arXiv subjects

Samara Keshavarz

Publications and source records attributed to Samara Keshavarz.

5 recordsLinked to original sources

arXiv2018

A systematic study of the local minima in L(S)DA+U

We have performed a systematic study of the emergence of meta-stable states in density functional theory plus Hubbard U (DFT+U ) simulations of NiO, CoO, FeO. Particular attention is given to the spin-polarization of the exchange-correlation functional and the double counting term, and the role of the spin-orbit coupling. The method of occupation matrix control is extended to use constrained random density matrices to map out the local minima in the total energy landscape. The extended scheme, random density matrix control, is successfully benchmarked against UO2, one of the most investigated systems in the field. When applied to the transition metal oxides it yields several meta- stable states which are well-characterized by their local spin and orbital moments. We find that the addition of spin-orbit coupling helps the simulations to converge to the global high-spin energy minimum. The random density matrix control scheme combined with LDA+U yields accurate magnetic moments for all the studied AFM transition metal oxides.

cond-mat.mtrl-sci↗

arXiv2018

Electronic structure, magnetism and exchange integrals in transition metal oxides: role of the spin polarization of the functional in DFT+$U$ calculations

Density functional theory augmented with Hubbard-$U$ corrections (DFT+$U$) is currently one of the widely used methods for first-principles electronic structure modeling of insulating transition metal oxides (TMOs). Since $U$ is relatively large compared to band widths, the magnetic excitations in TMOs are expected to be well described by a Heisenberg model. However, in practice the calculated exchange parameters $J_{ij}$ depend on the magnetic configuration from which they are extracted and on the functional used to compute them. In this work we investigate how the spin polarization dependence of the underlying exchange-correlation functional influences the calculated magnetic exchange constants of TMOs. We perform a systematic study of the predictions of calculations based on the local density approximation plus $U$ (LDA+$U$) and the local spin density approximation plus $U$ (LSDA+$U$) for the electronic structures, total energies and magnetic exchange interactions $J_{ij}$'s extracted from ferromagnetic (FM) and antiferromagnetic (AFM) configurations of several transition metal oxide materials. We report that, for realistic choices of Hubbard $U$ and Hund's $J$ parameters, LSDA+$U$ and LDA+$U$ calculations result in different values of the magnetic exchange constants and band gap. The dependence of the band gap on the magnetic configuration is stronger in LDA+$U$ than in LSDA+$U$ and we argue that this is the main reason why the configuration dependence of the $J_{ij}$'s is found to be systematically more pronounced in LDA+$U$ than in LSDA+$U$ calculations. We report a very good correspondence between the computed total energies and the parameterized Heisenberg model for LDA+$U$ calculations, but not for LSDA+$U$, suggesting that LDA+$U$ is a more appropriate method for estimating exchange interactions.

physics.comp-ph↗

arXiv2018

Half-metallicity and magnetism in the Co$_2$MnAl/CoMnVAl heterostructure

We present a study of the electronic structure and magnetism of Co$_2$MnAl, CoMnVAl and their heterostructure. We employ a combination of density-functional theory and dynamical mean-field theory (DFT+DMFT). We find that Co$_2$MnAl is a half-metallic ferromagnet, whose electronic and magnetic properties are not drastically changed by strong electronic correlations, static or dynamic. Non-quasiparticle states are shown to appear in the minority spin gap without affecting the spin-polarization at the Fermi level predicted by standard DFT. We find that CoMnVAl is a semiconductor or a semi-metal, depending on the employed computational approach. We then focus on the electronic and magnetic properties of the Co$_2$MnAl/CoMnVAl heterostructure, predicted by previous first principle calculations as a possible candidate for spin-injecting devices. We find that two interfaces, Co-Co/V-Al and Co-Mn/Mn-Al, preserve the half-metallic character, with and without including electronic correlations. We also analyse the magnetic exchange interactions in the bulk and at the interfaces. At the Co-Mn/Mn-Al interface, competing magnetic interactions are likely to favor the formation of a non-collinear magnetic order, which is detrimental for the spin-polarization.

cond-mat.mtrl-sci↗

arXiv2016

Exchange interactions of CaMnO3 in the bulk and at the surface

In this work, we present electronic and magnetic properties of CaMnO3 (CMO) as obtained from ab initio calculations. We identify the preferable magnetic order by means of density functional theory plus Hubbard U calculations and extract the effective exchange parameters (Jij's) using the magnetic force theorem. We find that the effects of geometrical relaxation at the surface as well as the change of crystal field are very strong and are able to influence the lower energy magnetic configuration. In particular, our analysis reveals that the exchange interaction between the Mn atoms belonging to the surface and the subsurface layers is very sensitive to the structural changes. An earlier study [A. Filippetti and W.E. Pickett, Phys. Rev. Lett. 83, 4184 (1999)] suggested that this coupling is ferromagnetic and gives rise to the spin flip process on the surface of CMO. In our work we confirm their finding for an unrelaxed geometry, but once the structural relaxations are taken into account, this exchange coupling changes its sign. Thus, we suggest that the surface of CMO should have the same G-type antiferromagnetic order as in the bulk. Finally, we show that the suggested SF can be induced in the system by introducing an excess of electrons.

physics.comp-ph↗

arXiv2016

First principles studies of the Gilbert damping and exchange interactions for half-metallic Heuslers alloys

Heusler alloys have been intensively studied due to the wide variety of properties that they exhibit. One of these properties is of particular interest for technological applications, i.e. the fact that some Heusler alloys are half-metallic. In the following, a systematic study of the magnetic properties of three different Heusler families $\textrm{Co}_2\textrm{Mn}\textrm{Z}$, $\text{Co}_2\text{Fe}\text{Z}$ and $\textrm{Mn}_2\textrm{V}\textrm{Z}$ with $\text{Z}=\left(\text{Al, Si, Ga, Ge}\right)$ is performed. A key aspect is the determination of the Gilbert damping from first principles calculations, with special focus on the role played by different approximations, the effect that substitutional disorder and temperature effects. Heisenberg exchange interactions and critical temperature for the alloys are also calculated as well as magnon dispersion relations for representative systems, the ferromagnetic $\textrm{Co}_2\textrm{Fe}\textrm{Si}$ and the ferrimagnetic $\textrm{Mn}_2\textrm{V}\textrm{Al}$. Correlations effects beyond standard density-functional theory are treated using both the local spin density approximation including the Hubbard $U$ and the local spin density approximation plus dynamical mean field theory approximation, which allows to determine if dynamical self-energy corrections can remedy some of the inconsistencies which were previously reported for these alloys.

cond-mat.mtrl-sci↗