51 research outputs found
Magnetostatics and the rotational sense of cycloidal spin spirals
The magnetostatic energy of a cycloidal spin-spiral configuration is calculated. The free-standing spiral is compared to the case of spirals that are brought in contact to a magnetically polarizable substrate. While a free-standing layer is energetically degenerate with respect to the spiral's sense of rotation, it is shown that a polarizable substrate breaks this symmetry and lifts the degeneracy. Consequently, a strongly polarizable substrate can stabilize (destabilize) a spin spiral that would be unstable (stable) without considering the magnetostatics of the substrateFunding by CONSOLIDER-INGENIO EN NANOCIENCIA MOLECULAR (ref. CSD2007 − 00010), by the Comunidad de Madrid through Project No. S2009/MAT-1726, and the Deutsche Forschungsgemeinschaft in the framework of SFB 668 is acknowledge
Substrate polarization effects in two-dimensional magnetic arrays
The magnetostatic energy of a two-dimensional (2D) periodic array of magnetic particles (or a thin film with periodic magnetization) is evaluated, including additional energy terms due to a polarizable substrate. The polarization of the substrate is solved self-consistently using surface charges. This requires describing the magnetic potential of the 2D array in terms of an equivalent surface charge distribution. Analytic expressions for the magnetostatic self-energy of the 2D array as well as the energy due to the interaction of the magnetic structure and polarizable substrate are presented. It is shown how substrates with large susceptibility significantly alter the stray-field energy and, hence, the magnetic properties of the array, even promoting a spin-reorientation transition. Our results suggest that system properties can be controlled in a simple way by exploiting substrates with tunable polarizabilityWe acknowledge funding by Consolider-Ingenio en Nanociencia Molecular Ref. No. CSD2007-00010, by the Comunidad de Madrid through Project No. S2009/MAT-1726,
and Project No. FIS 2010-18847 from MICIN
Tailoring magnetic anisotropy in epitaxial half metallic La0.7Sr0.3MnO3 thin films
We present a detailed study on the magnetic properties, including anisotropy,
reversal fields, and magnetization reversal processes, of well characterized
half-metallic epitaxial La0.7Sr0.3MnO3 (LSMO) thin films grown onto SrTiO3
(STO) substrates with three different surface orientations, i.e. (001), (110)
and (1-18). The latter shows step edges oriented parallel to the [110]
(in-plane) crystallographic direction. Room temperature high resolution
vectorial Kerr magnetometry measurements have been performed at different
applied magnetic field directions in the whole angular range. In general, the
magnetic properties of the LSMO films can be interpreted with just the uniaxial
term with the anisotropy axis given by the film morphology, whereas the
strength of this anisotropy depends on both structure and film thickness. In
particular, LSMO films grown on nominally flat (110)-oriented STO substrates
presents a well defined uniaxial anisotropy originated from the existence of
elongated in-plane [001]-oriented structures, whereas LSMO films grown on
nominally flat (001)-oriented STO substrates show a weak uniaxial magnetic
anisotropy with the easy axis direction aligned parallel to residual substrate
step edges. Elongated structures are also found for LSMO films grown on vicinal
STO(001) substrates. These films present a well-defined uniaxial magnetic
anisotropy with the easy axis lying along the step edges and its strength
increases with the LSMO thickness. It is remarkable that this step-induced
uniaxial anisotropy has been found for LSMO films up to 120 nm thickness. Our
results are promising for engineering novel half-metallic magnetic devices that
exploit tailored magnetic anisotropy.Comment: 10 pages, 10 figures, 1 tabl
Spin-spin correlations in ferromagnetic nanosystems
Using exact diagonalization, Monte-Carlo, and mean-field techniques,
characteristic temperature scales for ferromagnetic order are discussed for the
Ising and the classical anisotropic Heisenberg model on finite lattices in one
and two dimensions. The interplay between nearest-neighbor exchange, anisotropy
and the presence of surfaces leads, as a function of temperature, to a complex
behavior of the distance-dependent spin-spin correlation function, which is
very different from what is commonly expected. A finite experimental
observation time is considered in addition, which is simulated within the
Monte-Carlo approach by an incomplete statistical average. We find strong
surface effects for small nanoparticles, which cannot be explained within a
simple Landau or mean-field concept and which give rise to characteristic
trends of the spin-correlation function in different temperature regimes.
Unambiguous definitions of crossover temperatures for finite systems and an
effective method to estimate the critical temperature of corresponding infinite
systems are given.Comment: 7 pages, 5 figures, EPJB (in press
First-principles quantum transport modeling of spin-transfer and spin-orbit torques in magnetic multilayers
We review a unified approach for computing: (i) spin-transfer torque in
magnetic trilayers like spin-valves and magnetic tunnel junction, where
injected charge current flows perpendicularly to interfaces; and (ii)
spin-orbit torque in magnetic bilayers of the type
ferromagnet/spin-orbit-coupled-material, where injected charge current flows
parallel to the interface. Our approach requires to construct the torque
operator for a given Hamiltonian of the device and the steady-state
nonequilibrium density matrix, where the latter is expressed in terms of the
nonequilibrium Green's functions and split into three contributions. Tracing
these contributions with the torque operator automatically yields field-like
and damping-like components of spin-transfer torque or spin-orbit torque
vector, which is particularly advantageous for spin-orbit torque where the
direction of these components depends on the unknown-in-advance orientation of
the current-driven nonequilibrium spin density in the presence of spin-orbit
coupling. We provide illustrative examples by computing spin-transfer torque in
a one-dimensional toy model of a magnetic tunnel junction and realistic
Co/Cu/Co spin-valve, both of which are described by first-principles
Hamiltonians obtained from noncollinear density functional theory calculations;
as well as spin-orbit torque in a ferromagnetic layer described by a
tight-binding Hamiltonian which includes spin-orbit proximity effect within
ferromagnetic monolayers assumed to be generated by the adjacent monolayer
transition metal dichalcogenide.Comment: 22 pages, 9 figures, PDFLaTeX; prepared for Springer Handbook of
Materials Modeling, Volume 2 Applications: Current and Emerging Material
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