156 research outputs found
Cotunneling through a quantum dot coupled to ferromagnetic leads with noncollinear magnetizations
Spin-dependent electronic transport through a quantum dot has been analyzed
theoretically in the cotunneling regime by means of the second-order
perturbation theory. The system is described by the impurity Anderson
Hamiltonian with arbitrary Coulomb correlation parameter . It is assumed
that the dot level is intrinsically spin-split due to an effective molecular
field exerted by a magnetic substrate. The dot is coupled to two ferromagnetic
leads whose magnetic moments are noncollinear. The angular dependence of
electric current, tunnel magnetoresistance, and differential conductance are
presented and discussed. The evolution of a cotunneling gap with the angle
between magnetic moments and with the splitting of the dot level is also
demonstrated.Comment: accepted for publication in Eur. Phys. J.
Spin disorder in maghemite nanoparticles investigated using polarized neutrons and nuclear resonant scattering
The manuscript reports the investigation of spin disorder in maghemite nanoparticles of different shape by a combination of polarized small-angle neutron scattering (SANSPOL) and nuclear forward scattering (NFS) techniques. Both methods are sensitive to magnetization on the nanoscale. SANSPOL allows for investigation of the particle morphology and spatial magnetization distribution and NFS extends this nanoscale information to the atomic scale, namely the orientation of the hyperfine field experienced by the iron nuclei. The studied nanospheres and nanocubes with diameters of 7.4 nm and 10.6 nm, respectively, exhibit a significant spin disorder. This effect leads to a reduction of the magnetization to 44% and 58% of the theoretical maghemite bulk value, observed consistently by both techniques
The legends and myths of nanotechnologies: what is a real nature of elastic properties of nanocrystallites
Dynamics of ferrocene in molecular sieves probed by Mossbauer spectroscopy and nuclear resonant scattering
A detailed study on the slow dynamics of ferrocene in the unidimensional channels of the molecular sieves SSZ-24 and AlPO4-5 has been carried out, using Mössbauer spectroscopy (MS), nuclear forward scattering (NFS) and synchrotron radiation-based perturbed angular correlations (SRPAC). In both host systems, anisotropic rotational dynamics is observed above 100 K. For SSZ-24, this anisotropy persists even above the bulk melting temperature of ferrocene. Various theoretical models are exploited for the study of anisotropic discrete jump rotations for the first time. The experimental data can be described fairly well by a jump model that involves reorientations of the molecular axis on a cone mantle with an opening angle dependant on temperature
Structural and magnetic properties of co-sputtered Fe0.8C0.2 thin films
We studied the structural and magnetic properties of \FeC~thin films
deposited by co-sputtering of Fe and C targets in a direct current magnetron
sputtering (dcMS) process at a substrate temperature (\Ts) of 300, 523 and
773\,K. The structure and morphology was measured using x-ray diffraction
(XRD), x-ray absorption near edge spectroscopy (XANES) at Fe and C
-edges and atomic/magnetic force microscopy (AFM, MFM), respectively. An
ultrathin (3\,nm) \FeC~layer, placed between relatively thick
\FeC~layers was used to estimate Fe self-diffusion taking place during growth
at different \Ts~using depth profiling measurements. Such \FeC~layer was
also used for Fe conversion electron M\"{o}ssbauer spectroscopy (CEMS)
and nuclear resonance scattering (NRS) measurements, yielding the magnetic
structure of this ultrathin layer. We found from XRD measurements that the
structure formed at low \Ts~(300\,K) is analogous to Fe-based amorphous alloy
and at high \Ts~(773\,K), pre-dominantly a \tifc~phase has been formed.
Interestingly, at an intermediate \Ts~(523\,K), a clear presence of
\tefc~(along with \tifc~and Fe) can be seen from the NRS spectra. The
microstructure obtained from AFM images was found to be in agreement with XRD
results. MFM images also agrees well with NRS results as the presence of
multi-magnetic components can be clearly seen in the sample grown at \Ts~=
523\,K. The information about the hybridization between Fe and C, obtained from
Fe and C -edges XANES also supports the results obtained from other
measurements. In essence, from this work, experimental realization of \tefc~has
been demonstrated. It can be anticipated that by further fine-tuning the
deposition conditions, even single phase \tefc~phase can be realized which
hitherto remains an experimental challenge.Comment: 11 pages, 9 figure
Ab initio and nuclear inelastic scattering studies of FeSi/GaAs heterostructures
The structure and dynamical properties of the FeSi/GaAs(001) interface
are investigated by density functional theory and nuclear inelastic scattering
measurements. The stability of four different atomic configurations of the
FeSi/GaAs multilayers is analyzed by calculating the formation energies and
phonon dispersion curves. The differences in charge density, magnetization, and
electronic density of states between the configurations are examined. Our
calculations unveil that magnetic moments of the Fe atoms tend to align in a
plane parallel to the interface, along the [110] direction of the FeSi
crystallographic unit cell. In some configurations, the spin polarization of
interface layers is larger than that of bulk FeSi. The effect of the
interface on element-specific and layer-resolved phonon density of states is
discussed. The Fe-partial phonon density of states measured for the FeSi
layer thickness of three monolayers is compared with theoretical results
obtained for each interface atomic configuration. The best agreement is found
for one of the configurations with a mixed Fe-Si interface layer, which
reproduces the anomalous enhancement of the phonon density of states below 10
meVComment: 14 pages, 9 figures, 4 table
Origin of a Simultaneous Suppression of Thermal Conductivity and Increase of Electrical Conductivity and Seebeck Coefficient in Disordered Cubic Cu2ZnSnS4
The parameters governing the thermoelectric efficiency of a material, Seebeck coefficient, electrical, and thermal conductivities, are correlated and their reciprocal interdependence typically prevents a simultaneous optimization. Here, we present the case of disordered cubic kesterite CuZnSnS, a phase stabilized by structural disorder at low temperature. With respect to the ordered form, the introduction of disorder improves the three thermoelectric parameters at the same time. The origin of this peculiar behavior lies in the localization of some Sn lone pair electrons, leading to “rattling” Sn ions. On one hand, these rattlers remarkably suppress thermal conductivity, dissipating lattice energy via optical phonons located below 1.5 THz; on the other, they form electron-deficient Sn—S bonds leading to a p-type dopinglike effect and highly localized acceptor levels, simultaneously enhancing electrical conductivity and the Seebeck coefficient. This phenomenon leads to a 3 times reduced thermal conductivity and doubling of both electrical conductivity and the Seebeck coefficient, resulting in a more than 20 times increase in figure of merit, although still moderate in absolute terms
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