254 research outputs found
Observation of Josephson coupling through an interlayer of antiferromagnetically ordered chromium
The supercurrent transport in metallic Josephson tunnel junctions with an
additional interlayer made up by chromium, being an itinerant antiferromagnet,
was studied. Uniform Josephson coupling was observed as a function of the
magnetic field. The supercurrent shows a weak dependence on the interlayer
thickness for thin chromium layers and decays exponentially for thicker films.
The diffusion constant and the coherence length in the antiferromagnet were
estimated. The antiferromagnetic state of the barrier was indirectly verified
using reference samples. Our results are compared to macroscopic and
microscopic models.Comment: Phys. Rev. B (2009), in prin
Thickness dependence of linear and quadratic magneto-optical Kerr effect in ultrathin Fe(001) films
Magneto-optical Kerr effect (MOKE) magnetometry is one of the most widely
employed techniques for the characterization of ferromagnetic thin-film
samples. Some information, such as coercive fields or anisotropy strengths can
be obtained without any knowledge of the optical and magneto-optical (MO)
properties of the material. On the other hand, a quantitative analysis, which
requires a precise knowledge of the material's index of refraction n and the MO
coupling constants K and G is often desirable, for instance for the comparison
of samples, which are different with respect to ferromagnetic layer
thicknesses, substrates, or capping layers. While the values of the parameters
n and the linear MO coupling parameter K reported by different authors usually
vary considerably, the relevant quadratic MO coupling parameters G of Fe are
completely unknown. Here, we report on measurements of the thickness dependence
(0-60nm) of the linear and quadratic MOKE in epitaxial bcc-Fe(001) wedge-type
samples performed at a commonly used laser wavelength of 670nm. By fitting the
thickness dependence we are able to extract a complete set of parameters n, K,
(G11 - G12), and G44 for the quantitative description of the MOKE of
bcc-Fe(001). We find sizable different n, K, and G parameters for films thinner
than about 10nm as compared to thicker films, which is indicative of a
thickness dependence of the electronic properties or of surface contributions
to the MOKE. The effect size of the quadratic MOKE is found to be about a third
of the record values recently reported for Co2FeSi.Comment: 8 pages, 5 figure
Antiferromagnetic interlayer exchange coupling across an amorphous metallic spacer layer
By means of magneto-optical Kerr effect we observe for the first time
antiferromagnetic coupling between ferromagnetic layers across an amorphous
metallic spacer layer. Biquadratic coupling occurs at the transition from a
ferromagnetically to an antiferromagnetically coupled region. Scanning
tunneling microscopy images of all involved layers are used to extract
thickness fluctuations and to verify the amorphous state of the spacer. The
observed antiferromagnetic coupling behavior is explained by RKKY interaction
taking into account the amorphous structure of the spacer material.Comment: Typset using RevTex, 4 pages with 4 figures (.eps
Interfacial magnetization in exchange-coupled Fe/Cr/Fe structures investigated by second harmonic generation
We have studied the magnetic field dependences of magnetic optical second harmonic generation (SHG) in MBE-grown Fe/Cr/Fe/Ag/GaAs(100) heterostructures displaying both bilinear and biquadratic interlayer exchange coupling. The magnetic field H was applied in the (100) surface plane along both easy ([001]) and hard ([110]) axes of the in-plane fourfold magnetic anisotropy. The SHG has been measured in reflection at near normal incidence for different polarization combinations (pp,ps,ss,sp) of the fundamental and second harmonic light in longitudinal and transversal geometries. The magnetic field variation of the SHG signal clearly reflects the field-induced transformations of the magnetic state at the interfaces in the trilayer. It strongly depends on the configuration of light polarization, experimental geometry (longitudinal or transversal), and orientation of the magnetic field H relative to the crystal axes. In contrast to linear magneto-optical Kerr effect, which is odd in magnetic field, magnetic SHG is either even in H or does not display a definite parity at all, depending on the polarization configuration. We interpret the data based on a model accounting for nonmagnetic and magnetic contributions to SHG from the surface and interfaces described by C-4v point symmetry. Taking into account the changes of the mutual orientation of interfacial magnetizations allows us to describe the general features of the measured field dependences of SHG
Control of interlayer exchange coupling in Fe/Cr/Fe trilayers by ion beam irradiation
The manipulation of the antiferromagnetic interlayer coupling in the
epitaxial Fe/Cr/Fe(001) trilayer system by moderate 5 keV He ion beam
irradiation has been investigated experimentally. It is shown that even for
irradiation with very low fluences (10^14 ions/cm^2) a drastic change in
strength of the coupling appears. For thin Cr-spacers (below 0.6 - 0.7 nm) the
coupling strength decreases with fluence, becoming ferromagnetic for fluences
above (2x10^14 ions/cm^2). The effect is connected with the creation of
magnetic bridges in the layered system due to atomic exchange events caused by
the bombardment. For thicker Cr spacers (0.8 - 1.2 nm) an enhancement of the
antiferromagnetic coupling strength is found. A possible explanation of the
enhancement effect is given.Comment: Submitted to PR
Influence of torrefaction on the grindability and reactivity of woody biomass
7 pages, 5 figures, 4 tables.-- Available online Oct 23, 2007.The use of biomass to produce energy is becoming more and more frequent as it helps to achieve a sustainable environmental scenario. However the exploitation of this fuel source does have drawbacks that need to be solved. In this work, the torrefaction of woody biomass (eucalyptus) was studied in order to improve its properties for pulverised systems. The process consisted in a heating treatment at moderate temperature (240, 260, 280°C) under an inert atmosphere. The grindability of raw biomass and the treated samples was compared and an improvement in the grindability characteristics was observed after the torrefaction process. Thermogravimetric analysis of the samples was carried out in order to study their reactivity in air. The DTG curves of the torrefied biomass showed a double peak nature. The kinetic parameters were calculated for each reaction stage. The torrefaction process was found to influence the parameters of the first stage, whereas those corresponding to the second remained unaffected.This work was carried out with financial support from the Spanish CDTI (Project CENIT PiIBE) and ELCOGAS, S.A. M.G.P. and C.P. acknowledge the support from the CSIC I3P Program co-financed by the European Social Fund, and J.F. from the Plan Regional de Investigacion del Principado de Asturias.Peer reviewe
Multistate Current-Induced Magnetization Switching in Au/Fe/MgO(001) Epitaxial Heterostructures
Magnetization switching using in-plane charge current recently has been widely investigated in heavy metal/ferromagnet bilayers with the switching mechanism usually attributed to the action of the spin-orbit coupling. Here we study in-plane current induced magnetization switching in model epitaxial bilayers that consist of Au(001) and Fe(001) grown on MgO(001). We use the planar Hall effect combined with magnetooptical Kerr effect (MOKE) microscopy to investigate magnetic properties of the bilayers and current-induced switching. We show that a current density beyond 1.4×107 A/cm2 can be employed for reproducible electrical switching of the magnetization between multiple stable states that correspond to different arrangements of magnetic domains with magnetization direction along one of the in-plane easy magnetization axes of the Fe(001) film. Lower current densities result in stable intermediate transversal resistances which are interpreted based on MOKE-microscopy investigations as resulting from the current-induced magnetic domain structure that is formed in the area of the Hall cross. We find that the physical mechanism of the current-induced magnetization switching of the Au/Fe/MgO(001) system at room temperature can be fully explained by the Oersted field, which is generated by the charge current flowing mostly through the Au layer. © 2021 authors. Published by the American Physical Society
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