Heusler's alloy

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As Heusler alloys , named after Friedrich Heusler , be in the metallurgy alloys denotes that a Heusler phase containing as essential component. Heusler phases are intermetallic phases with a special composition and lattice structure. They are ferromagnetic , although the alloying elements they contain do not have this property. Heusler first described this phenomenon in 1903 using Cu 2 MnAl.

Heusler compounds show different properties than one would assume from the combination of the alloy components: The first Heusler compound consisted of the non-ferromagnetic elements copper, manganese and aluminum. However, Cu 2 MnAl is a ferromagnet even at room temperature.

Structure and properties

With complete Heusler alloys with the composition X 2 YZ ( e.g .: Co 2 MnSi) two X positions (L21 structure) are occupied, with half Heusler alloys XYZ a face-centered cubic partial lattice remains unoccupied (C1b structure).

There are two types of Heusler alloys. The half Heusler alloys generally have a composition XYZ (each letter stands for an alloy element), while the complete Heusler alloys are composed according to the formula X 2 YZ. X and Y are transition metals , while Z is an element of III.-V. Main group is. The alloying elements form phases of order , so that the crystal structure consists of four face-centered cubic sub- lattices nested in one another (one of the XYZ types is empty) . The interactions between the atoms of the partial lattices, which have not yet been fully understood in detail, result in an almost complete alignment ( spin polarization ) of the magnetic dipole moments of the “correctly polarized” part (e.g. spin ) of the electrons, while the “incorrectly polarized” electrons (e.g. . B. ) are all non-metallic. Which manifests itself macroscopically at sufficiently low temperatures (e.g. room temperature and below) as ferromagnetism . So you are dealing with so-called semi - metal ferromagnetism.

Use as a topological insulator

In 2006, Shoucheng Zhang at Stanford University predicted a new quantum state of matter for the semiconductor mercury telluride (HgTe). Physicists working with Laurens W. Molenkamp confirmed the assumption experimentally at the University of Würzburg. Topological insulators consist of materials that are insulators or semiconductors, but have a metallic effect on their surface or at interfaces. They differ from normal metals because, like superconductors, the electrons on the surface or at interfaces do not interact with the environment because they are in a new quantum state. Unlike in superconductors, topological insulators show two non-interacting currents, one for each spin direction. Both spin currents, which perceive neither defects nor impurities in the material, can be used for information processing. Of the more than 1000 known Heusler compounds, more than 50 features are assumed to be topological insulators .

List of some Heusler phases

  • Cu 2 MnAl, Cu 2 MnIn, Cu 2 MnSn,
  • Ni 2 MnAl, Ni 2 MnIn, Ni 2 MnSn, Ni 2 MnSb, Ni 2 MnGa
  • Co 2 MnAl, Co 2 MnSi, Co 2 MnGa, Co 2 MnGe
  • Pd 2 MnAl, Pd 2 MnIn, Pd 2 MnSn, Pd 2 MnSb
  • Co 2 FeSi, Co 2 FeAl
  • Fe 2 VAl
  • Mn 2 VGa, Co 2 FeGe

References and footnotes

  1. ↑ G. Sauthoff : Intermetallics, Wiley-VCH, Weinheim 1995, p. 83 u. 90.
  2. ↑ F. Heusler: Verh. Deutsche Physikalische Gesellschaft 5 (1903), p. 219 ff.
  3. ↑ Otto-Albrecht Neumüller (Ed.): Römpps Chemie-Lexikon. Volume 3: H-L. 8th revised and expanded edition. Franckh'sche Verlagshandlung, Stuttgart 1983, ISBN 3-440-04513-7 , p. 1694.
  4. ↑ As is well known, for every electron state there are two spin polarizations, and , which are usually equally frequent and have the same energy. In ferromagnetic material, however, electrons with the “correct polarization” are more common, since they have lower energy than corresponding states with the wrong polarization.
  5. ↑ K. Ramesh Kumar, K. Kamala Bharathi, J. Arout Chelvane, S. Venkatesh, G. Markandeyulu, N. Harishkumar: First-Principles Calculation and Experimental Investigations on Full-Heusler Alloy Co 2 FeGe . In: IEEE Transactions on Magnetics . 45, No. 10, 2009, p. 3997. bibcode : 2009ITM .... 45.3997K . doi : 10.1109 / TMAG.2009.2022748 .

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