Magnetization dynamics in disordered Fex_xCo1−x_{1-x} alloys : A first-principles augmented space approach and atomistic spin dynamics simulations

In this paper, we present a general method to study magnetization dynamics in chemically disordered alloys. This computationally feasible technique, which seamlessly combines three approaches : the density functional based linear muffin-tin orbitals (LMTO) for self-consistently obtaining a sparse Hamiltonian; the generalized recursion method to obtain the one and two-particle Green functions and augmented space approach to deal with disorder averaging. The same formalism applied to both spectral and response properties should make the errors compatible in different studies. %The underlying computational routines are optimized and parallelized for ease of handling. We have demonstrated a successful application to the binary chemically disordered Fe$_x$Co$_{1-x}$ alloys to explain several experimental features in magnon spectra. Our study captures significant magnon softening due to magnon-electron scattering for chemically disordered Fe$_x$Co$_{1-x}$ alloys within linear spin wave regime. As a complementary study, we have done atomistic spin dynamics simulations by solving Landau-Lifshitz-Gilbert equation with parameters obtained from ab initio multiple scattering theory to compare with the results obtained from augmented space approach.Comment: arXiv admin note: text overlap with arXiv:1102.4551, arXiv:1304.7091 by other author

All-thermal switching of amorphous Gd-Fe alloys: analysis of structural properties and magnetization dynamics

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.Comment: Accepted in Physical Review B as a regular article. It contains 14 pages and 14 figure

Optical Properties of Materials Calculated from First Principles Theory

In this project work, we performed ab-inito calculations for 20 different non-magnetic materials (band gap ranging between 0.5 eV to 13 eV ) and three different magnetic materials such as NiO, EuO and GdN using density functional theory (DFT). Generalized gradient approximation (PBE) and hybrid functional (HSE06, PBE0) within projector-augmented wave (PAW) methodology were adopted to investigate the electronic properties, whereas only PBE approximation was used to study optical properties of these materials. Furthermore, for magnetic materials, PBE+U method was employed to treat the strongly correlated d and f electrons. Subtle difference in f electron state at the Fermi level in EuO and GdN for different approximations was thoroughly evaluated here. Using HSE06 we have showed that for non-magnetic materials the band-gap values were comparable with the experimental values. ForEuO we have observed a band-gap of 0.8 eV by using PBE+U approximation. However, in the HSE06 approximation, no band-gap was observed at Fermi level. The optical properties for non-magnetic systems were evaluated by calculating the dynamic dielectric functions such as absorption, reflection and energy-loss spectroscopy with the help of self-developed numerical codes. The static dielectric matrices of materials were calculated using density functional perturbation theory. The static dielectric constant values were calculated by different approaches: i) by including local field effects in both DFT and random phase approximations (RPA) and ii) excluding local field effects and including local field effects in DFT. The static constant values of the materials including local field effects for RPA approximation yielded better results than other methods

Optical Properties of Materials Calculated from First Principles Theory

In this project work, we performed ab-inito calculations for 20 different non-magnetic materials (band gap ranging between 0.5 eV to 13 eV ) and three different magnetic materials such as NiO, EuO and GdN using density functional theory (DFT). Generalized gradient approximation (PBE) and hybrid functional (HSE06, PBE0) within projector-augmented wave (PAW) methodology were adopted to investigate the electronic properties, whereas only PBE approximation was used to study optical properties of these materials. Furthermore, for magnetic materials, PBE+U method was employed to treat the strongly correlated d and f electrons. Subtle difference in f electron state at the Fermi level in EuO and GdN for different approximations was thoroughly evaluated here. Using HSE06 we have showed that for non-magnetic materials the band-gap values were comparable with the experimental values. ForEuO we have observed a band-gap of 0.8 eV by using PBE+U approximation. However, in the HSE06 approximation, no band-gap was observed at Fermi level. The optical properties for non-magnetic systems were evaluated by calculating the dynamic dielectric functions such as absorption, reflection and energy-loss spectroscopy with the help of self-developed numerical codes. The static dielectric matrices of materials were calculated using density functional perturbation theory. The static dielectric constant values were calculated by different approaches: i) by including local field effects in both DFT and random phase approximations (RPA) and ii) excluding local field effects and including local field effects in DFT. The static constant values of the materials including local field effects for RPA approximation yielded better results than other methods

