100 research outputs found
Ab initio theory of electron-phonon mediated ultrafast spin relaxation of laser-excited hot electrons in transition-metal ferromagnets
We report a computational theoretical investigation of electron spin-flip
scattering induced by the electron-phonon interaction in the transition-metal
ferromagnets bcc Fe, fcc Co and fcc Ni. The Elliott-Yafet electron-phonon
spin-flip scattering is computed from first-principles, employing a generalized
spin-flip Eliashberg function as well as ab initio computed phonon dispersions.
Aiming at investigating the amount of electron-phonon mediated demagnetization
in femtosecond laser-excited ferromagnets, the formalism is extended to treat
laser-created thermalized as well as nonequilibrium, nonthermal hot electron
distributions. Using the developed formalism we compute the phonon-induced spin
lifetimes of hot electrons in Fe, Co, and Ni. The electron-phonon mediated
demagnetization rate is evaluated for laser-created thermalized and
nonequilibrium electron distributions. Nonthermal distributions are found to
lead to a stronger demagnetization rate than hot, thermalized distributions,
yet their demagnetizing effect is not enough to explain the experimentally
observed demagnetization occurring in the subpicosecond regime.Comment: 14 pages, 8 figures, to appear in PR
Influence of laser-excited electron distributions on the x-ray magnetic circular dichroism spectra: Implications for femtosecond demagnetization in Ni
In pump-probe experiments an intensive laser pulse creates non-equilibrium
excited electron distributions in the first few hundred femtoseconds after the
pulse. The influence of non-equilibrium electron distributions caused by a pump
laser on the apparent X-ray magnetic circular dichroism (XMCD) signal of Ni is
investigated theoretically here for the first time, considering electron
distributions immediately after the pulse as well as thermalized ones, that are
not in equilibrium with the lattice or spin systems. The XMCD signal is shown
not to be simply proportional to the spin momentum in these situations. The
computed spectra are compared to recent pump-probe XMCD experiments on Ni. We
find that the majority of experimentally observed features considered to be a
proof of ultrafast spin momentum transfer to the lattice can alternatively be
attributed to non-equilibrium electron distributions. Furthermore, we find the
XMCD sum rules for the atomic spin and orbital magnetic moment to remain valid,
even for the laser induced non-equilibrium electron distributions.Comment: 6 pages, 3 figure
Atomistic simulations of magnetoelastic effects on sound velocity
In this work, we leverage atomistic spin-lattice simulations to examine how
magnetic interactions impact the propagation of sound waves through a
ferromagnetic material. To achieve this, we characterize the sound wave
velocity in BCC iron, a prototypical ferromagnetic material, using three
different approaches that are based on the oscillations of kinetic energy,
finite-displacement derived forces, and corrections to the elastic constants,
respectively. Successfully applying these methods within the spin-lattice
framework, we find good agreement with the Simon effect including high order
terms. In analogy to experiments, morphic coefficients associated with the
transverse and longitudinal waves propagating along the [001] direction are
extracted from fits to the fractional change in velocity data. The present
efforts represent an advancement in magnetoelastic modelling capabilities which
can expedite the design of future magneto-acoustic devices
Ab-initio investigation of phonon dispersion and anomalies in palladium
In recent years, palladium has proven to be a crucial component for devices
ranging from nanotube field effect transistors to advanced hydrogen storage
devices. In this work, I examine the phonon dispersion of fcc Pd using first
principle calculations based on density functional perturbation theory. While
several groups in the past have studied the acoustic properties of palladium,
this is the first study to reproduce the phonon dispersion and associated
anomaly with high accuracy and no adjustable parameters. In particular, I focus
on the Kohn anomaly in the [110] direction.Comment: 19 pages, preprint format, 7 figures, added new figures and
discussio
M\"ossbauer studies of spin- and charge-modulations in BaFe2(As1-xPx)2
The BaFe2(As1-xPx)2 compounds with x = 0 (parent), x = 0.10 (under-doped), x
= 0.31, 0.33, 0.53 (superconductors with Tc = 27.3 K, 27.6 K, 13.9 K,
respectively) and x = 0.70, 0.77 (over-doped) have been investigated versus
temperature using 57Fe M\"ossbauer spectroscopy. Special attention was paid to
regions of the spin-density-wave (SDW) antiferromagnetic order, spin-nematic
phase, and superconducting transition. The BaFe2(As0.90P0.10)2 compound
exhibits a reduced amplitude of SDW as compared to the parent compound and
preserved universality class of two-dimensional magnetic planes with
one-dimensional spins. The spin-nematic phase region for x = 0.10 is
characterized by an incoherent magnetic order. BaFe2(As0.69P0.31)2 shows
coexistence of a weak magnetic order and superconductivity due to the vicinity
of the quantum critical point. The charge density modulations in the
BaFe2(As0.67P0.33)2 and BaFe2(As0.47P0.53)2 superconductors are perturbed near
Tc. Pronounced hump of the average quadrupole splitting across superconducting
transition is observed for the system with x = 0.33. The phosphorus
substitution increases the Debye temperature of the BaFe2(As1-xPx)2 compound.
