846 research outputs found
Field sweep rate dependence of the coercive field of single-molecule magnets: a classical approach with applications to the quantum regime
A method, based on the Neel-Brown model of thermally activated magnetization
reversal of a magnetic single-domain particle, is proposed to study the field
sweep rate dependence of the coercive field of single-molecule magnets (SMMs).
The application to Mn12 and Mn84 SMMs allows the determination of the important
parameters that characterize the magnetic properties: the energy barrier, the
magnetic anisotropy constant, the spin, tau_0, and the crossover temperature
from the classical to the quantum regime. The method may be particularly
valuable for large SMMs that do not show quantum tunneling steps in the
hysteresis loops.Comment: 6 pages, 6 figure
Understanding and engineering phonon-mediated tunneling into graphene on metal surfaces
Metal-intercalated graphene on Ir(111) exhibits phonon signatures in
inelastic elec- tron tunneling spectroscopy with strengths that depend on the
intercalant. Extraor- dinarily strong graphene phonon signals are observed for
Cs intercalation. Li interca- lation likewise induces clearly discriminable
phonon signatures, albeit less pronounced than observed for Cs. The signal can
be finely tuned by the alkali metal coverage and gradually disappears upon
increasing the junction conductance from tunneling to con- tact ranges. In
contrast to Cs and Li, for Ni-intercalated graphene the phonon signals stay
below the detection limit in all transport ranges. Going beyond the
conventional two-terminal approach, transport calculations provide a
comprehensive understanding of the subtle interplay between the
graphene{electrode coupling and the observation of graphene phonon
spectroscopic signatures
Quantum nucleation in a single-chain magnet
The field sweep rate (v=dH/dt) and temperature (T) dependence of the
magnetization reversal of a single-chain magnet (SCM) is studied at low
temperatures. As expected for a thermally activated process, the nucleation
field (H_n) increases with decreasing T and increasing v. The set of H_n(T,v)
data is analyzed with a model of thermally activated nucleation of
magnetization reversal. Below 1 K, H_n becomes temperature independent but
remains strongly sweep rate dependent. In this temperature range, the reversal
of the magnetization is induced by a quantum nucleation of a domain wall that
then propagates due to the applied field.Comment: 5 pages, 4 figure
Thermodynamics of Two Dimensional Magnetic Nanoparticles
A two dimensional magnetic particle in the presence of an external magnetic
field is studied. Equilibrium thermodynamical properties are derived by
evaluating analytically the partition function. When the external field is
applied perpendicular to the anisotropy axis the system exhibits a second order
phase transition with order parameter being the magnetization parallel to the
field. In this case the system is isomorph to a mechanical system consisting in
a particle moving without friction in a circle rotating about its vertical
diameter. Contrary to a paramagnetic particle, equilibrium magnetization shows
a maximum at finite temperature. We also show that uniaxial anisotropy in a
system of noninteracting particles can be missinterpreted as a ferromagnetic or
antiferromagnetic coupling among the magnetic particles depending on the angle
between anisotropy axis and magnetic field.Comment: 4 pages 6 figures 19 reference
Quantized Conductance of a Single Magnetic Atom
A single Co atom adsorbed on Cu(111) or on ferromagnetic Co islands is
contacted with non-magnetic W or ferromagnetic Ni tips in a scanning tunneling
microscope. When the Co atom bridges two non-magnetic electrodes conductances
of 2e^2/h are found. With two ferromagnetic electrodes a conductance of e^2/h
is observed which may indicate fully spin-polarized transport.Comment: 3 pages, 2 figure
Superparamagnetic behaviour of antiferromagnetic DyPO4 nanoparticles
We report on the low-temperature magnetic ac-susceptibility of
antiferromagnetic DyPO4 nanoparticles with a very high surface to volume ratio.
