70 research outputs found
Growth modes of nanoparticle superlattice thin films
We report about the fabrication and characterization of iron oxide
nanoparticle thin film superlattices. The formation into different film
morphologies is controlled by tuning the particle plus solvent-to-substrate
interaction. It turns out that the wetting vs. dewetting properties of the
solvent before the self-assembly process during solvent evaporation plays a
major role to determine the resulting film morphology. In addition to layerwise
growth also three-dimensional mesocrystalline growth is evidenced. The
understanding of the mechanisms ruling nanoparticle self-assembly represents an
important step toward the fabrication of novel materials with tailored optical,
magnetic or electrical transport properties
Depth profile of the ferromagnetic order in a YBaCuO / LaCaMnO superlattice on a LSAT substrate: a polarized neutron reflectometry study
Using polarized neutron reflectometry (PNR) we have investigated a
YBa2Cu3O7(10nm)/La2/3Ca1/3MnO3(9nm)]10 (YBCO/LCMO) superlattice grown by pulsed
laser deposition on a La0.3Sr0.7Al0.65Ta0.35O3 (LSAT) substrate. Due to the
high structural quality of the superlattice and the substrate, the specular
reflectivity signal extends with a high signal-to-background ratio beyond the
fourth order superlattice Bragg peak. This allows us to obtain more detailed
and reliable information about the magnetic depth profile than in previous PNR
studies on similar superlattices that were partially impeded by problems
related to the low temperature structural transitions of the SrTiO3 substrates.
In agreement with the previous reports, our PNR data reveal a strong magnetic
proximity effect showing that the depth profile of the magnetic potential
differs significantly from the one of the nuclear potential that is given by
the YBCO and LCMO layer thickness. We present fits of the PNR data using
different simple block-like models for which either a ferromagnetic moment is
induced on the YBCO side of the interfaces or the ferromagnetic order is
suppressed on the LCMO side. We show that a good agreement with the PNR data
and with the average magnetization as obtained from dc magnetization data can
only be obtained with the latter model where a so-called depleted layer with a
strongly suppressed ferromagnetic moment develops on the LCMO side of the
interfaces. The models with an induced ferromagnetic moment on the YBCO side
fail to reproduce the details of the higher order superlattice Bragg peaks and
yield a wrong magnitude of the average magnetization. We also show that the PNR
data are still consistent with the small, ferromagnetic Cu moment of 0.25muB
that was previously identified with x-ray magnetic circular dichroism and x-ray
resonant magnetic reflectometry measurements on the same superlattice.Comment: 11 pages, 7 figure
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Grazing incidence neutron diffraction from large scale 2D structures
The distorted wave Born approximation (DWBA) is applied to evaluate the diffraction pattern of neutrons (or X-rays) from a 2D array of dots deposited onto a dissimilar substrate. With the radiation impinging on the surface at a grazing incidence angle {alpha}, the intensities diffracted both in and out the plane of specular reflection are calculated as a function of the periodicity of the array, height and diameter of the dots. The results are presented in the form of diffracted intensity contours in a plane with coordinates {alpha} and {alpha}{prime}, the latter being the glancing angle of scattering. The optimization of the experimental conditions for polarized neutron experiments on submicron dots is discussed. The feasibility of such measurements is confirmed by a test experiment
Magnetization reversal and exchange bias effects in hard/soft ferromagnetic bilayers with orthogonal anisotropies
