The opposing nanoscale and macroscale effects of selected nanoparticle addition to AZ91/ZK60A hybrid magnesium alloy

10.1007/s11051-013-1938-1Journal of Nanoparticle Research159

Nucleation of amorphous shear bands at nanotwins in boron suboxide

The roles of grain boundaries and twin boundaries in mechanical properties are well understood for metals and alloys. However, for covalent solids, their roles in deformation response to applied stress are not established. Here we characterize the nanotwins in boron suboxide (B(6)O) with twin boundaries along the [Image: see text] planes using both scanning transmission electron microscopy and quantum mechanics. Then, we use quantum mechanics to determine the deformation mechanism for perfect and twinned B(6)O crystals for both pure shear and biaxial shear deformations. Quantum mechanics suggests that amorphous bands nucleate preferentially at the twin boundaries in B(6)O because the twinned structure has a lower maximum shear strength by 7.5% compared with perfect structure. These results, which are supported by experimental observations of the coordinated existence of nanotwins and amorphous shear bands in B(6)O, provide a plausible atomistic explanation for the influence of nanotwins on the deformation behaviour of superhard ceramics

Density Functional Theory for Magnetism and Magnetic Anisotropy

Density functional theory and its application for the simulation of magnetic properties of condensed matter is introduced. This includes vector-spin density functional theory for the evaluation of spin-spin interactions and relativistic extensions to capture effects like the magnetocrystalline anisotropy. The role of the different approximations to the exchange-correlation functional, e.g., the local density approximation, or the generalized gradient approximation, is investigated, showing successes and limitations of the present functionals. Special techniques to determine, e.g., the magnetic ground state or finite temperature properties based on density functional theory are shortly discussed

Spin Excitations in Solid from Many-Body Perturbation Theory

Electronic spin excitations are low-energy excitations that influence the properties of magnetic materials substantially. Two types of spin excitations can be identified, single-particle Stoner excitations and collective spin-wave excitations. They can be treated on the same footing within many-body perturbation theory. In this theory, the collective spin excitations arise from the correlated motion of electron-hole pairs with opposite spins. We present the theory in detail and discuss several aspects of an implementation within the full-potential linearized augmented plane-wave method. The pair propagation is described by the transverse magnetic susceptibility, which we calculate from first principles employing the ladder approximation for the T matrix. The four-point T matrix is represented in a basis of Wannier functions. By using an auxiliary Wannier set with suitable Bloch character, the magnetic response function can be evaluated for arbitrary k points, allowing fine details of the spin-wave spectra to be studied. The energy of the acoustic spin-wave branch should vanish in the limit k →0, which is a manifestation of the Goldstone theorem. However, this condition is often violated in the calculated acoustic magnon dispersion, which can partly be traced back to the choice of the Green function. In fact, the numerical gap error is considerably reduced when a renormalized Green function is used. As an alternative simple correction scheme, we suggest an adjustment of the Kohn-Sham exchange splitting. We present spin excitation spectra for the elementary ferromagnets Fe, Co, and Ni as illustrative examples and compare to model calculations of the homogeneous electron gas

Many-Body Spin Excitations in Ferromagnets from First Principles

Electronic spin excitations are low-energy excitations that influence the properties of magnetic materials substantially. Two types of spin excitations can be identified, single-particle Stoner excitations and collective spin-wave excitations. They can be treated on the same footing within many-body perturbation theory. In this theory, the collective spin excitations arise from the correlated motion of electron-hole pairs with opposite spins. We present the theory in detail and discuss several aspects of an implementation within the full-potential linearized augmented plane-wave method. The pair propagation is described by the transverse magnetic susceptibility, which we calculate from first principles employing the ladder approximation for the T matrix. The four-point T matrix is represented in a basis of Wannier functions. By using an auxiliary Wannier set with suitable Bloch character, the magnetic response function can be evaluated for arbitrary k points, allowing fine details of the spin-wave spectra to be studied. The energy of the acoustic spin-wave branch should vanish in the limit k →0, which is a manifestation of the Goldstone theorem. However, this condition is often violated in the calculated acoustic magnon dispersion, which can partly be traced back to the choice of the Green function. In fact, the numerical gap error is considerably reduced when a renormalized Green function is used. As an alternative simple correction scheme, we suggest an adjustment of the Kohn-Sham exchange splitting. We present spin excitation spectra for the elementary ferromagnets Fe, Co, and Ni as illustrative examples and compare to model calculations of the homogeneous electron ga

Spectral Density Functional Approach to Electronic Correlations and Magnetism in Crystals

A novel approach to electronic correlations and magnetism of crystals based on realistic electronic structure calculations is reviewed. In its simplest form it is a combination of the ``local density approximation'' (LDA) and the dynamical mean field theory (DMFT) approaches. Using numerically exact QMC solution to the effective DMFT multi-orbital quantum-impurity problem, a successful description of electronic structure and finite temperature magnetism of transition metals has been achieved. We discuss a simplified perturbation LDA+DMFT scheme which combines the T-matrix and fluctuation-exchange approximation (TM-FLEX). We end with a discussion of cluster generalization of the non-local DMFT scheme and its applications to the magnetism and superconductivity of high-Tc superconductors.Comment: 37 pages, to be published in: "Electron Correlations and Materials Properties 2" ed. by A. Gonis (Kluwer, NY

Spin Excitations in Solids from Many-Body Perturbation Theory

Collective spin excitations form a fundamental class of excitations in magnetic materials. As their energy reaches down to only a few meV, they are present at all temperatures and substantially influence the properties of magnetic systems. To study the spin excitations in solids from first principles, we have developed a computational scheme based on many-body perturbation theory within the full-potential linearized augmented plane-wave (FLAPW) method. The main quantity of interest is the dynamical transverse spin susceptibility or magnetic response function, from which magnetic excitations, including single-particle spin-flip Stoner excitations and collective spin-wave modes as well as their lifetimes, can be obtained. In order to describe spin waves we include appropriate vertex corrections in the form of a multiple-scattering T matrix, which describes the coupling of electrons and holes with different spins. The electron–hole interaction incorporates the screening of the many-body system within the random-phase approximation. To reduce the numerical cost in evaluating the four-point T matrix, we exploit a transformation to maximally localized Wannier functions that takes advantage of the short spatial range of electronic correlation in the partially filled d or f orbitals of magnetic materials. The theory and the implementation are discussed in detail. In particular, we show how the magnetic response function can be evaluated for arbitrary k points. This enables the calculation of smooth dispersion curves, allowing one to study fine details in the k dependence of the spin-wave spectra. We also demonstrate how spatial and time-reversal symmetry can be exploited to accelerate substantially the computation of the four-point quantities. As an illustration, we present spin-wave spectra and dispersions for the elementary ferromagnet bcc Fe, B2-type tetragonal FeCo, and CrO2 calculated with our scheme. The results are in good agreement with available experimental data