Heusler compounds were discovered in 1903 by Friedrich Heusler. It was found that elements whose valence electrons occupied sporbitals, such as, Aluminum(Al), Indium (In), Tin (Sn), Strontium (Sb) and Bismuth (Bi),turned Copper –Manganese (Cu –Mn) alloy into a ferromagnetic material, even though the alloy does not containanyof the ferromagnetic elements (Hillebrands and Felser, 2006). In this section, a detailed treatiseof priorstudies on Heusler compounds will be discussed.
Heusler structure is formed from the ordered combination of two binary B2 alloys XY and XZ crystallising in the Ceasium Chloride (CsCl) type crystal structure (Buschow, 2003)shown in Figure 2.1(Pauling et al., 1993). The X element is normally a 3dtransition metal elementsuch as Cr, Mn, Fe, Co, Ni, while 3d, 4dor 5delementsenter as the Y element. spelements enter the Z position.
TheCsCl structure is an interpenetrating primitive cubic structure. Each of the constituent atoms forms theirown primitive cubic lattice. The separate primitive lattices interpenetrate such that an atom of one type occupies the centre of the cube of the other type. It is similar to the bcc structure, the difference being thatthe atoms at the different lattice sites alternate. It can also be viewed as asimple cubic with a different atom at the centre of the cube.At the stoichiometric composition, full Heusler alloys (X2YZ) crystallise in the L21structure in the cubic space group Fm-3m (similar to NaCl), space group no. 225, prototype Cu2MnAl (Grafet al., 2011), which is shown in Figure 2.2(Pauling et al., 1993).The L21structurehas a unit cell that consistsof four interpenetrating fcc sublattices with the Wyckoff positions 8c (¼, ¼, ¼) for X atoms,4b (½, ½, ½) for Y and 4a (000) for the Z atom.It,therefore,consists of 4 interpenetrating fcc sublattices, two of which are equally occupied by X atoms.
A rock salt type (NaCl) lattice is formed by the Y and Z elements, which are the most and least electropositiveelements respectively,due to their ionic characterandare octahedrally coordinated. The X atoms fill all the tetrahedral holes. The structure can be viewed as a zinc blende type sub lattice, madeup ofone X and Z. The remaining tetrahedral holesare occupied by the second X, whereas Y fills the octahedral holes. At stoichiometric compositions, the atoms may partially interchange their positions, resulting in disorder. When the Y and Z atoms partially occupy each other’s sub latticean L21–B2 type disorder results. When half of Y and Z atoms interchange, a B2 type structure can be obtained, which results in an antiferromagnetic ordering due to smaller inter-atomic distances in B2 type structure. The ratio of L21/B2 depends on the heat treatments. Y, Z disorder does not affect properties of half-metals significantly (Graf et al., 2011).
The Z atom provides the sp states and,therefore,the energy of the pelectrons is dependent on the Z atoms. It hybridises with d-electrons and determines the degree of occupation of the p-dorbitals. They,therefore,have a role in determining the total number of occupied and empty states which leads to the positioning of the Fermi level within the band gap (Trudel et al., 2010). The magnetic order and the formation of the magnetic moment are dependent on the spelectron concentration (Liu et al., 2008). In particular, it plays a major role in the formation of the magnetic moments at the X site by changing the lattice parameter (Kulkova et al., 2006).
The half-Heusler compounds have a C1bstructure. The Clbstructure can be obtained from the L21 by leaving half of the tetrahedral sites vacantor by systematically replacing half of the X sites witha different atom.
The half-metallic gap in full Heusler compounds occurs as a result of the strong d-dhybridisation between d-states of the transition metal atoms, as well as a p-dhybridisation involving the main group element, such that a gap for only one spin direction is formed, with the Fermi level positioned in it. The energy gap width decreases with expanded lattice and with increasing atomic number (Luo et al., 2008b).
Mn element enters as the Y elementin most of the Heusler alloys. Mn in X is rare for example in Mn2VAl and Mn2VGa, in which together with CoMnSb, ferrimagnetism has been detected. Heusler compounds, having Cobalt as the X element have exceptional electronic structure and outstanding properties that have made them the subject of a great amount of research activity in spintronics. Magnetism is morecommon in 3dand 4f materials than in 4dand 5dcompounds butwhen itoccurs, it’s very interesting. Strong-magneto crystalline and magneto-optical effects may be observed due to more spin –orbit effects (Solovyev et al., 2003).
2- Theoretical and Experimental Studies on Heusler Compounds
Theoretical studies using electronic band structure calculations and experimental investigations have been carried out by several researchers on the structure, electronic and magnetic properties of Heuslercompounds. Mn2VAl is one of the Heusler systems that hasreceived considerable attention, both theoretically and experimentally. A study done by Kawakamietal.(1981),onthe magnetic properties of Mn2VAl using Nuclear Magnetic Resonance (NMR) concluded that itis a simple ferromagnet based on the magnetic field dependence of the NMR frequencies andthe observed high field susceptibility.DFTcalculations conducted on Mn2VAl employing the Generalised Gradient Approximation (GGA), led to a prediction of half-metallic ferrimagnetism, with only the spin down electrons available for conduction. Atomic moments of 1.5𝜇𝐵and −0.9𝜇𝐵, for Mn and V atoms respectively were reported(Weht and Pickett, 1999). However, magnetisation and resistivity measurements for Mn2VAl prepared by arc melting, found that the dipole moment was 1.94𝜇𝐵/𝑓.𝑢at 5K and the residual resistivity was 1.3×10−8Ω𝑚for temperature dependence below 60 K. In this study, low magneto resistance of <0.1 % at 1T at 77K, measured on the samples or pressed powder made it difficult for the samples to be classified as half-metallic as had been predicted byelectronic band structure calculations (Jiang et al., 2001).
Reference: Thesis
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