Moreover, experimental electron charge densities at Fe nuclei in this material
conclusively show that it should be recognized as a hole-doped system. The
measured M\"ossbauer spectral shift and spectral area are not affected by
transition to the superconducting state. This indicates that neither the
average electron density at Fe nuclei nor the dynamical properties of the
Fe-sublattice in BaFe2(As1-xPx)2 are sensitive to the superconducting
transition. Theoretical calculations of hyperfine parameters determining the
patterns of M\"ossbauer spectra of BaFe2(As1-xPx)2 with x = 0, 0.31, 0.5, and
1.0 are performed within the framework of the density functional theory
MAELAS 2.0: A new version of a computer program for the calculation of magneto-elastic properties
MAELAS is a computer program for the calculation of magnetocrystalline
anisotropy energy, anisotropic magnetostrictive coefficients and magnetoelastic
constants in an automated way. The method originally implemented in version 1.0
of MAELAS was based on the length optimization of the unit cell, proposed by Wu
and Freeman, to calculate the anisotropic magnetostrictive coefficients. We
present here a revised and updated version (v2.0) of MAELAS, where we added a
new methodology to compute anisotropic magnetoelastic constants from a linear
fitting of the energy versus applied strain. We analyze and compare the
accuracy of both methods showing that the new approach is more reliable and
robust than the one implemented in version 1.0, especially for non-cubic
crystal symmetries. This analysis also help us to find that the accuracy of the
method implemented in version 1.0 could be improved by using deformation
gradients derived from the equilibrium magnetoelastic strain tensor, as well as
potential future alternative methods like the strain optimization method.
Additionally, we clarify the role of the demagnetized state in the fractional
change in length, and derive the expression for saturation magnetostriction for
polycrystals with trigonal, tetragonal and orthorhombic crystal symmetry. In
this new version, we also fix some issues related to trigonal crystal symmetry
found in version 1.0
Automated calculations of exchange magnetostriction
We present a methodology based on deformations of the unit cell that allows
to compute the isotropic magnetoelastic constants, isotropic magnetostrictive
coefficients and spontaneous volume magnetostriction associated to the exchange
magnetostriction. This method is implemented in the python package MAELAS
(v3.0), so that it can be used to obtain these quantities by first-principles
calculations and classical spin-lattice models in an automated way. We show
that the required reference state to obtain the spontaneous volume
magnetostriction combines the equilibrium volume of the paramagnetic state and
magnetic order of the ground state. The presented computational tool may be
helpful to provide a better understanding and characterization of the
relationship between the exchange interaction and magnetoelasticity
Crystal field splitting is limiting the stability and strength of ultra-incompressible orthorhombic transition metal tetraborides
PubMed ID: 26976479The lattice stability and mechanical strengths of the supposedly superhard transition metal tetraborides (TmB4, Tm = Cr, Mn and Fe) evoked recently much attention from the scientific community due to the potential applications of these materials, as well as because of general scientific interests. In the present study, we show that the surprising stabilization of these compounds from a high symmetry to a low symmetry structure is accomplished by an in-plane rotation of the boron network, which maximizes the in-plane hybridization by crystal field splitting between d orbitals of Tm and p orbitals of B. Studies of mechanical and electronic properties of TmB4 suggest that these tetraborides cannot be intrinsically superhard. The mechanical instability is facilitated by a unique in-plane or out-of-plane weakening of the three-dimensional covalent bond network of boron along different shear deformation paths. These results shed a novel view on the origin of the stability and strength of orthorhombic TmB4, highlighting the importance of combinational analysis of a variety of parameters related to plastic deformation of the crystalline materials when attempting to design new ultra-incompressible, and potentially strong and hard solids.Web of Science6art. no. 2308
Elastic constants and volume changes associated with two high-pressure rhombohedral phase transformations in vanadium
We present results from ab initio calculations of the mechanical properties
of the rhombohedral phase (beta) of vanadium metal reported in recent
experiments, and other predicted high-pressure phases (gamma and bcc), focusing
on properties relevant to dynamic experiments. We find that the volume change
associated with these transitions is small: no more than 0.15% (for beta -
gamma). Calculations of the single crystal and polycrystal elastic moduli
(stress-strain coefficients) reveal a remarkably small discontinuity in the
shear modulus and other elastic properties across the phase transitions even at
zero temperature where the transitions are first order.Comment: 6 pages, 3 figure
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