Josef Kudrnovsky

Exchange interactions and critical temperatures in diluted magnetic semiconductors

A first-principles approach to magnetic properties of diluted magnetic semiconductors (DMS) is presented that is based on the local spin-density approximation (LSDA) as implemented in the framework of the tight-binding linear muffin-tin orbital method, while the effect of randomness is described by the coherent potential approximation. Application of a real-space Green-function formalism yields the exchange pair interactions between distant magnetic atoms that are needed for quantitative studies of magnetic excitations including the Curie temperatures. We have found that the pair exchange interactions exhibit a strong directional dependence and are exponentially damped with increasing distance between magnetic atoms due to disorder and the half-metallic character of the DMS. As a case study we consider (Ga, Mn) As, (Ga, Mn)N, and (Zn, Cr)Te alloys. The calculations demonstrate that inclusion of disorder and, in particular, realistic distances among magnetic impurities, are needed to obtain critical temperatures which are in good agreement with available experiments.

Electronic and magnetic properties of quaternary (Cu,Ni)MnSb alloys

Magnetic properties and Curie temperatures of semi-Heusler alloys Cu1−x Ni x MnSb are calculated as a function of the alloy composition. The transition from the ferromagnetic state (NiMnSb) to the antiferromagnetic state (CuMnSb) gives rise to an abrupt change in the concentration dependence of alloy magnetization at about x Cu = 0.7 while the Curie temperature decreases monotonically with the Cu-content. Such behaviour can be understood as being due to the onset of disorder in orientations of Mn-spins at x Cu = 0.7. A simple account of magnetic disorder as based on the uncompensated disorder local moment picture provides also a good quantitative understanding of available experimental data.

Transport Properties of (Ga,Mn)As Diluted Magnetic Semiconductors in the Bulk and in Layered Systems

Transport properties of systems based on diluted magnetic semiconductors (Ga,Mn)As are investigated theoretically by means of the Kubo linear response theory. The underlying electronic structure is obtained within the local spin-density approximation using the scalarrelativistic tight-binding linear muffin-tin orbital (TB-LMTO) method. The effect of substitutional randomness on the electronic structure and on the transport properties is systematically described in the coherent potential approximation (CPA). The quantities studied include the residual resistivities of the bulk alloys as well as conductances of epitaxial trilayers Cr/(Ga,Mn)As/Cr (001) in the current-perpendicular-to-plane (CPP) orientation. The results witness that various compensating defects such as As antisite atoms and Mn interstitials have much stronger detrimental effect on the spin polarization of the CPP conductances as compared to their influence on the spin polarization of the bulk conductivities.

Efficient ab initio technique for spin-wave stiffness of random ferromagnets

The spin-wave stiffness D of ferromagnetic systems represents an important micromagnetic parameter closely related to exchange interactions between local magnetic moments.

First-principles study of thermodynamical properties of random magnetic overlayers on fcc-Cu(001) substrate

We present a theoretical study of the thermodynamical properties of a fcc-Cu(001) substrate covered by an iron-cobalt monolayer as well as by an incomplete iron layer. The effective two-dimensional Heisenberg Hamiltonian is constructed from first principles and the properties of exchange interactions are investigated. The Curie temperatures are estimated using the Monte Carlo (MC) simulations and compared with a simplified approach using the random-phase approximation (RPA) in connection with the virtual-crystal approach (VCA) to treat randomness in exchange integrals. Calculations indicate a weak maximum of the Curie temperature as a function of composition of the iron-cobalt overlayer. While a good quantitative agreement between RPA-VCA and MC was found for the iron-cobalt monolayer, the RPA-VCA approach fails quantitatively for low coverage due to the magnetic percolation effect. We also present a study of the effect of alloy disorder on the shape of magnon spectra of random overlayers.

Exchange Interactions and Magnetic Percolation in Diluted Magnetic Semiconductors

We propose a theory that combines first-principles evaluations of inter-atomic exchange interactions with a classical Heisenberg model and Monte Carlo simulations. Exchange interactions are determined using the magnetic force theorem and the one-electron Green functions. The magnetic properties of diluted magnetic semiconductors are dominated by short ranged interatomic exchange interactions that; have a strong directional dependence. We show that; critical temperatures of a broad range of diluted magnetic semiconductors, involving Mn-doped GaAs and GaN as well as Cr-doped ZnTe, arc reproduced with a good accuracy only when the magnetic atoms arc randomly positioned on the Ga (Zn) sites, whereas in ordered structure of the magnetic atoms results in critical temperatures that are too high. This suggests that the ordering of diluted magnetic semiconductors is heavily influenced by magnetic percolation, and that the measured critical ternperatures should be very sensitive to the details of the sample preparation, in agreement with observations.