No Arabic abstract
The structure, magnetic properties, and lattice dynamics of ordered Fe-Pt alloys with three stoichiometric compositions, Fe$_3$Pt, FePt and FePt$_3$, have been investigated using the density functional theory. Additionally, the existing experimental data have been complemented by new measurements of the Fe projected phonon density of states performed for the Fe$_3$Pt and FePt$_3$ thin films using the nuclear inelastic scattering technique. The calculated phonon dispersion relations and phonon density of states have been compared with the experimental data. The dispersion curves are very well reproduced by the calculations, although, the softening of the transversal acoustic mode TA$_1$ leads to some discrepancy between the theory and experiment in Fe$_3$Pt. A very goood agreement between the measured spectra and calculations performed for the tetragonal structure derived from the soft mode may signal that the tetragonal phase with the space group $P4/mbm$ plays an important role in the martensitic transformation observed in Fe$_3$Pt. For FePt$_3$, the antiferromagnetic order appearing with decreasing temperature has been also investigated. The studies showed that the phonon density of states of FePt$_3$ very weakly depends on the magnetic configuration.
The finite-temperature magnetic properties of Fe$_x$Pd$_{1-x}$ and Co$_x$Pt$_{1-x}$ alloys have been investigated. It is shown that the temperature-dependent magnetic behaviour of alloys, composed of originally magnetic and non-magnetic elements, cannot be described properly unless the coupling between magnetic moments at magnetic atoms (Fe,Co) mediated through the interactions with induced magnetic moments of non-magnetic atoms (Pd,Pt) is included. A scheme for the calculation of the Curie temperature ($T_C$) for this type of systems is presented which is based on the extended Heisenberg Hamiltonian with the appropriate exchange parameters $J_{ij}$ obtained from {em ab-initio} electronic structure calculations. Within the present study the KKR Greens function method has been used to calculate the $J_{ij}$ parameters. A comparison of the obtained Curie temperatures for Fe$_x$Pd$_{1-x}$ and Co$_x$Pt$_{1-x}$ alloys with experimental data shows rather good agreement.
The composition-dependent behavior of the Dzyaloshinskii-Moriya interaction (DMI), the spin-orbit torque (SOT), as well as anomalous and spin Hall conductivities of Mn$_{1-x}$Fe$_x$Ge alloys have been investigated by first-principles calculations using the relativistic multiple scattering Korringa-Kohn-Rostoker (KKR) formalism. The $D_{rm xx}$ component of the DMI exhibits a strong dependence on the Fe concentration, changing sign at $x approx 0.85$ in line with previous theoretical calculations as well as with experimental results demonstrating the change of spin helicity at $x approx 0.8$. A corresponding behavior with a sign change at $x approx 0.5$ is predicted also for the Fermi sea contribution to the SOT, as this is closely related to the DMI. In the case of anomalous and spin Hall effects it is shown that the calculated Fermi sea contributions are rather small and the composition-dependent behavior of these effects are determined mainly by the electronic states at the Fermi level. The spin-orbit-induced scattering mechanisms responsible for both these effects suggest a common origin of the minimum of the AHE and the sign change of the SHE conductivities.
For powder samples of CuAl$_{1-x}$Fe$_x$O$_2$ ($x =$ 0, 0.01, 0.05, and 0.1), measured optical properties are compared with model simulations and phonon spectra are compared with simulations based on weighted dynamical matrix approach.
In recent years, there has been an intense interest in understanding the microscopic mechanism of thermally induced magnetization switching driven by a femtosecond laser pulse. Most of the effort has been dedicated to periodic crystalline structures while the amorphous counterparts have been less studied. By using a multiscale approach, i.e. first-principles density functional theory combined with atomistic spin dynamics, we report here on the very intricate structural and magnetic nature of amorphous Gd-Fe alloys for a wide range of Gd and Fe atomic concentrations at the nanoscale level. Both structural and dynamical properties of Gd-Fe alloys reported in this work are in good agreement with previous experiments. We calculated the dynamic behavior of homogeneous and inhomogeneous amorphous Gd-Fe alloys and their response under the influence of a femtosecond laser pulse. In the homogeneous sample, the Fe sublattice switches its magnetization before the Gd one. However, the temporal sequence of the switching of the two sublattices is reversed in the inhomogeneous sample. We propose a possible explanation based on a mechanism driven by a combination of the Dzyaloshiskii-Moriya interaction and exchange frustration, modeled by an antiferromagnetic second-neighbour exchange interaction between Gd atoms in the Gd-rich region. We also report on the influence of laser fluence and damping effects in the all-thermal switching.
We study magnon properties in terms of spin stiffness, Curie temperatures and magnon spectrum of Fe-Ni, Co-Ni and Fe-Co random alloys using a combination of electronic structure calculations and atomistic spin dynamics simulations. Influence of the disorder are studied in detail by use of large supercells with random atomic arrangement. It is found that disorder affects the magnon spectrum in vastly different ways depending on the system. Specifically, it is more pronounced in Fe-Ni alloys compared to Fe-Co alloys. In particular, the magnon spectrum at room temperature in Permalloy (Fe$_{20}$Ni$_{80}$) is found to be rather diffuse in a large energy interval while in Fe$_{75}$Co$_{25}$ it forms sharp branches. Fe-Co alloys are very interesting from a technological point of view due to the combination of large Curie temperatures and very low calculated Gilbert damping of $sim$0.0007 at room temperature for Co concentrations around 20--30%.