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We describe our theoretical formalism and computational scheme for making ab initio calculations of the dynamic paramagnetic spin susceptibilities of metals and alloys at finite temperatures. Its basis is timedependent density functional theory within an electronic multiple scattering, imaginary time Green function formalism. Results receive a natural interpretation in terms of overdamped oscillator systems making them suitable for incorporation into spin fluctuation theories. For illustration we apply our method to the nearly ferromagnetic metal Pd and the nearly antiferromagnetic chromium alloy Cr95V5. We compare and contrast the spin dynamics of these two metals and in each case identify those fluctuations with relaxation times much longer than typical electronic ‘‘hopping times.’’ There is renewed interest in spin fluctuations in materials close to magnetic order. This is due in part to a realization that nearly critical magnetic fluctuations may be important factors governing the nonconventional properties of a wide range of materials which include the high-Tc ~HTC! superconducting cuprates and heavy fermion systems. 1 The strongly correlated electrons in many of these systems, however, have meant that most theoretical work has concentrated on parametrized models in which the electronic motion is treated rather simply. Another complementary approach is to use an ab initio theory such as time-dependent density functional theory ~TDDFT !~ Ref. 2! but apply it to materials where it can be expected to work, i.e., where the effects of electron correlations are not so important, but which otherwise have important similarities to the systems in question. For example, with its perovskite structure containing transition metal ~TM!-oxygen planes Sr2RuO4 has several aspects in common with the HTC materials. But the presence of the 4d TM Ru rather than the narrower band 3d TM Cu means that electron correlation effects are smaller and therefore DFT-based calculations can provide a valuable starting point. Moreover, its exotic superconductivity at low temperature seems likely to be affected by spin fluctuations. 3‐5 Concerning another example, the transition temperature separating paramagnetic and magnetically ordered phases of the cubic transition metal compound MnSi, which has the B20 crystal structure, is driven down to zero temperature upon the application of pressure. In the vicinity of the critical pressure for this quantum phase transition the system exhibits

We have identified the pieces of the Fermi surface responsible for the long period oscillations of magnetic coupling across Cr and Cr(1 x)Vx alloy spacers in metallic multilayers. Analysing experiments and results of KKR-CPA calculations we find that the periods are determined by the extremal wave vectors of the hole pockets centered on the N-point in the Brillouin zone.

The cubic lattice symmetry of ferromagnetic homogeneously disordered alloys is broken when a compositional modulation is imposed. This can have a profound influence on the magnetocrystalline anisotropy energy (MAE). We describe our ab initio theory of this effect and use the framework of concentration waves with the electronic structure described within the spin-polarized relativistic Korringa-Kohn-Rostoker coherent-potential approximation. We find that ordering produces a 2 order of magnitude increase in the MAE as well as altering the equilibrium direction of magnetization. Using the same theoretical framework we also examine directional compositional order produced by magnetic annealing with an explicit study of permalloy. [S0031-9007(99)09484-3]

