The first material to be predicted from first-principles calculations as half-metallic was NiMnSb, and the research on this material has been intense due to its possible applications in spintronics devices. The failure of many experiments to measure spin polarization to more than a fraction of the predicted 100% has partly been blamed on structural defects. In this work a complete first-principles treatise of point defects, including nonstoichiometric antisites, interstitial and vacancy defects, as well as stoichiometric atomic swap defects in NiMnSb, is presented. We find that the formation energies of the defects span a large scale from 0.2 to 14.4 eV. The defects with low formation energies preserve the half-metallic character of the material. We also find that some of the defects increase the magnetic moment and thus can explain the experimentally observed increase of magnetic moments in some samples of NiMnSb. Most interesting in this respect are Mn interstitials which increase the magnetic moment, have a low formation energy, and keep the half-metallic character of the material.
Transition-metal (TM)/V superlattices (TM=Cr,Fe,Mo) show certain peculiarities under hydrogen uptake. Here we investigate the influence of an induced magnetization of the V layers on the hydrogen dissolution by means of first-principles calculations. We find that below a certain value for the magnetic moment of the V host the hydrogen solubility is slightly reduced, whereas for larger moments the hydrogen dissolution becomes favored. The actual position of this transition depends on the tetragonal distortion of the V layers.
Wehave performed ab initio calculations of the mixing enthalpy forthe Mo-Ru alloy system. Both completely random alloys on thefcc, bcc, and hcp lattices as well as ordered andpartially ordered structures based on the hcp lattice and a phase have been examined. Further, we have performed aground-state search for the Ru-rich region using ab initio derivedeffective interactions, and find a series of structures below thetie line of the simple compounds. Using the structures fromthis ground-state search, we are able to make an estimationof the contribution to the total energy due to orderingeffects in this system. We find unusually large deviations betweencalculated and experimental values of the mixing enthalpy for Ru-richhcp alloys. Our calculations indicate, in agreement with experiment, thatthere are ordering trends in the system. However, even underassumption of maximal order theoretical results differ substantially from theexperiment. Possible reasons for the disagreement are discussed.
Ab initio calculations of the enthalpy of formation of bcc, fcc, and hcp Ru–Mo alloys have been performed for random, ordered, and partially ordered structures. The lattice stability of the bcc and hcp forms of Mo is isolated in order to compare the hcp–bcc difference calculated by ab initio and CALPHAD methods with experimental measurements of the enthalpy of formation of Ru–Mo alloys. The significance of this comparison in calculating the Mo–Ru phase diagram is illustrated. The results of these considerations suggest a rational method for coupling ab initio and CALPHAD techniques might be utilization of the ab initio methods for calculation of the isostructural energies of formation for binary bcc, hcp, and fcc solutions while retaining the CALPHAD lattice stabilities in the calculation of phase diagrams.
We have calculated the conductivities of the 3d, 4d, and 5d transition metals by using a quasiclassical Boltzmann approach which utilizes self-consistent energy bands. Electron scattering is characterized by a constant scattering strength which treats all sources of resistivity on an equal footing. This results in a single parameter theory with which we are able to reproduce, qualitatively, trends and anomalies in conductivity throughout the transition metal series. By the choice of an appropriate scattering strength at each temperature, we are able to reproduce these trends over the experimentally observable temperature range of 10-1000 K. In particular, the well known dip in conductivity at the middle of the series, especially pronounced for Mn, is understood in terms of the host electronic structure. © 2008 The American Physical Society.
It is shown that, using the generalized perturbation method (GPM) with screened Coulomb interactions that ensures its consistency with the force theorem, one is able to obtain effective interactions that yield an accurate and physically transparent description of configurational energetics in the framework of the Korringa-Kohn-Rostoker method within the atomic sphere and coherent potential approximations. This is demonstrated with calculations of ordering energies, short-range order parameters, and transition temperatures in the CuZn, CuAu, CuPd, and PtCo systems. Furthermore, we show that the GPM can be used to obtain Heisenberg exchange interaction parameters, which, for instance, capture very well the magnetic configurational energy in bcc Fe.
We discuss the use of the magnetic force theorem using different reference states upon which the perturbative approach is based. Using a fixed spin disordered local moment state one finds good Curie (or Neél) temperatures, and good energetics for planar spin spirals in the 3d magnets Fe, fcc Co, Ni, Mn, and Cr, though worse agreement for small θ spin spirals. On the other hand, the ferromagnetic reference state provides excellent energetics for small θ spin spirals in Fe, fcc Co, and Ni, and by extension magnon energies under the assumption of adiabacity. However, planar spin spiral energetics and transition temperatures show worse agreement. The reasons for this, and for the case of fcc Co where both approaches work very well, are discussed. We further provide an extension of the mapping of the quantum problem to include longitudinal fluctuations within force theorem based approaches, and discuss the role they will play in magnetic phase transitions. This construction is tested using planar spin spirals where q is fixed but the moment is allowed to relax. It is demonstrated that results from this approach and directly calculated ab initio values agree very well.
We investigate the complex magnetic ordering in face-centered cubic (fcc) Fe (γ-Fe) using a mapping of the electronic structure problem to an effective Heisenberg model via Green's function perturbation theory. We find that low-moment ([approximate]0.6µB) spin spirals (SSs) are accurately captured by this Hamiltonian, but not SSs of intermediate-moment ([approximate]2.0µB). We attribute this latter result to a hybridization gap at the Fermi level, resulting from the specific symmetry of the SS, which is difficult to capture within a perturbative approach. Using a generalization of the usual Heisenberg approach, in which we include the moment size as well as orientational degrees of freedom, we find that the strong volume dependence of this system enters through the intrasite exchange term only. We confirm this result with direct calculations of fixed moment and relaxed moment spin spirals. This highlights the importance of intrasite magnetism in this system, and points toward the moment and not the volume as being the crucial quantity to compare with experiments.