195 research outputs found
Ab initio Evidence for Giant Magnetoelectric Responses Driven by Structural Softness
We show that inducing structural softness in regular magnetoelectric (ME)
multiferroics -- i.e., tuning the materials to make their structure strongly
reactive to applied fields -- makes it possible to obtain very large ME
effects. We present illustrative first-principles results for BiFeO3 thin
films.Comment: 4 pages with 3 figures embedded. More information at
http://www.icmab.es/dmmis/leem/jorg
Dissipationless Spin Current between Two Coupled Ferromagnets
We demonstrate the general principle which states that a dissipationless spin
current flows between two coupled ferromagnets if their magnetic orders are
misaligned. This principle applies regardless the two ferromagnets are metallic
or insulating, and also generally applies to bulk magnetic insulators. On a
phenomenological level, this principle is analogous to Josephson effect, and
yields a dissipationless spin current that is independent from scattering. The
microscopic mechanisms for the dissipationless spin current depend on the
systems, which are elaborated in details. A uniform, static magnetic field is
further proposed to be an efficient handle to create the misaligned
configuration and stabilize the dissipationless spin current.Comment: 10 pages, 5 figure
Magnetoelectric response of multiferroic BiFeO3 and related materials
We present a first-principles scheme for computing the magnetoelectric
response of multiferroics. We apply our method to BiFeO3 (BFO) and related
compounds in which Fe is substituted by other magnetic species. We show that
under certain relevant conditions -- i.e., in absence of incommensurate spin
modulation, as in BFO thin films and some BFO-based solid solutions -- these
materials display a large linear magnetoelectric response. Our calculations
reveal the atomistic origin of the coupling and allow us to identify the most
promising strategies to enhance it.Comment: 4 pages with 1 figure embedded. More information at
http://www.icmab.es/dmmis/leem/jorg
Prediction for new magnetoelectric fluorides
We use symmetry considerations in order to predict new magnetoelectric
fluorides. In addition to these magnetoelectric properties, we discuss among
these fluorides the ones susceptible to present multiferroic properties. We
emphasize that several materials present ferromagnetic properties. This
ferromagnetism should enhance the interplay between magnetic and dielectric
properties in these materials.Comment: 12 pages, 4 figures, To appear in Journal of Physics: Condensed
Matte
Multiferroic properties of an \aa kermanite SrCoSiO single crystal in high magnetic fields
The magnetic and dielectric properties of \aa kermanite SrCoSiO
single crystals in high magnetic fields were investigated. We have observed
finite induced electric polarization along the c axis in high fields, wherein
all Co spins were forcibly aligned to the magnetic field direction. Existence
of the induced polarization in the spin-polarized state accompanied with the
finite slope in the magnetization curve suggests the possible role of the
orbital angular momenta in the excited states as its microscopic origin. The
emergence of the field-induced polarization without particular magnetic order
can be regarded as the magnetoelectric effects of the second order from the
symmetry point of view. A low magnetic field-driven electric polarization flip
induced by a rotating field, even at room temperature, has been successfully
demonstrated.Comment: 12 pages, 4 figure
Magneto-electric effect in NdCrTiO5
We have measured the dielectric constant and the pyroelectric current of
orthorhombic (space group ) NdCrTiO polycrystalline samples. The
dielectric constant and the pyroelectric current show features associated with
ferroelectric transitions at the antiferromagnetic transition temperature
( = 21 K). The effect of magnetic fields is to enhance the
features almost linearly up to the maximum measured field (7 T) with a
spontaneous polarization value of C/m. Two possible
scenarios, the linear magnetoelectric effect and multiferroicity
(antiferromagnetism + ferroelectricity), are discussed as possible explanations
for the observations.Comment: 7 pages, 6 figure
U(1)-Symmetry breaking and violation of axial symmetry in TlCuCl3 and other insulating spin systems
We describe the Bose-Einstein condensate of magnetic bosonic quasiparticles
in insulating spin systems using a phenomenological standard functional method
for T = 0. We show that results that are already known from advanced
computational techniques immediately follow. The inclusion of a perturbative
anisotropy term that violates the axial symmetry allows us to remarkably well
explain a number of experimental features of the dimerized spin-1/2 system
TlCuCl3. Based on an energetic argument we predict a general intrinsic
instability of an axially symmetric magnetic condensate towards a violation of
this symmetry, which leads to the spontaneous formation of an anisotropy gap in
the energy spectrum above the critical field. We, therefore, expect that a true
Goldstone mode in insulating spin systems, i.e., a strictly linear
energy-dispersion relation down to arbitrarily small excitations energies,
cannot be observed in any real material.Comment: 6 pages, 3 figure
Spin echo in spinor dipolar Bose-Einstein condensates
We theoretically propose and numerically realize spin echo in a spinor
Bose--Einstein condensate (BEC). We investigate the influence on the spin echo
of phase separation of the condensate. The equation of motion of the spin
density exhibits two relaxation times. We use two methods to separate the
relaxation times and hence demonstrate a technique to reveal magnetic
dipole--dipole interactions in spinor BECs.Comment: 4 pages, 5 figure
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
Observation of Coherent Precession of Magnetization in Superfluid 3He A-phase
We report the first observation of coherent quantum precession of
magnetization (CQP) in superfluid 3He-A in aerogel. The coherent precession in
bulk 3He A-phase is unstable due to the positive feedback of spin supercurrent
to the gradient of phase of precession. It was predicted that the homogeneous
precession will be stable if the orbital momentum of 3He-A could be oriented
along the magnetic field. We have succeeded to prepare this configuration by
emerging 3He in uniaxially-deformed anisotropic aerogel. The dissipation rate
of coherent precession states in aerogel is much larger then one in bulk 3He-B.
We propose a mechanism of this dissipation.Comment: 4 pages, 4 figure
- …