The magnetization reversal processes are discussed for exchange-coupled ferromagnetic hard/soft bilayers made from Co[subscript 0.66]Cr[subscript 0.22]Pt[subscript 0.12] (10 and 20 nm)/Ni (from 0 to 40 nm) films with out-of-plane and in-plane magnetic easy axes respectively, based on room temperature hysteresis loops and first-order reversal curve analysis. On increasing the Ni layer thicknesses, the easy axis of the bilayer reorients from out-of-plane to in-plane. An exchange bias effect, consisting of a shift of the in-plane minor hysteresis loops along the field axis, was observed at room temperature after in-plane saturation. This effect was associated with specific ferromagnetic domain configurations experimentally determined by polarized neutron reflectivity. On the other hand, perpendicular exchange bias effect was revealed from the out-of-plane hysteresis loops and it was attributed to residual domains in the magnetically hard layer.National Science Foundation (U.S.)MIT-Spain/La Cambra de Barcelona Seed Fun
Magnetization reversal and exchange bias effects in hard/soft ferromagnetic bilayers with orthogonal anisotropies
21 p.The magnetization reversal processes are discussed for exchange-coupled ferromagnetic hard/soft bilayers made from Co0.66Cr0.22Pt0.12 (10 and 20 nm)/Ni (from 0 to 40 nm) films with out-of-plane and in-plane magnetic easy axes respectively, based on room temperature hysteresis loops and first-order reversal curve analysis. On increasing the Ni layer thicknesses, the easy axis of the bilayer reorients from out-of-plane to in-plane. An exchange bias effect, consisting of a shift of the in-plane minor hysteresis loops along the field axis, was observed at room temperature after in-plane saturation. This effect was associated with specific ferromagnetic domain configurations experimentally determined by polarized neutron reflectivity. On the other hand, perpendicular exchange bias effect was revealed from the out-of-plane hysteresis loops and it was attributed to residual domains in the magnetically hard layer.CAR and DN gratefully acknowledge the support of the National Science Foundation and the MIT-Spain/La Cambra de Barcelona Seed Fund. CR and DN thank the Ministerio de Economia y Competitividad for financial support (MAT2010-20798-C05-02)
Ferromagnetic Domain Distribution in Thin Films During Magnetization Reversal
We have shown that polarized neutron reflectometry can determine in a
model-free way not only the mean magnetization of a ferromagnetic thin film at
any point of a hysteresis cycle, but also the mean square dispersion of the
magnetization vectors of its lateral domains. This technique is applied to
elucidate the mechanism of the magnetization reversal of an exchange-biased
Co/CoO bilayer. The reversal process above the blocking temperature is governed
by uniaxial domain switching, while below the blocking temperature the reversal
of magnetization for the trained sample takes place with substantial domain
rotation
Magnetization and magnetization reversal in epitaxial Fe/Cr/Co asymmetric spin-valve systems
Magnetic Proximity Effect in YBa₂Cu₃O₇/La<sub>2/3</sub>Ca<sub>1/3</sub>MnO₃ and YBa₂Cu₃O₇/LaMnO₃₊ Superlattices
Using neutron reflectometry and resonant x-ray techniques we studied the magnetic proximity effect (MPE) in superlattices composed of superconducting YBa₂Cu₃O₇ and ferromagnetic-metallic La0.67Ca0.33MnO₃ or ferromagnetic-insulating LaMnO₃₊. We find that the MPE strongly depends on the electronic state of the manganite layers, being pronounced for the ferromagnetic-metallic La0.67Ca0.33MnO₃ and almost absent for ferromagnetic-insulating LaMnO₃₊. We also detail the change of the magnetic depth profile due to the MPE and provide evidence for its intrinsic nature
Spatial Fluctuations of Loose Spin Coupling in CuMn/Co Multilayers
A detailed investigation of magnetic impurity-mediated interlayer exchange coupling observed in Cu 0.94 Mn 0.06 / Co multilayers using polarized neutron reflectometry and magnetic x-ray techniques is reported. Excellent descriptions of temperature and magnetic field dependent biquadratic coupling are obtained using a variant of the loose spin model that takes into account the distribution of the impurity Mn ions in three dimensions. Positional disorder of the magnetic impurities is shown to enhance biquadratic coupling via a new contribution J fluct 2 , leading to a temperature dependent canting of magnetic domains in the multilayer. These results provide measurable effects on RKKY coupling associated with the distribution of impurities within planes parallel to the interfaces
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