Magnetization dynamics of complex magnetic materials by atomistic spin dynamics simulations

In recent years, there has been an intense interest in understanding the microscopic mechanism of laser induced ultrafast magnetization dynamics in picosecond time scales. Magnetization switching on such a time scale has potential to be a significant boost for the data storage industry. It is expected that the writing process will become ~1000 times faster by this technology, compared to existing techniques. Understanding the microscopic mechanisms and controlling the magnetization in such a time scale is of paramount importance at present. In this thesis, laser induced ultrafast magnetization dynamics has been studied for Fe, Co, GdFe, CoMn and Heusler alloys. A multiscale approach has been used, i.e., first-principles density functional theory combined with atomistic spin dynamics utilizing the Landau –Lifshitz-Gilbert equation, along with a three-temperature phenomenological model to obtain the spin temperature. Special attention has been paid to the calculations of exchange interaction and Gilbert damping parameters. These parameters play a crucial role in determining the ultrafast magnetization dynamics under laser fluence of the considered materials. The role of longitudinal and transversal excitations was studied for elemental ferromagnets, such as Fe and Co. A variety of complex temporal behavior of the magnetic properties was observed, which can be understood from the interplay between electron, spin, and lattice subsystems. 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 was studied. We have shown that the ultrafast thermal switching process can happen above the compensation temperature in GdFe alloys. It is demonstrated that the exchange frustration via Dzyaloshinskii-Moriya interaction between the atomic Gd moments, in Gd rich area of these alloys, leads to Gd demagnetization faster than the Fe sublattice. In addition, we show that Co is a perfect Heisenberg system. Both Co and CoMn alloys have been investigated with respect to ultrafast magnetization dynamics. Also, it is predicted that ultrafast switching process can happen in the Heulser alloys when they are doped with heavy elements. Finally, we studied multiferroic CoCr2O4 and Ca3CoMnO4 systems by using the multiscale approach to study magnetization dynamics. In summary, our approach is able to capture crucial details of ultrafast magnetization dynamics in technologically important materials

Magnetization Dynamics in Fe_xCo_1-x in Presence of Chemical Disorder

In this paper, we present a theoretical formulation of magnetization dynamics in disordered binary alloys, based on the Kubo linear response theory, interfaced with a seamless combination of three approaches: density functional-based tight-binding linear muffin-tin orbitals, generalized recursion and augmented space formalism. We applied this method to study the magnetization dynamics in chemically disordered FexCo1−x (x = 0.2, 0.5, 0.8) alloys. We found that the magnon energies decreased with an increase in Co concentration. Significant magnon softening was observed in Fe20Co80 at the Brillouin zone boundary. Magnon–electron scattering increased with increasing Co content, which in turn modified the hybridization between the Fe and Co atoms. This reduced the exchange energy between the atoms and softened down the magnon energy. The lowest magnon lifetime was found in Fe50Co50, where disorder was at a maximum. This clearly indicated that the damping of magnon energies in FexCo1−x was governed by hybridization between Fe and Co, whereas the magnon lifetime was controlled by disorder configuration. Our atomistic spin dynamics simulations show reasonable agreement with our theoretical approach in magnon dispersion for different alloy compositions

Magnetization Dynamics in FexCo1-x in Presence of Chemical Disorder

In this paper, we present a theoretical formulation of magnetization dynamics in disordered binary alloys, based on the Kubo linear response theory, interfaced with a seamless combination of three approaches: density functional-based tight-binding linear muffin-tin orbitals, generalized recursion and augmented space formalism. We applied this method to study the magnetization dynamics in chemically disordered FexCo1−x (x = 0.2, 0.5, 0.8) alloys. We found that the magnon energies decreased with an increase in Co concentration. Significant magnon softening was observed in Fe20Co80 at the Brillouin zone boundary. Magnon–electron scattering increased with increasing Co content, which in turn modified the hybridization between the Fe and Co atoms. This reduced the exchange energy between the atoms and softened down the magnon energy. The lowest magnon lifetime was found in Fe50Co50, where disorder was at a maximum. This clearly indicated that the damping of magnon energies in FexCo1−x was governed by hybridization between Fe and Co, whereas the magnon lifetime was controlled by disorder configuration. Our atomistic spin dynamics simulations show reasonable agreement with our theoretical approach in magnon dispersion for different alloy compositions