We have modelled the phase diagram of magnetic shape memory alloys of the Heusler type by using the phenomenological Ginzburg–Landau theory. When fixing the parameters by realistic values taken from experiment we are able to reproduce most details of, for example, the phase diagram of Ni2+xMn1−xGa in the (T, x) plane. We present the results of ab initio calculations of the electronic and phonon properties of several ferromagnetic Heusler alloys, which allow one to characterize the structural changes associated with the martensitic instability leading to the modulated and tetragonal phases. From the ab initio investigations emerges a complex pattern of the interplay of magic valence electron per atom numbers (Hume–Rothery rules for magnetic ternary alloys), Fermi surface nesting and phonon instability. As the main result, we find that the driving force for structural transformations is considerably enhanced by the extremely low lying optical modes of Ni in the Ni-based Heusler alloys, which interfere with the acoustical modes enhancing phonon softening of the TA2 mode. In contrast, the ferromagnetic Co-based Heusler alloys show no tendency for phonon softening.

https://iopscience.iop.org/article/10.1088/0022-3727/39/5/S13/pdf

Modelling the phase diagram of magnetic shape memory Heusler alloys

Phase equilibria in alloys to a great extent are governed by the ordering behavior of alloy species. One of the important goals of alloy theory is therefore to be able to simulate these kinds of phenomena on the basis of first principles. Unfortunately, it is impossible, even with present day total energy software, to calculate entirely from first principles the changes in the internal energy caused by changes of the atomic configurations in systems with several thousand atoms at the rate required by statistical thermodynamics simulations. The time-honored solution to this problem that we shall review in this paper is to obtain the configurational energy needed in the simulations from an Ising-type Hamiltonian with so-called effective cluster interactions associated with specific changes in the local atomic configuration. Finding accurate and reliable effective cluster interactions, which take into consideration all relevant thermal excitations, on the basis of first-principles methods is a formidable task. However, it pays off by opening new exciting perspectives and possibilities for materials science as well as for physics itself. In this paper we outline the basic principles and methods for calculating effective cluster interactions in metallic alloys. Special attention is paid to the source of errors in different computational schemes. We briefly review first-principles methods concentrating on approximations used in density functional theory calculations, Green's function method and methods for random alloys based on the coherent potential approximation. We formulate criteria for the validity of the supercell approach in the calculations of properties of random alloys. The generalized perturbation method, which is an effective and accurate tool for obtaining cluster interactions, is described in more detail. Concentrating mostly on the methodological side we give only a few examples of applications to the real systems. In particular, we show that the ground state structure of Au3Pd alloys should be a complex long-period superstructure, which is neither DO22 nor DO23 as has been recently predicted.

http://iopscience.iop.org/article/10.1088/0034-4885/71/4/046501/pdf

Configurational thermodynamics of alloys from first principles : effective cluster interactions

We report on the results of a systematic ab initio study of the magnetic structure of Fe rich fcc FeNi binary alloys for Ni concentrations up to 50 at. %. Calculations are carried out within densit ...

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Competition between Magnetic Structures in the Fe-Rich FCC FeNi Alloys

We review recent developments in the field of first-principles simulations of magnetic materials above the magnetic order disorder transition temperature, focusing mainly on 3d-transition metals, t ...

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Recent progress in simulations of the paramagnetic state of magnetic materials

We show that the large negative magnetic contribution to the thermal expansion in disordered Fe-Pt alloys can be understood within the disordered local moment (DLM) approach. On the basis of first principles calculations we quantitatively describe the spontaneous volume magnetostriction for various Pt concentrations. It is found that the Invar effect in these alloys is entirely related to the state of thermal magnetic disorder modeled by the DLM states. We also show that the experimentally observed anomaly in the temperature dependence of the magnetization is due to a spontaneous reduction of the local magnetic moments rather than to "hidden excitations."

Large negative magnetic contribution to the thermal expansion in iron-platinum alloys: quantitative theory of the Invar effect.

Zero- and finite-temperature (T) first-principles calculations versus composition (c) show that magnetochemical effects lead to Invar anomalies in Fe-(Ni, Co, Pt) alloys. Chemical short- or long-range order and negative interatomic exchange interaction of electrons in antibonding majority-spin states force the face-centered-cubic lattice to compete simultaneously for a smaller volume (from antiferromagnetic tendencies) and a larger volume (from Stoner ferromagnetic tendencies). The resulting additional negative lattice anharmonicity is very large for Fe-(Ni, Co) while absent for Fe-Pt. Our results explain the T- and c-dependent behavior of Invar properties, including the lattice softening and thermal expansion of Fe-Ni. In addition, the occurrence of a noncollinear spin structure at $T=0 \mathrm{K}$ near Invar can be understood on the basis of our results.

/pdf/magnetochemical-origin-for-invar-anomalies-in-iron-nickel-gy9zrmfh0v.pdf

Magnetochemical origin for Invar anomalies in iron-nickel alloys

Electronic properties of Cu–Zn–Al alloys

The electronic and magnetic properties of ferromagnet-semiconductor (FM/SC) heterostructure systems have been studied by means of fully relativistic LMTO as well as scalar-relativistic KKR-CPA band structure calculations. As a structural model for our calculations periodic multilayer systems have been assumed, taking for the ferromagnet Fe and for the semiconductor GaAs, respectively. In addition the influence of interdiffusion at the interface was investigated. Concerning the resulting magnetisation profiles the most important features are that no magnetically dead layers occurred and an appreciable induced magnetisation was found in the semiconductor subsystem. To stimulate corresponding experimental work the magnetic circular dichroism has been investigated for the L 2,3 -edges absorption spectra of Ga as well as As. In all cases the circular dichroism was found to be pronounced enough to be detectable. Investigations on the magneto-crystalline anisotropy energy revealed that all of the studied systems...

Electronic and Magnetic Properties of Ferromagnet-Semiconductor Heterostructure Systems

First-principles calculations using the Korringa-Kohn-Rostocker method and the Coherent-Potential Approximation for Fe-Ni, Fe-Ni-Co and Fe-Pt alloys show that several features are responsible for the Invar anomalies. Atomic short range ordering in the alloys is responsible for the appearance of antiferromagnetic and non-colinear magnetic moments. The antiferromagnetic contributions are responsible for two effects, the negative anharmonicity due to the tendency of the alloys to have a smaller lattice constant, as well as the tendency to have a larger lattice constant because of additional density of states of antibonding majority-spin orbitals at the Fermi level, which simultaneously stabilizes the antiferromagnetic moments.

First-Principles Calculations of the Instabilities in Fe-(Ni, Co, Pt) Alloys

Details of the reactive processes by which TiC and TiN microclusters are formed from precursor molecules in the gas phase are not clearly understood. We have performed ab initio calculations based on density functional theory methods in order to get some insight into the chemical reactions of precursor molecules. Using microcanonical molecular-dynamics we have simulated scattering processes between molecules consisting of the elements Ti, Cl, N and H. The simulations show that fluctuations of the kinetic energy can be large enough to break up bonds at moderate temperatures.

V. Crisan

Papers

Magnetochemical origin for Invar anomalies in iron-nickel alloys

Electronic properties of Cu–Zn–Al alloys

Electronic and Magnetic Properties of Ferromagnet-Semiconductor Heterostructure Systems

First-Principles Calculations of the Instabilities in Fe-(Ni, Co, Pt) Alloys

First-Principles Investigations of Precursor Molecules for Microclusters