922 research outputs found
Geometrical effects on spin injection: 3D spin drift diffusion model
We discuss a three-dimensional (3D) spin drift diffusion (SDD) model to
inject spin from a ferromagnet (FM) to a normal metal (N) or semiconductor
(SC). Using this model we investigate the problem of spin injection into
isotropic materials like GaAs and study the effect of FM contact area and SC
thickness on spin injection. We find that in order to achieve detectable spin
injection a small contact area or thick SC samples are essential for direct
contact spin injection devices. We investigate the use of thin metal films (Cu)
proposed by S.B. Kumar et al. and show that they are an excellent substitute
for tunnelling barriers (TB) in the regime of small contact area. Since most
tunnelling barriers are prone to pinhole defects, we study the effect of
pinholes in AlO tunnelling barriers and show that the reduction in the
spin-injection ratio () is solely due to the effective area of the
pinholes and there is no correlation between the number of pinholes and the
spin injection ratio.Comment: 5 pages, 6 figures. Accepted by JA
Ab initio exchange interactions and magnetic properties of Gd2Fe17 iron sublattice: rhombohedral vs. hexagonal phases
In the framework of the LSDA+U method electronic structure and magnetic
properties of the intermetallic compound Gd2Fe17 for both rhombohedral and
hexagonal phases have been calculated. On top of that, ab initio exchange
interaction parameters within the Fe sublattice for all present nearest and
some next nearest Fe ions have been obtained. It was found that for the first
coordination sphere direct exchange interaction is ferromagnetic. For the
second coordination sphere indirect exchange interaction is observed to be
weaker and of antiferromagnetic type. Employing the theoretical values of
exchange parameters Curie temperatures Tc of both hexagonal and rhombohedral
phases of Gd2Fe17 within Weiss mean-field theory were estimated. Obtained
values of Tc and its increase going from the hexagonal to rhombohedral crystal
structure of Gd2Fe17 agree well with experiment. Also for both structures
LSDA+U computed values of total magnetic moment coincide with experimental
ones.Comment: 20 pages, 2 figures; V2 as published in PR
312 MAX Phases: Elastic Properties and Lithiation
Interest in the Mn+1AXn phases (M = early transition metal; A = group 13–16 elements, and X = C or N) is driven by their ceramic and metallic properties, which make them attractive candidates for numerous applications. In the present study, we use the density functional theory to calculate the elastic properties and the incorporation of lithium atoms in the 312 MAX phases. It is shown that the energy to incorporate one Li atom in Mo3SiC2, Hf3AlC2, Zr3AlC2, and Zr3SiC2 is particularly low, and thus, theoretically, these materials should be considered for battery applications
Origin of Middle-Infrared Peaks in Cerium Compounds
We have demonstrated that the middle-infrared (mid-IR) peaks in the optical
conductivity spectra of Ce ( = Pd, Sn, In) can be explained by
first-principle band structure calculation with the spin-orbit interaction. The
mid-IR peak shapes in these materials are not identical to one another:
CePd, CeSn, and CeIn have a triple-peak structure, double-peak
structure and broad single-peak structure, respectively. These peaks can be
theoretically explained by the optical transition from the occupied state to
the spin-orbit splitted Ce state. This result indicates that the mid-IR
peaks originate from the simple band picture with the Ce state near the
Fermi level, not from the conventional cf hybridization gap based on the
periodic Anderson model.Comment: 5 pages, 6 figures. To be published in J. Phys. Soc. Jpn. 78(1)
(2009
Ferromagnetic order in U(Rh,Co)Ge
We report the variation of ferromagnetic order in the pseudo-ternary
compounds URh_{1-x}Co_{x}Ge (0 \leq x \leq 1). Magnetization and transport data
taken on polycrystalline samples show that the Curie temperature T_{C}
gradually increases with increasing Co content from a value of 9.5 K for URhGe
to a maximum value of 20 K for x = 0.6 and then steadily decreases to 3 K for
