Generalized perturbation theory in disordered transitional alloys: Applications to the calculation of ordering energies

Colossal Magnetoresistive Switching Induced by d0 Ferromagnetism of MgO in a Semiconductor Nanochannel Device with Ferromagnetic Fe/MgO Electrodes

Exploring potential spintronic functionalities in resistive switching (RS) devices is of great interest for creating new applications, such as multifunctional resistive random-access memory and novel neuromorphic computing devices. In particular, the importance of the spin-triplet state of cation vacancies in oxide materials, which is induced by localized and strong O-2p on-site Coulomb interactions, in RS devices has been overlooked. d0 ferromagnetism sometimes appears due to the spin-triplet state and ferromagnetic Zener's double exchange interactions between cation vacancies, which are occasionally strong enough to make nonmagnetic oxides ferromagnetic. Here, for the first time, anomalous and colossal magneto-RS (CMRS) with very high magnetic field dependence is demonstrated by utilizing an unconventional RS device composed of a Ge nanochannel with all-epitaxial single-crystalline Fe/MgO electrodes. The device shows colossal and unusual behavior as the threshold voltage and ON/OFF ratio strongly depend on a magnetic field, which is controllable with an applied voltage. This new phenomenon is attributed to the formation of d0 -ferromagnetic filaments by attractive Mg vacancies due to the spin-triplet states with ferromagnetic double exchange interactions and the ferromagnetic proximity effect of Fe on MgO. The findings will allow the development of energy-efficient CMRS devices with multifield susceptibility.

Simulating short-range order in compositionally complex materials

In multicomponent materials, short-range order (SRO) is the development of correlated arrangements of atoms at the nanometer scale. Its impact in compositionally complex materials has stimulated an intense debate within the materials science community. Understanding SRO is critical to control the properties of technologically relevant materials, from metallic alloys to functional ceramics. In contrast to long-range order, quantitative characterization of the nature and spatial extent of SRO evades most of the experimentally available techniques. Simulations at the atomistic scale have full access to SRO but face the challenge of accurately sampling high-dimensional configuration spaces to identify the thermodynamic and kinetic conditions at which SRO is formed and what impact it has on material properties. Here we highlight recent progress in computational approaches, such as machine learning-based interatomic potentials, for quantifying and understanding SRO in compositionally complex materials. We briefly recap the key theoretical concepts and methods.

Anderson transition in stoichiometric Fe2VAl: high thermoelectric performance from impurity bands

Discovered more than 200 years ago in 1821, thermoelectricity is nowadays of global interest as it enables direct interconversion of thermal and electrical energy via the Seebeck/Peltier effect. In their seminal work, Mahan and Sofo mathematically derived the conditions for ’the best thermoelectric’—a delta-distribution-shaped electronic transport function, where charge carriers contribute to transport only in an infinitely narrow energy interval. So far, however, only approximations to this concept were expected to exist in nature. Here, we propose the Anderson transition in a narrow impurity band as a physical realisation of this seemingly unrealisable scenario. An innovative approach of continuous disorder tuning allows us to drive the Anderson transition within a single sample: variable amounts of antisite defects are introduced in a controlled fashion by thermal quenching from high temperatures. Consequently, we obtain a significant enhancement and dramatic change of the thermoelectric properties from p-type to n-type in stoichiometric Fe₂VAl, which we assign to a narrow region of delocalised electrons in the energy spectrum near the Fermi energy. Based on our electronic transport and magnetisation experiments, supported by Monte-Carlo and density functional theory calculations, we present a novel strategy to enhance the performance of thermoelectric materials.

The mathematical conditions for the best thermoelectric is well known but never realised in real materials. Here, the authors propose the Anderson transition in a narrow impurity band as a physical realisation of this seemingly unrealisable scenario.

First-principles study of order-disorder transitions in multicomponent solid-solution alloys

In this review, we will focus on the recent development of the order-disorder transition in metallic materials. The past decades have witnessed fast development in the first-principles methodologies and their applications to ordering transitions in multi-component alloys, particularly the high-entropy alloys. The driving force for the proceedings comes from (i) the advance of algorithms and increasingly cheaper hardware, and also (ii) the great passion to model alloys with increasing number of components. The review starts with a brief introduction of the history for the ordering transitions. More detailed scientific proceedings prior to the 1970s had been well summarized in Krivoglaz and Smirnov (1965 The Theory of Order-Disorder in Alloys (New York: Elsevier)) and Stoloff and Davies (1968 Prog. Mater. Sci. 13 1-84). In the second part, the methods to study the ordering transitions, primarily on the theoretic methods are introduced. These will include (i) KKR-CPA method and supercell methods for energetic calculations; and (ii) thermodynamic and statistical methods to compute the transition temperatures. The third part will focus on representative applications in alloys, including our own work and many others. This part supplies the primary information of this review to the readers. The fourth part will summarize the connections between ordering transitions and broader physical properties (e.g. the mechanical properties). In the last part, some concluding remarks and perspectives will be given.