The results are interpreted in terms of superparamagnetic relaxation of the
Neel vector arising from a relatively large number approx 0.2 Na of
uncompensated spins probably existing on the surface of the nanoparticles. The
activation energy of the relaxation process is found to be Ea / kB = (2.6+-0.1)
K within a model taking into account the magnetic interaction between
nanoparticles.Comment: 3 pages, 1 figure, Espcrc2, ICM0
Effect of temperature-dependent shape anisotropy on coercivity with aligned Stoner-Wohlfarth soft ferromagnets
The temperature variation effect of shape anisotropy on the coercivity,
HC(T), for the aligned Stoner-Wohlfarth (SW) soft ferromagnets, such as fcc Ni,
fcc Co and bcc Fe, are investigated within the framework of Neel-Brown (N-B)
analysis. An extended N-B equation is thus proposed,by introducing a single
dimensionless correction function, the reduced magnetization, m(\tao) =
MS(T)/MS(0), in which \tao = T/TC is the reduced temperature, MS(T) is the
saturation magnetization, and TC is the Curie temperature. The factor, m(\tao),
accounts for the temperature-dependent effect of the shape anisotropy. The
constants, H0 and E0, are for the switching field at zero temperature and the
potential barrier at zero field, respectively. According to this newly derived
equation, the blocking temperature above which the properties of
superparamagnetism show up is described by the expression, TB =
E0m^2(\tao)/[kBln(t/t0)], with the extra correction factor m^2(\tao). The
possible effect on HC(T) and the blocking temperature, TB, attributed to the
downshift of TC resulting from the finite size effect has been discussed also.Comment: 22 pages, 2 figures, 1 table, Accepted by Phys. Rev.
Shape-induced phenomena in the finite size antiferromagnets
It is of common knowledge that the direction of easy axis in the finite-size
ferromagnetic sample is controlled by its shape. In the present paper we show
that a similar phenomenon should be observed in the compensated
antiferromagnets with strong magnetoelastic coupling. Destressing energy which
originates from the long-range magnetoelastic forces is analogous to
demagnetization energy in ferromagnetic materials and is responsible for the
formation of equilibrium domain structure and anisotropy of macroscopic
magnetic properties. In particular, crystal shape may be a source of additional
uniaxial magnetic anisotropy which removes degeneracy of antiferromagnetic
vector or artificial 4th order anisotropy in the case of a square cross-section
sample. In a special case of antiferromagnetic nanopillars shape-induced
anisotropy can be substantially enhanced due to lattice mismatch with the
substrate. These effects can be detected by the magnetic rotational torque and
antiferromagnetic resonance measurements.Comment: 7 pages, 5 figures, to appear in Phys. Rev. B, v.75, N17, 200
Anomalous Ferromagnetism of Monatomic Co Wire at the Pt(111) Surface Step Edge
A first-principles investigation of the anomalous ferromagnetism of a
quasi-one-dimensional Co chain at the Pt(111) step edge is reported. Our
calculations show that the symmetry breaking at the step leads to an easy
magnetization axis at an odd angle of {\em towards} the Pt
step, in agreement with experiment [P. Gambardella {\em et al.}, {\em Nature}
{\bf 416}, 301 (2002)]. Also, the Co spin and orbital moments become
noncollinear, even in the case of a collinear ferromagnetic spin arrangement. A
significant enhancement of the Co orbital magnetic moment is achieved when
modest electron correlations are treated within LSDA+ calculations.Comment: Presented at MRS Meeting in Boston, Dec. 2003; 4 pages including 3
figure
Epitaxial strain effects in the spinel ferrites CoFe2O4 and NiFe2O4 from first principles
The inverse spinels CoFe2O4 and NiFe2O4, which have been of particular
interest over the past few years as building blocks of artificial multiferroic
heterostructures and as possible spin-filter materials, are investigated by
means of density functional theory calculations. We address the effect of
epitaxial strain on the magneto-crystalline anisotropy and show that, in
agreement with experimental observations, tensile strain favors perpendicular
anisotropy, whereas compressive strain favors in-plane orientation of the
magnetization. Our calculated magnetostriction constants of
about -220 ppm for CoFe2O4 and -45 ppm for NiFe2O4 agree well with available
experimental data. We analyze the effect of different cation arrangements used
to represent the inverse spinel structure and show that both LSDA+U and GGA+U
allow for a good quantitative description of these materials. Our results open
the way for further computational investigations of spinel ferrites
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