UCoGe. The magnetic interaction strength varies smoothly across the series. For
all samples the electrical resistivity for T < T_{C} follows the behaviour \rho
= \rho_{0} + AT^2. The A coefficient is dominated by scattering at spin waves
and is strongly enhanced for x = 0 and 1.Comment: 12 pages (4 figures), submitted to SS
The Stability of Al11Sm3 (Al4Sm) Phases in the Al-Sm Binary System
The relative stability of Al11Sm3 (Al4Sm) intermetallic phases was experimentally investigated through a series of heat treatments followed by microstructural, microchemical, and X-ray diffraction (XRD) analyses. The principal findings are that the high-temperature tetragonal phase is stable from 1655 to 1333 K and that the low-temperature orthorhombic phases, α and γ, have no range of full stability but are metastable with respect to the crystalline Al and Sm reference states down to 0 K. Thermodynamic modeling is used to describe the relative energetics of stable and metastable phases along with the associated two-phase mixtures. Issues regarding transition energetics and kinetics are discussed
Geometrical quadrupolar frustration in DyB
Physical properties of DyB have been studied by magnetization, specific
heat, and ultrasonic measurements. The magnetic entropy change and the
ultrasonic properties in the intermediate phase II indicate that the degeneracy
of internal degrees of freedom is not fully lifted in spite of the formation of
magnetic order. The ultrasonic attenuation and the huge softening of
in phase II suggests existence of electric-quadrupolar (orbital) fluctuations
of the 4-electron. These unusual properties originate from the geometrical
quadrupolar frustration.Comment: 4 pages, 4 figures, accepted for publication in Journal of the
Physical Society of Japa
Phase diagram of superconductivity and antiferromagnetism in single crystals of Sr(Fe1-xCox)2As2 and Sr1-yEuy(Fe0.88Co0.12)2As2
We report magnetic susceptibility, resistivity and heat capacity measurements
on single crystals of the Sr(Fe1-xCox)2As2 and Sr1-yEuy(Fe0.88Co0.12)2As2
series. The optimal Co concentration for superconductivity in Sr(Fe1-xCox)2As2
is determined to be x ~ 0.12. Based on this we grew members of the
Sr1-yEuy(Fe0.88Co0.12)2As2 series so as to examine the effects of well defined,
local magnetic moments, on the superconducting state. We show that
superconductivity is gradually suppressed by paramagnetic Eu2+ doping and
coexists with antiferromagnetic ordering of Eu2+ as long as Tc > TN. For y >=
0.65, TN crosses Tc and the superconducting ground state (as manifested by zero
resistivity) abruptly disappears with evidence for competition between
superconductivity and local moment antiferromagnetism for y up to 0.72. It is
speculated that the suppression of the antiferromagnetic fluctuations of Fe
sublattice by coupling to the long range order of Eu2+ sublattice destroys bulk
superconductivity when TN > Tc.Comment: 10 pages, 12 figure
Thermal relaxation of magnetic clusters in amorphous Hf_{57}Fe_{43} alloy
The magnetization processes in binary magnetic/nonmagnetic amorphous alloy
Hf_{57}Fe_{43} are investigated by the detailed measurements of magnetic
hysteresis loops, temperature dependence of magnetization, relaxation of
magnetization and magnetic ac susceptibility, including a nonlinear term.
Blocking of magnetic moments at lower temperatures is accompanied with the slow
relaxation of magnetization and magnetic hysteresis loops. All of the observed
properties are explained with the superparamagnetic behaviour of the single
domain magnetic clusters inside the nonmagnetic host, their blocking by the
anisotropy barriers and thermal fluctuation over the barriers accompanied by
relaxation of magnetization. From magnetic viscosity analysis based on thermal
relaxation over the anisotropy barriers it is found out that magnetic clusters
occupy the characteristic volume from 25 up to 200 nm3 . The validity of the
superparamagnetic model of Hf_{57}Fe_{43} is based on the concentration of iron
in the Hf_{100-x}Fe_{43} system that is just below the threshold for the long
range magnetic ordering. This work throws more light on magnetic behaviour of
other amorphous alloys, too
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