Shell-driven magnetic stability in core-shell nanoparticles

The magnetic properties of ferromagnetic-antiferromagnetic Co-CoO core-shell nanoparticles are investigated as a function of the in-plane coverage density from 3.5% to 15%. The superparamagnetic blocking temperature, the coercivity, and the bias field radically increase with increasing coverage. This behavior cannot be attributed to the overall interactions between cores. Rather, it can be semiquantitatively understood by assuming that the shells of isolated core-shell nanoparticles have strongly degraded magnetic properties, which are rapidly recovered as nanoparticles come into contact

Metastable Random Field Ising model with exchange enhancement: a simple model for Exchange Bias

We present a simple model that allows hysteresis loops with exchange bias to be reproduced. The model is a modification of the T=0 random field Ising model driven by an external field and with synchronous local relaxation dynamics. The main novelty of the model is that a certain fraction f of the exchange constants between neighbouring spins is enhanced to a very large value J_E. The model allows the dependence of the exchange bias and other properties of the hysteresis loops to be analyzed as a function of the parameters of the model: the fraction f of enhanced bonds, the amount of the enhancement J_E and the amount of disorder which is controlled by the width sigma of the Gaussian distribution of the random fields.Comment: 8 pages, 11 figure

Magnetization of nanoparticle systems in a rotating magnetic field

The investigation of a sizable thermal enhancement of magnetization is put forward for uniaxial ferromagnetic nanoparticles that are placed in a rotating magnetic field. We elucidate the nature of this phenomenon and evaluate the resonant frequency dependence of the induced magnetization. Moreover, we reveal the role of magnetic dipolar interactions, point out potential applications and reason the feasibility of an experimental observation of this effect.Comment: 10 pages, 2 figure

Inhomogeneous ferrimagnetic-like behavior in Gd2/3Ca1/3MnO3 single crystals

We present a study of the magnetic properties of Gd2/3Ca1/3MnO3 single crystals at low temperatures. We show that this material behave as an inhomogeneous ferrimagnet. In addition to small saturation magnetization at 5 K, we have found history dependent effects in the magnetization and the presence of exchange bias. These features are compatible with microscopic phase separation in the clean Gd2/3Ca1/3MnO3 system studied.Comment: 7 pages, 6 figures, submitted Journal of Magnetism and Magnetic Material

Limited carbon inputs from plants into soils in arid ecosystems: a study of changes in the d13C in the soil-root interface

Background and aims. The tracing of C assimilation and the subsequent partitioning among plant organs has been a central focus of studies utilising Free Air CO2 Enrichment (FACE) facilities. The approach makes use of the fossil origin of this carbon, which is depleted in 13C. However, there is little data for desert environments. The Nevada Desert FACE Facility (NDFF), located in the Mojave Desert, has been one of the main facilities for the study of C dynamics in arid ecosystems and how they respond to rising atmospheric CO2 concentrations. In this experiment, we studied the incorporation of fixed CO2 during the previous two years (detectable by its lower 13C) in the soil fraction surrounding roots. Methods. The soil was collected monthly in direct vicinity to the roots during a complete growth season, at two depths (5 and 15 cm). Soil samples were dried and fractionated by size (> 50 μm and 50 μm), 13C values ranged between -1 and -2¿ for carbonates and between -23 and -25¿ for soil organic matter. These values did not significantly change throughout the experiment and were not affected by depth (5 or 15 cm). In contrast, 13C values for both organic and inorganic carbon in the fine fraction ( 50 μm). The 13C values for organic C ranged mostly between -20¿ and -27¿, and were roughly maintained throughout the sampling period. For inorganic C, the 13C values were mostly between 0¿ and -15¿, and tended to become less negative during the course of the sampling period. Overall the effect of [CO2] on 13C values of either organic or inorganic carbon was not significant for any experimental condition (plant species, depth, fraction). Conclusion. Little or no signs of recently fixed CO2 (13C-depleted) were detected in the soils close to the roots, in the coarse fraction (> 50 μm), the fine fraction (< 50 μm), the organic matter, or in carbonates. This indicates a slow C turnover 45 in the studied soils, which can result from a highly conservative use of photoassimilates by plants, including a very low release of organic matter into the soil in the form of dead roots or root exudates, and from a conservative use of available C reserves

Imprinting vortices into antiferromagnets

Premi a l'excel·lència investigadora. Àmbit de les Ciències Tecnològiques. 2008The effect of imprinting symmetric and displaced vortex structures into an antiferromagnetic material is investigated in micron-sized disks consisting of exchange coupled ferromagnetic-antiferromagnetic bilayers. The imprint of displaced vortices manifests itself by the occurrence of a new type of asymmetric hysteresis loops characterized by curved, reversible, central sections with nonzero remanent magnetization. Such an imprint is achieved by cooling the disks through the blocking temperature of the system in small fields. Micromagnetic simulations reveal that asymmetric vortexlike loops naturally result from the competition between the different energies involved in the system

Thickness dependence of the exchange bias in epitaxial manganite bilayers

Exchange bias has been studied in a series of La2/3Ca1/3MnO3 / La1/3Ca2/3MnO3 bilayers grown on (001) SrTiO3 substrates by ozone-assisted molecular beam epitaxy. The high crystalline quality of the samples and interfaces has been verified using high-resolution X-ray diffractometry and Z-contrast scanning transmission electron microscopy with electron energy loss spectroscopy. The dependence of exchange bias on the thickness of the antiferromagnetic layer has been investigated. A critical value for the onset of the hysteresis loop shift has been determined. An antiferromagnetic anisotropy constant has been obtained by fitting the results to the generalized Meiklejohn-Bean model.Comment: 7 pages. 8 figure

Enhanced exchange bias effects in a nanopatterned system consisting of two perpendicularly coupled ferromagnets

This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics.The effect of patterning on the shift of the hysteresis loop HE and coercivityHC in a system composed of two perpendicularly exchange-coupled ferromagnets (NiFe sputtered onto a [Pt/Co] multilayer) is investigated in long stripes and square dots. Setting the exchange bias coupling along the stripes results in a threefold increase of HE compared to the continuous films. HC increases dramatically when the coupling is set perpendicular to the stripes and also in the dots. Magnetic force microscopy studies and micromagnetic simulations suggest that differences in the number and orientation of the magnetic domains can account for the observed effects