Alloys Heusler

Figure 6 [001] HRTEM image of the distorted austenite of L2i Ni2MnGa Heusler alloy clearly revealing modulated ISO s. The modulation wavelength is appr. 6(110) planes (courtesy Zheludev et al. ).

A1 Mn Cu Cu MnCu2Al (Heusler alloy) Fm3m LiNi2Sn, TiCo2Si... 

An example where, due to ordering, we observe perhaps in a more immediate way, the increase of the unit cell size (formation of a multiple cell) is the MnCu2Al-type structure (representative of the so-called Heusler alloys) which can be considered a derivative structure (superstructure) of the cP2-CsCl type, which in turn is a superstructure of the W-type structure, corresponding to a non-primitive cubic cI2... 

In the Heusler alloy ErPd2Sn superconductivity and antiferromagnetic order coexist although there is no clear separation between the superconducting and the magnetic sublattices and Tc 1.17 K is not much different from 7 n 1 K (Shelton et al. 1986 Stanley et al. 1987). However, the focus on this interesting compound was short lived because of the discovery of the high Tc cuprate superconductors (Lynn 2001). 

Keywords ferromagnetism, martensitic transformations, Heusler alloys, phase diagram. 

The structural (martensitic) transition in Ni2MnGa-based Heusler alloys was described as driven by a band Jahn-Teller effect [ff, 12], In the framework of this model, the results of theoretical [12] and experimental [If] studies of the redistribution of magnetization upon martensitic transformation in Ni2MnGa alloys have been discussed. 

S. Joseph Poon, Electronic and Thermoelectric Properties of Half-Heusler Alloys Terry M. Tritt, A. L. Pope, and J. W. Kolis, Overview of the Thermoelectric Properties of Quasicrystalline Materials and Their Potential for Thermoelectric Applications Alexander C. Ehrlich and Stuart A. Wolf, Military Applications of Enhanced Thermoelectrics David J. Singh, Theoretical and Computational Approaches for Identifying and Optimizing Novel Thermoelectric Materials... 

In Table XXIII predictions for several ordered alloys are compared with available experimental data. Many of these have already been discussed. Attention is here directed to the Heusler alloys (Cu2MnM, with M = Al, Ga, In, Sn, As, Sb, Bi), some perovskite-type nitrides and carbides, and the Mn-Au alloys. These examples are discussed because they illustrate how the chemical intuition that has been developed can provide insights into the electron correlations of relatively complex compounds. 

Chemical stability indicates that in the cubic, metallic perovskites the interstitial C and N are probably neutral. They represent, therefore, an M atom with half filled p or s-p orbitals, and in the cubic structure the metal-M-metal interaction is defined by Figure 86. [This is to be contrasted with low-temperature CrN, which has considerable ionic character and an ordering of its covalent character along a given axis.] In contrast to the M atoms of the Heusler alloys, the p electrons of C and N correlate with, and therefore spin pair, the near-neighbor eg electrons. The metal- -metal interactions are determined by the Ug electron-spin correlations since Hu 2.76 A < Rc. 

Heusler alloys have a rich variety of apphcations, owing to some of their unique properties. Some of these phases are half-metallic ferromagnets, exhibiting semiconductor properties for the majority-spin electrons and normal metallic behavior for the minority-spin electrons. Therefore, the conduction electrons are completely polarized. The Ni2MnGa phase is used as a magnetic shape memory alloy and single crystals of Cu2MnAl are used to produce monochromatic beams of polarized neutrons. 

The surface of a half-metallic semi-Heusler alloy NiMnSb 001 ... 

Amongst the magnetic alloys, two groups in particular have been subjected to considerable attention in recent years the so-called Heusler and semi-Heusler alloys. These materials were first described in two short papras [123,124] published in 1903, and take the general compositioiuil formulae X2MnY and XMnY respectively, where X represents an element from the t/-block of the periodic table, and Y an element from the />-block. For a wide range of combinations of X and Y, these alloys crystallise in well-ordered structures based upon the fee lattice. 

Despite their long pedigree, however, the ewent interest in the Heusler and semi-Heusler alloys was only sparked in 1983 upon the publication of a theoretical paper by de Groot et al [125] in which the prototypical semi-Heusler alloy NiMnSb was predicted to display half-metallic properties. The concept of a half-metal needs to be carefully distinguished from the similar-sounding but entirely different conc ts of Ae semi-metal and the semiconductor. The property of half-metallicity is inextricably linked with the permanent magnetic polarisation of a material, and refers to the case when the electrons at the Fermi level display 100% spin-polarisation. In other words, while the electronic bands of one spin-species cross the Fermi level, those of the other spin-species do not. In the case of half-metallic semi-Heusler alloys, the majority-spin electrons behave as if the material were a good metal, while the minority-spin electrons behave as if the material were an insulator or semiconductor. 

Subsequent to the initial work of de Groot et al [125] on NiMnSb, a number of other semi-Heusler alloys have also been predicted to be half-metallic. Their potential in spintronic applications cannot be overstressed. In effect, semi-Heusler alloys could one day be employed as perfect spin-valves, for injection of polarised clarge carriers into semiconductor devices. Before such potential can be realised, however, it will be necessary to understand in detail die electronic... 

It is fair to say that the surface science of the half-metal semi-Heusler alloys is in its infancy. The majority of studies on these materials have focussed upon the bulk properties, and surface effects have been viewed largely as unwanted complications. Nevertheless, a small amount of experimental data on the electronic nature of the NiMnSb surface is now available, and DFT calculations have a considerable role to play, both in interpreting the results and in suggesting routes for further investigation. 

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