A scheme for making ab initio calculations of the dynamic paramagnetic spin susceptibilities of solids at finite temperatures is described. It is based on time-dependent density functional theory and employs an electronic multiple scattering formalism. Incommensurate and commensurate antiferromagnetic spin fluctuations in paramagnetic Cr and compositionally disordered Cr 95V5 and Cr95Re5 alloys are studied together with the connection with the nesting of their Fermi surfaces. We find that the spin fluctuations can be described rather simply in terms of an overdamped oscillator model. Good agreement with inelastic neutron scattering data is obtained. [S0031-9007(99)08949-8] PACS numbers: 75.40.Gb, 71.15.Mb, 75.10.Lp, 75.20.En Chromium is the archetypal itinerant antiferromagnet (AF) whose famous incommensurate spin density wave (SDW) ground state is determined by the nesting wave vectors qnest identified in the Fermi surface [1]. Chromium alloys also have varied AF properties [1] and their paramagnetic states have recently attracted attention owing, in part, to analogies drawn with the high temperature superconducting cuprates especially sLacSr12cd2CuO4. The incommensurate SDW fluctuations in these materials [2] are rather similar to those seen in the paramagnetic phase of Cr close to TN . Moreover “parent” materials La 2CuO4, in the one instance, and Cr95Mn5 or Cr95Re5, in the other, are simple commensurate AF materials which on lowering the electron concentration by suitable doping develop incommensurate spin fluctuations which may be promoted by imperfectly nested Fermi surfaces. Here we examine the nature of damped diffusive spin fluctuations in chromium above the Neel temperature TN which are precursory to the SDW ground state. We also study dilute chromium alloys, Cr95Re5 and Cr95V5, and obtain good agreement with experimental data. For example, recent inelastic neutron scattering experiments [1,3] have measured incommensurate AF “paramagnons,” persisting up to high frequencies in the latter system. We explore the temperature dependence, the variation with dopant concentration, and the evolution of the spin fluctuations in these systems from incommensurability to commensurability with increasing frequency and provide the first ab initio description of these effects. To this end we describe a new scheme for calculating the wave vector and frequency dependent dynamic spin susceptibility of metals which is based on the time-dependent density functional theory (TDDFT) of Gross et al. [4] and as such is an allelectron theory. For the first time the temperature dependent dynamic spin susceptibility of metals and alloys is calculated from this basis. There have been several simple parametrized models to describe the magnetic properties of Cr and its alloys [5]. All of these have concentrated on the approximately nested electron “jack” and slightly larger octahedral hole pieces of the Fermi surface [1] and, at best, have included only the effects of all the remaining electrons via an electron reservoir. We find similarities between our results and results from such models but show that a complete picture is obtained only when an electronic band-filling effect which favors a simple AF ordering at low temperature is also considered. We also find that the spin fluctuations are given an accurate description as over

The technological performances of metallic compounds are largely influenced by atomic ordering. Although there is a general consensus that successful theories of metallic systems should account for the quantum nature of the electronic glue, existing non-perturbative high-temperature treatments are based on effective classical atomic Hamiltonians. We propose a solution for the above paradox and offer a fully quantum mechanical, though approximate, theory that on equal footing deals with both electrons and ions. By taking advantage of a coarse grained formulation of the density functional theory [Bruno et al., Phys. Rev. B 77, 155108 (2008)] we develop a MonteCarlo technique, based on an ab initio Hamiltonian, that allows for the efficient evaluation of finite temperature statistical averages. Calculations of the relevant thermodynamic quantities and of the electronic structures for CuZn and Ni$_3$V support that our theory provides an appropriate description of order-disorder phase transitions., Comment: 4 pages, 4 figures

Self-consistent ‘band theory’, based on density functional theory, is a useful approach to describing the electron glue which holds solids together. However, its powerful group theoretic and numerical techniques cannot be deployed for disordered states of matter. The self-consistent KKR-CPA is an analogous method which is able to deal with some of these interesting cases. In particular, we show how it describes random metallic alloys, treating all the classic Hume-Rothery factors: size-effect, electronegativity and electrons per atom ratio ( e:a ) on more or less equal footing and from first principles. Moreover, we use the KKR-CPA framework to analyse the instability of the disorder state to compositional ordering processes and hence provide a first principle description of the forces which drive order-disorder transformations.

For many years the Korringa-Kohn-Rostoker coherent-potential approximation (KKR-CPA) has been widely used to describe the electronic structure of disordered systems based upon a first-principles description of the crystal potential. However, as a single-site theory the KKR-CPA is unable to account for important environmental effects such as short-range order (SRO) in alloys and spin fluctuations in magnets, amongst others. Using the recently devised KKR-NLCPA (where NL stands for nonlocal), we show how to remedy this by presenting explicit calculations for the effects of SRO on the electronic structure of the bcc Cu_{50}Zn_{50} solid solution., 8 pages, 6 figures, Revised version

A comprehensive investigation of the Fermi surface topologies of a series of Cr1-xVx and Cr1-xMox alloys highlights the role of nesting in determining the periods of the oscillatory exchange coupling observed in multilayer systems. The long-standing controversy over the origin of the long-period oscillations across Cr and Cr-based alloys is resolved, with the evolution of one dimension of the N-hole ellipsoid Fermi surface sheet being directly associated with the evolution of the period.

