26 research outputs found
Spin waves damping in nanometre-scale magnetic materials
Spin dynamics in magnetic nanostructured materials is a topic of great current interest. To describe spin motions
in such magnetic systems, the phenomenological Landau–Lifshitz (LL), or the LL–Gilbert (LLG), equation
is widely used. Damping term is one of the dominant features of magnetization dynamics and plays an essential
role in these equations of motion. The form of this term is simple; however, an important question arises whether
it provides a proper description of the magnetization coupling to the thermal bath and the related magnetic fluctuations
in the real nanometre-scale magnetic materials. It is now generally accepted that for nanostructured systems
the damping term in the LL (LLG) equation fails to account for the systematics of the magnetization relaxation,
even at the linear response level. In ultrathin films and nanostructured magnets particular relaxation
mechanisms arise, extrinsic and intrinsic, which are relevant at nanometre-length scales, yet are not so efficient
in bulk materials. These mechanisms of relaxation are crucial for understanding the magnetization dynamics that
results in a linewidth dependence on the nanomagnet’s size. We give an overview of recent efforts regarding the
description of spin waves damping in nanostructured magnetic materials. Three types of systems are reviewed:
ultrathin and exchange-based films, magnetic nanometre-scale samples and patterned magnetic structures. The
former is an example of a rare case where consideration can be done analytically on microscopic footing. The
latter two are typical samples when analytical approaches hardly have to be developed and numerical calculations
are more fruitful. Progress in simulations of magnetization dynamics in nanometre-scale magnets gives
hopes that a phenomenological approach can provide us with a realistic description of spin motions in expanding
diverse of magnetic nanostructures
Electrically active magnetic excitations in antiferromagnets (Review Article)
The magnetic resonance operation by electric field is highly nontrivial but the most demanding function in
the future spin-electronics. Recently observed in a variety of multiferroics materials named the collective electrically
active magnetic excitations, frequently referred to as “electromagnons”, reveal a possible way to implement
such a function. Experimental advances in terahertz spectroscopy of electromagnons in multiferroics as well as
related theoretical models are reviewed. The earlier theoretical works, where the existence of electric-dipole active
magnetic excitations in antiferro- and ferrimagnets with collinear spin structure has been predicted, are also
discussed. Multi-sublattice magnets with electrically active magnetic excitations at room temperature give a direct
possibility to transform one type of excitation into another in a terahertz time-domain. This is of crucial importance
for the magnon-based spintronics as only the short-wavelength exchange magnons allow the signal
processing on the nanoscale distance
Enhancement of the Josephson current by magnetic field in superconducting tunnel structures with a paramagnetic spacer
The dc Josephson critical current of a (S/M)IS tunnel structure in a parallel magnetic field is
investigated (here S is a superconductor, S/M is a proximity-coupled S and paramagnetic metal M
bilayer, and I is an insulating barrier). We consider the case when, due to Hund’s rule, in the
metal M the effective molecular interaction aligns the spins of the conduction electrons
antiparallel to the localized spins of magnetic ions. It is predicted that for the tunnel structures
under consideration there are conditions such that the destructive action of the internal and the
applied magnetic fields on Cooper pairs is weakened, and increase of the applied magnetic field
causes field-induced enhancement of the critical tunnel current. The experimental realization of
this interesting effect of the interplay between superconductivity and magnetism is also discussed
Point-contact Andreev-reflection spectroscopy of doped manganites: Charge carrier spin-polarization and proximity effects (Review Article)
Materials with spin-polarized charge carriers are the most demanded in the spin-electronics. Particularly requested
are the so-called half-metals which have the maximum attainable value of carrier spin polarization. Doped
manganites are in the list of compounds with, potentially, half-metallic properties. The point-contact (PC) Andreevreflection
(AR) spectroscopy is a robust and direct method to measure the degree of current spin polarization. In this
report, advances in PCAR spectroscopy of ferromagnetic manganites are reviewed. The experimental results obtained
on “classic” s-wave superconductor — ferromagnetic manganites PCs, as well as related theoretical models
applied to deduce the actual value of charge carrier spin-polarization, are discussed. Data obtained on “proximity affected”
contacts is also outlined. Systematic and repeatable nature of a number of principal experimental facts detected
in the AR spectrum of proximity affected contacts suggests that some new physical phenomena have been
documented here. Different models of current flow through a superconductor–half-metal ferromagnet interface, as
well as possibility of unconventional superconducting proximity effect, have been discussed
Andreev-spectroscopy study of unconventional superconductivity in MgB₂:(La,Sr)MnO₃ nanocomposite
Unconventional high-temperature superconductivity in MgB₂:La₀:₆₅Sr₀:₃₅MnO₃ (MgB:LSMO) nanocomposite
has been found recently [Phys. Rev. B 86, 10502 (2012)]. In this report, the symmetry of the nanocomposite
superconducting order parameter and plausible pairing mechanisms have been studied by the point-contact
Andreev-reflection (PCAR) spectroscopy. To clarify the experimental results obtained, we consider a model
of a ferromagnetic superconductor, which assumes a coexistence of itinerant ferromagnetism and mixed-parity
superconductivity. The Balian–Werthamer state, with quasiparticle gap topology of the same form as that
of the ordinary s-wave state, fits the experimental data reasonably well. Utilizing the extended Eliashberg formalizm,
we calculated the contribution of MgB₂ in the total composite’s conductivity and estimated the magnitude
