Heusler compound

Type of metallic alloy

Heusler compounds are magnetic intermetallics with face centered cubic crystal structure and a composition of XYZ (half Heuslers) or X2YZ (full Heuslers), where X and Y are transition metals and Z is in the p block. The term derives from the name of German mining engineer and chemist Friedrich Heusler, who studied such a compound (Cu2MnAl) in 1903. arises from the interpretation of Heuslers as intermetallics and is used predominantly in literature studying magnetic applications of Heuslers compounds. The XY2Z convention on the other hand is used mostly in thermoelectric materials and transparent conducting applications literature where semiconducting Heuslers (most half Heuslers are semiconductors) are used. This convention, in which the left most element on the periodic table comes first, uses the Zintl interpretation of semiconducting compounds where the chemical formula XY2Z is written in order of increasing electronegativity. In well known compounds such as Fe2VAl which were historically thought of as metallic (semi metallic) but were more recently shown to be small gap semiconductors one might find both styles being used. In the present article semiconducting compounds might sometimes be mentioned in the XY2Z style.

Although traditionally thought to form at compositions XYZ and X2YZ, studies published after 2015 have discovered and reliably predicted Heusler compounds with atypical compositions such as XY0.8Z and X1.5YZ. Besides these ternary compositions, quaternary Heusler compositions called the double Half Heusler X2YY'Z2 (e. g. Ti2FeNiSb2) and triple Half Heusler X2X'Y3Z3 The stable compositions and corresponding electrical properties for these compounds can be quite sensitive to temperature and their order disorder transition temperatures often occur below room temperatures. The X1.5YZ semiconducting composition is stabilized by the transition metal X playing a dual role (electron donor as well as acceptor) in the structure.

The magnetism of the early full Heusler compound Cu2MnAl varies considerably with heat treatment and composition. It has a room temperature saturation induction of around 8,000 gauss, which exceeds that of the element nickel (around 6100 gauss) but is smaller than that of iron (around 21500 gauss). For early studies see. In 1934, Bradley and Rogers showed that the room temperature ferromagnetic phase was a fully ordered structure of the L21 Strukturbericht type. This has a primitive cubic lattice of copper atoms with alternate cells body centered by manganese and aluminium. The lattice parameter is 5.95 Å. The molten alloy has a solidus temperature of about 910 °C. As it is cooled below this temperature, it transforms into disordered, solid, body centered cubic beta phase. Below 750 °C, a B2 ordered lattice forms with a primitive cubic copper lattice, which is body centered by a disordered manganese aluminium sublattice. Cooling below 610 °C causes further ordering of the manganese and aluminium sub lattice to the L21 form. In non stoichiometric alloys, the temperatures of ordering decrease, and the range of anealing temperatures, where the alloy does not form microprecipitates, becomes smaller than for the stoichiometric material. Neutron diffraction and other techniques have shown that a magnetic moment of around 3.7 Bohr magnetons resides almost solely on the manganese atoms. As these atoms are 4.2 Å apart, the exchange interaction, which aligns the spins, is likely indirect and is mediated through conduction electrons or the aluminium and copper atoms. Electron microscopy studies demonstrated that thermal antiphase boundaries (APBs) form during cooling through the ordering temperatures, as ordered domains nucleate at different centers within the crystal lattice and are often out of step with each other where they meet. These antiferromagnetic layers completely supersede the normal magnetic domain structure and stay with the APBs if they are grown by annealing the alloy. This significantly modifies the magnetic properties of the non stoichiometric alloy relative to the stoichiometric alloy which has a normal domain structure. Presumably this phenomenon is related to the fact that pure manganese is an antiferromagnet although it is not clear why the effect is not observed in the stoichiometric alloy. Similar effects occur at APBs in the ferromagnetic alloy MnAl at its stoichiometric composition. Some Heusler compounds also exhibit properties of materials known as ferromagnetic shape memory alloys. These are generally composed of nickel, manganese and gallium and can change their length by up to 10% in a magnetic field.

Understanding the mechanical properties of Heusler compounds is paramount for temperature sensitive applications (e. g. thermoelectrics) for which some sub classes of Heusler compounds are used. However, experimental studies are rarely encountered in literature. In fact, the commercialization of these compounds is limited by the material's ability to undergo intense, repetitive thermal cycling and resist cracking from vibrations. An appropriate measure for crack resistance is the material's toughness, which typically scales inversely with another important mechanical property: the mechanical strength. In this section, we highlight existing experimental and computational studies on the mechanical properties of Heusler alloys. Note that the mechanical properties of such a compositionally diverse class of materials is expectedly dependent on the chemical composition of the alloys themselves, and therefore trends in mechanical properties are difficult to identify without a case by case study. The elastic modulus values of half Heusler alloys range from 83 to 207 GPa, whereas the bulk modulus spans a tighter range from 100 GPa in HfNiSn to 130 GPa in TiCoSb. The decrease in modulus with respect to temperature is also observed in TiNiSn, ZrNiSn, and HfNiSn, where ZrNiSn has the highest modulus and Hf has the lowest. This phenomenon can be explained by the fact that the elastic modulus decreases with increasing interatomic separation: as temperature increases, the atomic vibrations also increase, resulting in a larger equilibrium interatomic separation. The mechanical strength is also rarely studied in Heusler compounds. One study has shown that, in off stoichiometric Ni2MnIn, the material reaches a peak strength of 475 MPa at 773 K, which drastically reduces to below 200 MPa at 973 K. In another study, a polycrystalline Heusler alloy composed of the Ni Mn Sn ternary composition space was found to possess a peak compressive strength of about 2000 MPa with plastic deformation up to 5%. However, the addition of Indium to the Ni Mn Sn ternary alloy not only increases the porosity of the samples, but it also reduces the compressive strength to 500 MPa. It is unclear from the study what percentage of the porosity increase from the indium addition reduces the strength. Note that this is opposite to the outcome expected from solid solution strengthening, where adding indium to the ternary system slows dislocation movement through dislocation solute interaction and subsequently increases the material's strength. The fracture toughness can also be tuned with composition modifications. For example, the average toughness of Ti1−x(Zr, Hf)xNiSn ranges from 1.86 MPa m1/2 to 2.16 MPa m1/2, increasing with Zr/Hf content. In their study, samples of Ti0.5Hf0.5Co0.5Ir0.5Sb1−xSnx were prepared using three different methods: a high temperature solid state reaction, high energy ball milling, and a combination of both. The study found higher fracture toughness in samples prepared without a high energy ball milling step of 2.7 MPa m1/2 to 4.1 MPa m1/2, as opposed to samples that were prepared with ball milling of 2.2 MPa m1/2 to 3.0 MPa m1/2. with the case of NiMnSb. Half metallicity leads to the full polarization of the conducting electrons. Half metallic ferromagnets are therefore promising for spintronics applications.