Direct observation of enhanced magnetism in individual size- and shape-selected 3d transition metal nanoparticles

Magnetic nanoparticles are critical building blocks for future technologies ranging from nanomedicine to spintronics. Many related applications require nanoparticles with tailored magnetic properties. However, despite significant efforts undertaken towards this goal, a broad and poorly understood dispersion of magnetic properties is reported, even within monodisperse samples of the canonical ferromagnetic 3d transition metals. We address this issue by investigating the magnetism of a large number of size- and shape-selected, individual nanoparticles of Fe, Co, and Ni using a unique set of complementary characterization techniques. At room temperature, only superparamagnetic behavior is observed in our experiments for all Ni nanoparticles within the investigated sizes, which range from 8 to 20 nm. However, Fe and Co nanoparticles can exist in two distinct magnetic states at any size in this range: (i) a superparamagnetic state, as expected from the bulk and surface anisotropies known for the respective materials and as observed for Ni, and (ii) a state with unexpected stable magnetization at room temperature. This striking state is assigned to significant modifications of the magnetic properties arising from metastable lattice defects in the core of the nanoparticles, as concluded by calculations and atomic structural characterization. Also related with the structural defects, we find that the magnetic state of Fe and Co nanoparticles can be tuned by thermal treatment enabling one to tailor their magnetic properties for applications. This paper demonstrates the importance of complementary single particle investigations for a better understanding of nanoparticle magnetism and for full exploration of their potential for applications

Interface and electronic characterization of thin epitaxial Co3O4 films

The interface and electronic structure of thin (~20-74 nm) Co3O4(110) epitaxial films grown by oxygen-assisted molecular beam epitaxy on MgAl2O4(110) single crystal substrates have been investigated by means of real and reciprocal space techniques. As-grown film surfaces are found to be relatively disordered and exhibit an oblique low energy electron diffraction (LEED) pattern associated with the O-rich CoO2 bulk termination of the (110) surface. Interface and bulk film structure are found to improve significantly with post-growth annealing at 820 K in air and display sharp rectangular LEED patterns, suggesting a surface stoichiometry of the alternative Co2O2 bulk termination of the (110) surface. Non-contact atomic force microscopy demonstrates the presence of wide terraces separated by atomic steps in the annealed films that are not present in the as-grown structures; the step height of ~ 2.7 A corresponds to two atomic layers and confirms a single termination for the annealed films, consistent with the LEED results. A model of the (1 * 1) surfaces that allows for compensation of the polar surfaces is presented.Comment: 8 pages, 7 figure

Effect of the Cu capping thickness on the magnetic properties of thin Ni/Cu(001) films

We have studied the effect of the Cu capping thickness (in the range 20 ≤ tcu ≤ 180 Å) on the magnetic and structural properties of ultrathin FCC 40Å Ni/Cu(001) films by means of magneto-optic Kerr effect (MOKE) magnetometry, polarised neutron reflection (PNR) and grazing incidence X-ray surface diffraction (GID) measurements. MOKE measurements show that perpendicular magnetic anisotropy (PMA) is observed for the whole of the Cu thickness range studied, while an increase of the coercive field with t Cu is found. The variation of the in-plane strain in the Ni film is shown to increase monotonously between 20 and 90Å Cu capping thickness from -1.60 ± 0.03% to -1.86 ± 0.07% as revealed by GID, while PNR measurements show a marked decrease in the Ni magnetic moment per atom with increasing Cu capping layer thickness, from 0.54 + 0.03 μB for tCu = 50Å to 0.45 ± 0.03 μ for tCu = 180Å. These results clearly show that a strain-induced reduction in the Ni magnetic moment per atom occurs. The increase in the coercive field is also attributed to the increase in the strain of the Ni film. © 2001 Published by Elsevier Science B.V

Imaging current induced magnetic domain wall motion in La0.7Sr0.3MnO3 nanowires by XMCD-PEEM

Présentation oraleInternational audienc

Automatable sample fabrication process for pump probe X ray holographic imaging

Soft X ray holography is a recently developed imaging technique with sub 50 nm spatial resolution. Key advantages of this technique are magnetic and elemental sensitivity, compatibility with imaging at free electron laser facilities, and immunity to in situ sample excitations and sample drift, which enables the reliable detection of relative changes between two images with a precision of a few nanometers. In X ray holography, the main part of the experimental setup is integrated in the sample, which consequently requires a large number of fabrication steps. Here we present a generic design and an automatable fabrication process for samples suitable, and optimized for, efficient high resolution X ray holographic dynamic imaging. The high efficiency of the design facilitates the acquisition of magnetic images in a few minutes and makes fully automatic image reconstruction possibl

Giant enhancement of orbital moments and perpendicular anisotropy in epitaxial Fe/GaAs(100)

The spin and orbital magnetic moments and the perpendicular magnetic anisotropy of 8 and 33 monolayer epitaxial bcc Fe films grown on GaAs(100)-4×6 have been measured using x-ray magnetic circular dichronism and polar magneto-optical Kerr effect. Both the films have approximately the same spin moments of about 2.0µB close to that of the bulk value. The ultrathin film shows a giant orbital moment enhancement of about 300% with respect to the bulk value and a perpendicular interface anisotropy field H/s(Fe-GaAs) of the order of –5×10(4) Oe. This may be partially due to an increased degree of localization of electronic states at the Fe/GaAs interface associated with the atomic scale interface structure. © 2001 American Institute of Physics

Magnetic and structural properties of stoichiometric thin Fe3Si/GaAs(001) films

[EN] In this study, we report experimental results on the structural and magnetic properties of epitaxial Fe3Si films grown on GaAs(0 0 1) by co-evaporation. X-ray reflectivity shows that relatively smooth interfaces are obtained (roughness approximate to0.7 nm for the Fe3Si/GaAs interface), while SQUID magnetometry yields a magnetic moment Of 0.9 mu(B)/atom at room temperature, close to the bulk value. From magneto-optic Kerr effect measurements the cubic anisotropy constant was estimated as K-1 = 3.1 X 10(4) erg/cm(3). The electrical transport properties were also investigated by I-V and photoexcitation measurements, which show a Schottky behaviour (with a barrier height of 0.51 eV) and that spin detection is possible in this system. (0 2004 Elsevier B.V. All rights reserved.Ionescu, A.; Vaz, C.; Trypiniotis, T.; Gürtler, CM.; Vickers, ME.; García Miquel, ÁH.; Bland, J. (2005). Magnetic and structural properties of stoichiometric thin Fe3Si/GaAs(001) films. Journal of Magnetism and Magnetic Materials. 286:72-76. doi:10.1016/j.jmmm.2004.09.042S727628