Václav Drchal

Bio: Václav Drchal is an academic researcher from Academy of Sciences of the Czech Republic. The author has contributed to research in topics: Coherent potential approximation & Ab initio. The author has an hindex of 32, co-authored 195 publications receiving 3797 citations. Previous affiliations of Václav Drchal include University of Vienna & Centre national de la recherche scientifique.

Ab initio calculations of exchange interactions, spin-wave stiffness constants, and Curie temperatures of Fe, Co, and Ni

Abstract: We have calculated Heisenberg exchange parameters for bcc Fe, fcc Co, and fcc Ni using the nonrelativistic spin-polarized Green-function technique within the tight-binding linear muffin-tin orbital method and by employing the magnetic force theorem to calculate total energy changes associated with a local rotation of magnetization directions. We have also determined spin-wave stiffness constants and found the dispersion curves for metals in question employing the Fourier transform of calculated Heisenberg exchange parameters. Detailed analysis of convergence properties of the underlying lattice sums was carried out and a regularization procedure for calculation of the spin-wave stiffness constant was suggested. Curie temperatures were calculated both in the mean-field approximation and within the Green-function random-phase approximation. The latter results were found to be in a better agreement with available experimental data.

Exchange interactions in III-V and group-IV diluted magnetic semiconductors

Abstract: Effective pair exchange interactions between Mn atoms in III-V and group-IV diluted magnetic semiconductors are determined from a two-step first-principles procedure. In the first step, the self-consistent electronic structure of a system is calculated for a collinear spin structure at zero temperature with the substitutional disorder treated within the framework of the coherent-potential approximation. The effective exchange pair interactions are then obtained in a second step by mapping the total energies associated with rotations of magnetic moments onto an effective classical Heisenberg Hamiltonian using the magnetic force theorem and one-electron Green functions. The formalism is applied to ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{As}$ alloys with and without As antisites, and to ${\mathrm{Ge}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}$ alloys recently studied experimentally. A detailed study of the behavior of pair exchange interactions as a function of the distance between magnetic atoms as well as a function of the concentrations of the magnetic atoms and compensating defects is presented. We have found that due to disorder and the half-metallic character of the system the pair exchange interactions are exponentially damped with increasing distance between the Mn atoms. The exchange interactions between Mn atoms are ferromagnetic for distances larger than the ones corresponding to the averaged nearest-neighbor Mn-Mn distance. The pair exchange interactions are also reduced with increasing concentrations of the Mn atoms and As antisites. As a simple application of the calculated exchange interactions we present mean-field estimates of Curie temperatures.

Magnetic Percolation in Diluted Magnetic Semiconductors

Abstract: We demonstrate that the magnetic properties of diluted magnetic semiconductors are dominated by short ranged interatomic exchange interactions that have a strong directional dependence By combining first principles calculations of interatomic exchange interactions with a classical Heisenberg model and Monte Carlo simulations, we reproduce the observed critical temperatures of a broad range of diluted magnetic semiconductors We also show that agreement between theory and experiment is obtained only when the magnetic atoms are randomly positioned This suggests that the ordering of diluted magnetic semiconductors is heavily influenced by magnetic percolation, and that the measured critical temperatures should be very sensitive to details in the sample preparation, in agreement with observations

Magnetism without magnetic impurities in oxides ZrO2 and TiO2

Abstract: We perform a theoretical study of the magnetism induced in transition metal dioxides ZrO2 and TiO2 by substitution of the cation by a vacancy or an impurity from the groups 1A or 2A of the periodic table, where the impurity is either K or Ca. In the present study both supercell and embedded cluster methods are used. It is demonstrated that the vacancy and the K-impurity leads to a robust induced magnetic moment on the surrounding O-atoms for both the cubic ZrO2 and rutile TiO2 host crystals. On the other hand it is shown that Ca-impurity leads to a non magnetic state. The native O-vacancy does not induce a magnetic moment in the host dioxide crystal.

Magnetism without magnetic impurities in ZrO2 oxide

Abstract: We present an ab initio study of the magnetism induced in ZrO2 dioxide by substitution of the cation by an impurity from the groups 1A or 2A of the Periodic Table (K and Ca). It is demonstrated that the K impurity leads to a robust induced magnetic moment on the surrounding O atoms in the cubic ZrO2 host whilst Ca impurity leads to a nonmagnetic groundstate. The estimated Curie temperature is above room temperature.

Electronic structure calculations with dynamical mean-field theory

Abstract: We present a review of the basic ideas and techniques of the spectral density functional theory which are currently used in electronic structure calculations of strongly{correlated materials where the one{electron description breaks down. We illustrate the method with several examples where interactions play a dominant role: systems near metal{insulator transition, systems near volume collapse transition, and systems with local moments.

Nonlocal magnetization dynamics in ferromagnetic heterostructures

Abstract: Two complementary effects modify the GHz magnetization dynamics of nanoscale heterostructures of ferromagnetic and normal materials relative to those of the isolated magnetic constituents. On the one hand, a time-dependent ferromagnetic magnetization pumps a spin angular-momentum flow into adjacent materials and, on the other hand, spin angular momentum is transferred between ferromagnets by an applied bias, causing mutual torques on the magnetizations. These phenomena are manifestly nonlocal: they are governed by the entire spin-coherent region that is limited in size by spin-flip relaxation processes. This review presents recent progress in understanding the magnetization dynamics in ferromagnetic heterostructures from first principles, focusing on the role of spin pumping in layered structures. The main body of the theory is semiclassical and based on a mean-field Stoner or spin-density-functional picture, but quantum-size effects and the role of electron-electron correlations are also discussed. A growing number of experiments support the theoretical predictions. The formalism should be useful for understanding the physics and for engineering the characteristics of small devices such as magnetic random-access memory elements.