We study Ni80Fe20-based permalloys with the relativistic spin-polarized Korringa-Kohn-Rostoker electronic structure method. Treating the compositional disorder with the coherent potential approximation, we investigate how the magnetocrystalline anisotropy, K, and magnetostriction, lambda, of Ni-rich Ni-Fe alloys vary with the addition of small amounts of non-magnetic transition metals, Cu and Mo. From our calculations we follow the trends in K and lambda and find the compositions of Ni-Fe-Cu and Ni-Fe-Mo where both are near zero. These high permeability compositions of Ni-Fe-Cu and Ni-Fe-Mo match well with those discovered experimentally. We monitor the connection of the magnetic anisotropy with the number of minority spin electrons, Nmin. By raising Nmin via artificially increasing the band-filling of Ni80Fe20, we are able to reproduce the key features that underpin the magnetic softening we find in the ternary alloys. The effect of band-filling on the dependence of magnetocrystalline anisotropy on atomic short-range order in Ni80Fe20 is also studied. Our calculations, based on a static concentration wave theory, indicate that the susceptibility of the high permeability of the Ni-Fe-Cu and Ni-Fe-Mo alloys to their annealing conditions is also strongly dependent on the alloys' compositions. An ideal soft magnet appears from these calculations., Comment: 20 pages, 6 figures

tions, we study the conditions that, for ZrZn 2, determine the proximity to this magnetic instability. More specifically, we discuss the role played by the geometrical arrangement of the lattice, the hybridization effects, and the presence of disorder, as well as the application of external pressure. These circumstances influence the width of the relevant Zr d bands whose narrowing, due to the reduction of the effective number of neighbors or to an increase of the cell volume, causes an enhancement of the density of states at the Fermi level. Finally, we highlight some general features that may aid the design of other materials close to magnetic instabilities.

Nature or Physics laws rule everything occurs within the Universe and on the Earth, including economical activities and social processes. Opposite to human laws, Physics laws cannot be transgressed, therefore environmental and economic sustainability can be pursued only through economy development models coherent with them, and it is insanity to think that politics and economy can preferentially respect free market laws. The first and the second Thermodynamics principles have a great economic and social relevance, because they imply that all human activities are transformations (nothing is created, nothing is destroyed, everything is transformed) with efficiency less than 100%, whose scrap is directly related to the environment pollution. This holds for economical activities too. In fact companies, in order to produce, need raw materials and energy, make wastes and run into debt; consumers, in order to consume, need energy, make wastes and run into debt; governments, in order to provide infrastructures and services for production and consumptions, need energy, make wastes and run into debt. The inputs of such process are then the planet resources while the outputs are debt and wastes that, in turn, require more energy and other resources to be treated. Because of the narrowness of the natural resources this model might get unstable and lead to the economical crisis and to consumption and degradation of the resources, as striking prescription of Thermodynamics laws. Any alternative model must realize that, since the pollution is unavoidable, it is wise to pollute as least as possible and as slowly as possible. Again the second Thermodynamics principle, not ideology, states that this is possible only through an ordered, fair and jointly liable access to the common assets, opposite to the predatory access onto which free market is based.

Almost immediately after the discovery of quantum mechanics, physicists and metallurgist began the task of trying to understand the origins of alloy phase stability in terms of the underlying electronic structure. Hume-Rothery [1] used the chemical concepts of electronegativity and atomic size supplemented with the solid state physics notion of electron to atom ratio to formulate a set of rules that could be used to rationalize the complex array of phases that form when two elemental metals are mixed to form an alloy. The Hume-Rothery rulesand their progeny [2], [3] have proved very successful in providing metallurgists and alloy designers with a basis for understanding alloy phase stability.

We have developed a hybrid, parallel computer code for calculating the electronic structure of both ordered and substitutionally disordered materials. By using PVM3.3, we can integrate into our local computer environment multiple parallel and vector supercomputers as well as high performance workstations. Without this approach, calculations of materials properties of large systems would be otherwise untenable due to a lack of computer resources. For example, we have determined the short-range order intensity and its electronic origin for the Ag-Mg alloy system, including an estimate of the order-disorder (spinodal) temperature. >