2,275 research outputs found
Hysteretic properties of a magnetic particle with strong surface anisotropy
We study the influence of surface anisotropy on the zero-temperature
hysteretic properties of a small single-domain magnetic particle, and give an
estimation of the anisotropy constant for which deviations from the
Stoner-Wohlfarth model are observed due to non-uniform reversal of the
particle's magnetisation. For this purpose, we consider a spherical particle
with simple cubic crystalline structure, a uniaxial anisotropy for core spins
and radial anisotropy on the surface. The hysteresis loop is obtained by
solving the local (coupled) Landau-Lifschitz equations for classical spin
vectors. We find that when the surface anisotropy constant is at least of the
order of the exchange coupling, large deviations are observed with respect to
the Stoner-Wohlfarth model in the hysteresis loop and thereby the
limit-of-metastability curve, since in this case the magnetisation reverses its
direction in a non-uniform manner via a progressive switching of spin clusters.
In this case the critical field, as a function of the particle's size, behaves
as observed in experiments.Comment: 12 pages, 15 eps figure
Validity of the scattering length approximation in strongly interacting Fermi systems
We investigate the energy spectrum of systems of two, three and four spin-1/2
fermions with short range attractive interactions both exactly, and within the
scattering length approximation. The formation of molecular bound states and
the ferromagnetic transition of the excited scattering state are examined
systematically as a function of the 2-body scattering length. Identification of
the upper branch (scattering states) is discussed and a general approach valid
for systems with many particles is given. We show that an adiabatic
ferromagnetic transition occurs, but at a critical transition point kF a much
higher than predicted from previous calculations, almost all of which use the
scattering length approximation. In the 4-particle system the discrepancy is a
factor of 2. The exact critical interaction strength calculated in the
4-particle system is consistent with that reported by experiment. To make
comparisons with the adiabatic transition, we study the quench dynamics of the
pairing instability using the eigenstate wavefunctions.Comment: 7 pages, 7 figure
Magnetism of 3d transition metal atoms on W(001): submonolayer films
We have investigated random submonolayer films of 3d transition metals on
W(001). The tight-binding linear muffin-tin orbital method combined with the
coherent potential approximation was employed to calculate the electronic
structure of the films. We have estimated local magnetic moments and the
stability of different magnetic structures, namely the ferromagnetic order, the
disordered local moments and the non-magnetic state, by comparing the total
energies of the corresponding systems. It has been found that the magnetic
moments of V and Cr decrease and eventually disappear with decreasing coverage.
On the other hand, Fe retains approximately the same magnetic moment throughout
the whole concentration range from a single impurity to the monolayer coverage.
Mn is an intermediate case between Cr and Fe since it is non-magnetic at very
low coverages and ferromagnetic otherwise.Comment: 5 pages, 3 figures in 6 files; presented at ICN&T 2006, Basel,
Switzerlan
Coupling between magnetic ordering and structural instabilities in perovskite biferroics: A first-principles study
We use first-principles density functional theory-based calculations to
investigate structural instabilities in the high symmetry cubic perovskite
structure of rare-earth (R La, Y, Lu) and Bi-based biferroic chromites,
focusing on and point phonons of states with para-, ferro-, and
antiferromagnetic ordering. We find that (a) the structure with G-type
antiferromagnetic ordering is most stable, (b) the most dominant structural
instabilities in these oxides are the ones associated with rotations of oxygen
octahedra, and (c) structural instabilities involving changes in Cr-O-Cr bond
angle depend sensitively on the changes in magnetic ordering. The dependence of
structural instabilities on magnetic ordering can be understood in terms of how
super-exchange interactions depend on the Cr-O-Cr bond angles and Cr-O bond
lengths. We demonstrate how adequate buckling of Cr-O-Cr chains can favour
ferromagnetism. Born effective charges (BEC) calculated using the Berry phase
expression are found to be anomalously large for the A-cations, indicating
their chemical relevance to ferroelectric distortions.Comment: 8 pages, 13 figure
Major loop reconstruction from switching of individual particles
Major hysteresis loops of groups of isolated 60 mm square garnet particles of a regular
two-dimensional array, have been measured magnetooptically. Individual loops for each particle
were measured, and the statistics of the distribution of coercivities and interaction fields was
determined. It is shown that from the measured coercivity distribution and calculated magnetostatic
interaction fields the major hysteresis loop can be reconstructed. The switching sequence, and the
major loop of an assembly of 535 particles were calculated numerically for two cases: first, when
calculating the magnetostatic interaction, the 25 particles were assumed to be isolated; second, the
major loop of the same 25 particles, embedded into a 939 square, was reconstructed taking into
account the interactions among all 81 particles. The numerically simulated major hysteresis loops
agree very well with the measured loops, demonstrating the reliability of numerical modeling
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
Decomposability of soil organic matter over time: the Soil Incubation Database (SIDb, version 1.0) and guidance for incubation procedures
Spin Josephson effect in ferromagnet/ferromagnet tunnel junctions
We consider the tunnel spin current between two ferromagnetic metals from a
perspective similar to the one used in superconductor/superconductor tunnel
junctions. We use fundamental arguments to derive a Josephson-like spin tunnel
current . Here the phases are
associated with the planar contribution to the magnetization,
. The crucial step in our
analysis is the fact that the -component of the spin is canonically
conjugate to the phase of the planar contribution: . This is
analogous to the commutation relation in superconductors, where
is the phase associated to the superconducting order parameter and
is the Cooper pair number operator. We briefly discuss the experimental
consequences of our theoretical analysis.Comment: LaTex, seven pages, no figures; version to appear in Europhys. Lett.;
in order to make room for a more extended microscopic analysis, the
phenomenological discussion contained in v2 was remove
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