I. Turek

Exchange interactions, spin waves, and transition temperatures in itinerant magnets

This contribution reviews an ab initio two-step procedure to determine exchange interactions, spin-wave spectra, and thermodynamic properties of itinerant magnets. In the first step, the self-consistent electronic structure of a system is calculated for a collinear spin structure at zero temperature. In the second step, parameters of an effective classical Heisenberg Hamiltonian are determined using the magnetic force theorem and the one-electron Green functions. The Heisenberg Hamiltonian and methods of statistical physics are employed in subsequent evaluation of magnon dispersion laws, spin-wave stiffness constants, and Curie/Néel temperatures. The applicability of the developed scheme is illustrated by selected properties of various systems such as transition and rare-earth metals, disordered alloys including diluted magnetic semiconductors, ultrathin films, and surfaces. A comparison to other ab initio approaches is presented as well.

Ab initio theory of exchange interactions and the Curie temperature of bulk Gd

An ab initio approach to the magnetic properties of bulk hexagonal Gd is developed that is based on the local spin-density approximation with the 4f electrons treated as localized core electrons. The effective one-electron problem is solved using the tight-binding linear muffin-tin orbital method in the atomic-sphere approximation with the valence basis consisting of s-, p-and d-type orbitals. The approach leads to a correct description of the ground-state properties like the stability of the ferromagnetic structure, the magnetic moment and the equilibrium lattice constant. Application of a real-space Green-function formalism yields the exchange pair interactions between distant neighbours that are inevitable for quantitative studies of magnetic excitations. The distance dependence and anisotropy of the exchange pair interactions are presented and the Curie temperature in the mean-field approximation is evaluated. The obtained value of 334 K is in much better agreement with the experimental value of 293 K than previous theoretical results. Depending on the atomic volume we find an unusually large dependence of the Curie temperature on the c/a ratio, which bears important consequences for the critical temperatures of thick strained Gd films as grown on various substrates.

Spin-dependent transparency of ferromagnet/superconductor interfaces.

We combine parameter-free calculations of the transmission and reflection matrices for clean and dirty interfaces with a scattering-theory formulation of Andreev reflection (AR) generalized to spin-polarized systems in order to critically evaluate the use of an extended Blonder-Tinkham-Klapwijk (BTK) model to extract values of the spin polarization for ferromagnetic metals from measurements of point-contact AR. Excellent agreement with the experimental conductance data is found for Pb/Cu but it is less good for Pb/Ni and poor for Pb/Co, indicating that the BTK formalism does not describe transport through superconducting/ferromagnetic interfaces correctly.

Ab Initio Study of Curie Temperatures of Diluted Magnetic Semiconductors

The Curie temperature of diluted (Ga,Mn)As magnetic semiconductors in the presence of As antisites is studied from first principles. We map total energies associated with rotations of Mn-magnetic moments onto the effective classical Heisenberg Hamiltonian which is treated in the mean-field approximation to find the Curie temperature. The presence of donors strongly reduces the Curie temperature and gives rise to a ground state with a partial disorder of local moments. We show that the observed dependence of the Curie temperature on the Mn concentration indicates that the concentration of As antisites increases with the Mn content.

Formation of a weak ferromagnetic state in Y(Co1-xAlx)2 compounds: a coherent potential approximation study

The band structure of substitutionally disordered Y(Co1-xAlx)2 has been calculated for various concentrations in the range 0≤x≤0.25 employing the coherent potential approximation embodied in an all-electron tight-binding linear muffin-tin orbital method. On the basis of the results, we provide a new explanation for the formation of weak ferromagnetic moments in these compounds. The discussion of the non-spin-polarized calculated densities of states is supported by direct evidence of the weak ferromagnetic states for certain lattice constants and Al concentrations. The roles of the disorder and the volume effects are discussed.

Nonlocal torque operators in ab initio theory of the Gilbert damping in random ferromagnetic alloys

We present an ab initio theory of the Gilbert damping in substitutionally disordered ferromagnetic alloys. The theory rests on introduced nonlocal torques which replace traditional local torque operators in the well-known torque-correlation formula and which can be formulated within the atomic-sphere approximation. The formalism is sketched in a simple tight-binding model and worked out in detail in the relativistic tight-binding linear muffin-tin orbital method and the coherent potential approximation (CPA). The resulting nonlocal torques are represented by nonrandom, non-site-diagonal, and spin-independent matrices, which simplifies the configuration averaging. The CPA-vertex corrections play a crucial role for the internal consistency of the theory and for its exact equivalence to other first-principles approaches based on the random local torques. This equivalence is also illustrated by the calculated Gilbert damping parameters for binary NiFe and FeCo random alloys, for pure iron with a model atomic-level disorder, and for stoichiometric FePt alloys with a varying degree of

Substrate-induced antiferromagnetism of a Fe monolayer on the Ir(001) surface

L{1}_{0}

Microscopic theory of magnetization processes in Y(Co1-xAlx)2

atomic long-range order.

Electronic properties ofα−UH3stabilized by Zr

We present detailed ab initio study of structural and magnetic stability of a Fe-monolayer on the fcc(001) surface of iridium. The Fe-monolayer has a strong tendency to order antiferromagnetically for the true relaxed geometry. On the contrary an unrelaxed Fe/Ir(001) sample has a ferromagnetic ground state. The antiferromagnetism is thus stabilized by the decreased Fe-Ir layer spacing in striking contrast to the recently experimentally observed antiferromagnetism of the Fe/W(001) system which exists also for an ideal bulk-truncated, unrelaxed geometry. The calculated layer relaxations for Fe/Ir(001) agree reasonably well with recent experimental LEED data. The present study centers around the evaluation of pair exchange interactions between Fe-atoms in the Fe-overlayer as a function of the Fe/Ir interlayer distance which allows for a detailed understanding of the antiferromagnetism of a Fe/Ir(001) overlayer. Furthermore, our calculations indicate that the nature of the true ground state could be more complex and display a spin spiral-like rather than a c(2x2)-antiferromagnetic order. Finally, the magnetic stability of the Fe monolayer on the Ir(001) surface is compared to the closely related Fe/Rh(001) system.

Fermi sea term in the relativistic linear muffin-tin-orbital transport theory for random alloys

Employing ab initio electronic structure calculations we study the development of the magnetic properties in Y (Co1−x Alx)2 for varying Al concentration. The effect of substitutional disorder is treated in the coherent-potential approximation implemented within a tight-binding linear muffin-tin orbital method. The experimentally observed reduction of the critical field of the itinerant electron metamagnetic phase transition with increasing content of non-magnetic Al is explained. It is shown, on the basis of a T = 0 K Stoner type itinerant magnetism theory, that the alloying-induced changes in the shape of the calculated density of states, caused by the Al substitution, lead to (i) a stabilization of the magnetic state, (ii) a smoothening of the first-order metamagnetic transition and (iii) a subsequent suppression of the metamagnetic transition around x = 0.15. Analysing the magnetization processes in Y (Co1−x Alx)2 by varying the strength of the exchange interaction, we provide a microscopical background to earlier phenomenological assumptions made in the literature.