saturating field; i.e., the magnetization reversal is asymmetric. The reversal of the sample magnetization from one saturated state to the other occurs via either domain wall motion or magnetization rotation on opposite sides of the same hysteresis loop

Two-stage magnetization reversal in exchange biased bilayers

MnF2/Fe bilayers exhibit asymmetric magnetization reversal that occurs by coherent rotation on one side of the loop and by nucleation and propagation of domain walls on the other side of the loop. Here, we show by polarized neutron reflectometry, magnetization, and magnetotransport measurements that for samples with good crystalline "quality" the rotation is a two-stage process, due to coherent rotation to a stable state perpendicular to the cooling field direction. The result is remarkably asymmetrically shaped hysteresis loops

Coercivity enhancement in exchange biased systems driven by interfacial magnetic frustration

We report the temperature and cooling field dependence of the coercivity of exchange biased MnF2/Fe bilayers. When the antiferromagnetic surface is in a state of maximum magnetic frustration and the net exchange bias is zero, we observe a strong enhancement of the coercivity, which is proportional to the exchange coupling between the layers. Hence, the coercivity can be tuned in a reproducible and repeatable fashion in the same sample. We propose that a frustrated interface provides local energy minima which effectively pin the propagating domain walls in the ferromagnet, leading to an enhanced coercivity

Lipidomics to analyze the influence of diets with different EPA:DHA ratios in the progression of Metabolic Syndrome using SHROB rats as a model

8 páginas, 4 tablasThe role of specific proportions of ω-3 EPA and DHA, in the modulation of inflammation and oxidative stress markers associated to the progression of Metabolic Syndrome was investigated. Potential inflammatory eicosanoids and docosanoids were discussed together to biomarkers of CVD, obesity, inflammation and oxidative stress in an animal model of metabolic disorders. Results evidenced a noteworthy health effect of 1:1 and 2:1 EPA:DHA proportions over 1:2 EPA:DHA based diets through a down-regulation in the production of strong pro-inflammatory ω-6 eicosanoids, a decrement of biomarkers of oxidative stress, and a modulation of fatty acid desaturase activities and plasma and membrane PUFAs towards greater anti-inflammatory profiles. Outcomes contribute to the general knowledge on the health benefits of marine lipids and their role on the progress of MetS, inflammation and oxidative stress. Results shed light on controversial protective mechanisms of EPA and DHA to better design dietary interventions aimed at reducing MetSThis work was supported by the Spanish Ministerio de Economía y Competitividad (AGL2009-12374-C3-1, -2, and -3, and AGL2013-49079-C2-1,2-R). The Consejo Superior de Investigaciones Científicas (CSIC) and the University of Santiago de Compostela (USC) are gratefully acknowledged for the doctoral fellowship to Gabriel Dasilva. Xunta de Galicia and European Social Fund are also thankfully recognized for the financial support of the postdoctoral contracts to M. P and E.G.-E., and ISCIII for the postdoctoral contract “Sara Borrell” to J.P.-J. (CD09/00068)Peer reviewe