Magnetism: the driving force of order in CoPt, a first-principles study

CoPt equiatomic alloy orders according to the tetragonal L1(0) structure which favors strong magnetic anisotropy. Conversely, magnetism can influence the chemical ordering. We present here ab initio calculations of the stability of the L1(0) and L1(2) structures of Co-Pt alloys in their paramagnetic and ferromagnetic states. They show that magnetism strongly reinforces the ordering tendencies in this system. A simple tight-binding analysis allows us to account for this behavior in terms of some pertinent parameters.

A Comparative Study of Short Range Order in Fe-Cr and Fe-V Alloys Around Equiatomic Composition

Configurational energies have been calculated for equiatomic Fe-Cr and Fe-V alloys possessing the high temperature bcc crystalline structure, within a first principles electronic band structure approach. In agreement with experimental facts, a tendency towards order, with a B2 ordered structure of CsCI type, is found for FeV whereas phase separation characterizes FeCr. These results suggest that the nature of short range order in the high temperature bcc solid solution is not the primary driving force for describing the structural transformation from bcc to sigma which takes place in both alloys upon decreasing temperature.

Structural Energy Differences in Al3Ti: The Role of Tetragonal Distortion in APB and Twin Energies

At stoichiometry, DO22 is the observed ground state of Al2Ti and has a C/A ratio of 2.23, but as a function of both concentration and temperature other ordered arrangements of APB's are observed. These phase transitions have sparked many studies in which the energy has been treated as that of chemical rearrangement on an fcc lattice. We have found that at the ideal C/A ratio, the Ll2 structure is lower in energy, but as the tetragonal distortion increases the DO22 energy drops below that of Ll2. The critical role played by the tetragonal distortion in the balance between Ll2 and DO22 energies precludes the use of any model based on the undistorted lattice.

The major impediment to the development of Al3Ti as a high-temperature material is its lack of ductility. The standard approach is to make alloy additions which transform the structure to Ll2. An alternate approach is to work toward the enhancement of ductility in the DO22 phase. As a first step we have calculated the twinning energy in Al3Ti.

On the Feasibility of AB Initio Calculations of Ordering Alloy Phase Diagrams

Many stable or metastable intermetallic phases useful to the alloy designer have crystal structures which are ordered superstructures of a parent disordered phase. A highly reliable statistical mechanical method (CVM) has now been developed for calculating such superstructure phase equilibria derived from say, the fcc parent lattice. To obtain phase diagrams, one needs certain physical parameters, such as effective pair interaction ratios. It is possible, in principle, to extract these parameters from band structure calculations in the coherent potential approximation (CPA), particularly from recently developed cluster-CPA techniques. If sufficient accuracy can be achieved, truly first-principles phase diagram calculations may soon become feasible.

A First-Principles Study of Short Range Order in Cu-Zn

Recently, measurements of short-range order (SRO) diffuse neutron scattering intensity have been performed on quenched Cu-Zn alloys with 22.4 to 31.1 atomic percent (a/o) Zn, and pair interactions were obtained by Inverse Monte Carlo simulation [1]. These results are compared to SRO intensities and effective pair interactions obtained from first-principles electronic structure calculations. The theoretical SRO intensities were calculated with the Cluster Variation Method (CVM) in the tetrahedron-octahedron approximation with first-principles pair interactions as input. More generally, phase stability in the Cu-Zn alloy system is discussed, using ab-initio energetic properties.

How Far to Use Tight-Binding Potentials for Bimetallic Surface Modelling?

Modelling in a realistic way both equilibrium and dynamical processes on bimetallic surfaces requires the availability of interatomic potentials sufficiently simple (i.e. analytical) although derived from the electronic structure. This is possible in the framework of Tight-Binding formalism. We present here a review of the applications of such potentials, together with some reflexions about their limitations.

Effective Interatomic Interactions VIA The TB-LMTO Method

The energetics of metallic alloys, their surfaces or interfaces, and magnetic multilayers is studied in terms of effective interatomic (or interlayer) interactions that are determined from ab initio electronic structure calculations using the TB-LMTO method combined with the coherent potential approximation and the method of surface Green functions. First the theoretical background (force theorem, Lloyd formula, generalized perturbation method for bulk and surfaces, vertex cancellation theorem, method of infinitesimal rotations) is discussed, and then the applications to the phase stability of bulk alloys, surface segregation in disordered alloys, magnetism-induced ordering in two- and three-dimensional systems, phase diagram of two-dimensional alloys, interlayer exchange coupling in metallic multilayers, and the construction of Heisenberg-like Hamiltonians for magnetic systems are presented.

First-principles KKR-CPA calculation of interactions between concentration fluctuations

The aim of this work is to develop the method of calculating atomic interactions in metals and semiconductors on the basis of first-principles electronic structure calculation. A new method to calculate the atomic interactions in the framework of KKR-CPA is proposed. In this approach two specific atoms embedded in a CPA medium are considered and the effects of both electron-electron interactions and multiple scattering, which are neglected in the generalized perturbation method (GPM), are fully taken into account. The calculated atomic interactions show that these effects are important for alloys containing transition-metal alloys such as FeAl. On the other hand, in the case of AuCu, where the d states lie considerably below the Fermi level, the effects are less important.