of the electron–phonon effects originated from MgB₂ in I–V characteristics of the composite at above-gap
energies. It was found that distinctive features observed in the PC spectra of the MgB:LSMO samples and conventionally
attributed to the electron–phonon interaction cannot be related to the MgB₂ phonons. It is argued that
the detected singularities may be a manifestation of the electron-spectrum renormalizations due to strong
magnetoelastic (magnon–phonon) interaction in LSMO
Layered ferromagnet-superconductor structures: the state and proximity effects
We investigate clean mutilayered structures of the SFS and SFSFS type, (where
the S layer is intrinsically superconducting and the F layer is ferromagnetic)
through numerical solution of the self-consistent Bogoliubov-de Gennes
equations for these systems. We obtain results for the pair amplitude, the
local density of states, and the local magnetic moment. We find that as a
function of the thickness of the magnetic layers separating adjacent
superconductors, the ground state energy varies periodically between two stable
states. The first state is an ordinary "0-state", in which the order parameter
has a phase difference of zero between consecutive S layers, and the second is
a "-state", where the sign alternates, corresponding to a phase difference
of between adjacent S layers. This behavior can be understood from simple
arguments. The density of states and the local magnetic moment reflect also
this periodicity.Comment: 12 pages, 10 Figure
Inhomogeneous magnetism induced in a superconductor at superconductor-ferromagnet interface
We study a magnetic proximity effect at superconductor (S) - ferromagnet (F)
interface. It is shown that due to an exchange of electrons between the F and S
metals ferromagnetic correlations extend into the superconductor, being
dependent on interface parameters. We show that ferromagnetic exchange field
pair breaking effect leads to a formation of subgap bands in the S layer local
density of states, that accommodate only one spin-polarized quasiparticles.
Equilibrium magnetization leakage into the S layer as function of SF interface
quality and a value of ferromagnetic interaction have also been calculated. We
show that a damped-oscillatory behavior versus distance from SF interface is a
distinguished feature of the exchange-induced magnetization of the S layer.Comment: 10 pages, 7 Postscript figure
Unconventional ferromagnetism and transport properties of (In,Mn)Sb dilute magnetic semiconductor
Narrow-gap higher mobility semiconducting alloys In_{1-x}Mn_{x}Sb were
synthesized in polycrystalline form and their magnetic and transport properties
have been investigated. Ferromagnetic response in In_{0.98}Mn_{0.02}Sb was
detected by the observation of clear hysteresis loops up to room temperature in
direct magnetization measurements. An unconventional (reentrant) magnetization
versus temperature behavior has been found. We explained the observed
peculiarities within the frameworks of recent models which suggest that a
strong temperature dependence of the carrier density is a crucial parameter
determining carrier-mediated ferromagnetism of (III,Mn)V semiconductors. The
correlation between magnetic states and transport properties of the sample has
been discussed. The contact spectroscopy method is used to investigate a band
structure of (InMn)Sb near the Fermi level. Measurements of the degree of
charge current spin polarization have been carried out using the point contact
Andreev reflection (AR) spectroscopy. The AR data are analyzed by introducing a
quasiparticle spectrum broadening, which is likely to be related to magnetic
scattering in the contact. The AR spectroscopy data argued that at low
temperature the sample is decomposed on metallic ferromagnetic clusters with
relatively high spin polarization of charge carriers (up to 65% at 4.2K) within
a cluster.Comment: 19 pages, 9 figures, 1 tabl
Proximity and Josephson effects in superconductor - antiferromagnetic Nb / \gamma-Fe50Mn50 heterostructures
We study the proximity effect in superconductor (S), antiferromagnetic (AF)
bilayers, and report the fabrication and measurement of the first trilayer
S/AF/S Josephson junctions. The disordered f.c.c. alloy \gamma-Fe50Mn50 was
used as the AF, and the S is Nb. Micron and sub-micron scale junctions were
measured, and the scaling of gives a coherence length in the AF of
2.4 nm, which correlates with the coherence length due to suppression of
in the bilayer samples. The diffusion constant for FeMn was found to be 1.7
\times 10 m s, and the density of states at the Fermi level was
also obtained. An exchange biased FeMn/Co bilayer confirms the AF nature of the
FeMn in this thickness regime.Comment: 6 pages, 5 figures, accepted for Phys. Rev.
Manifestation of triplet superconductivity in superconductor-ferromagnet structures
We study proximity effects in a multilayered superconductor/ferromagnet (S/F)
structure with arbitrary relative directions of the magnetization . If
the magnetizations of different layers are collinear the superconducting
condensate function induced in the F layers has only a singlet component and a
triplet one with a zero projection of the total magnetic moment of the Cooper
pairs on the direction. In this case the condensate penetrates the F
layers over a short length determined by the exchange energy . If
the magnetizations are not collinear the triplet component has, in
addition to the zero projection, the projections . The latter component
is even in the momentum, odd in the Matsubara frequency and penetrates the F
layers over a long distance that increases with decreasing temperature and does
not depend on (spin-orbit interaction limits this length). If the thickness
of the F layers is much larger than , the Josephson coupling between
neighboring S layers is provided only by the triplet component, so that a new
type of superconductivity arises in the transverse direction of the structure.
The Josephson critical current is positive (negative) for the case of a
positive (negative) chirality of the vector . We demonstrate that this
type of the triplet condensate can be detected also by measuring the density of
states in F/S/F structures.Comment: 14 pages; 9 figures. Final version, to be published in Phys. Rev.