The discovery of Heusler alloys dates back to 1903 when Heusler reported that the addition of sp elements (Al, In, Sn, Sb or Bi) turn Cu-Mn alloy into a ferromagnetic material eventhough the alloy contains none of the ferromagnetic elements.
The basic understanding of crystal structure
and composition of these alloys remained unknown for a long time. In 1929X-ray
measurements of Potter [38] on Cu-Mn-Al alloy revealed that all constituents of
thissystem was ordered on an fcc super lattice. Bradley and Rodgers [39]
investigated Cu-Mn-Alsystem in detail using X-ray and anomalous scattering. The
authors established a relationship between composition, chemical order and
magnetic properties. After complete understanding of crystal structure numerous
investigations were made. It is found that the Heusler structureis formed
essentially from the ordered combination of two binary B2 compounds XY and XZ.Both
compounds may have the CsCl type crystal structure, for instance CoMn and CoAl
yield Co2MnAl. Thus the ability of compounds to form B2 structure indicates the
possibility offorming new Heusler compounds. It was also discovered that it is
possible to leave one of thefour sublattices unoccupied (C1b structure). The
latter compounds are often called half- orsemi-Heusler alloys, while the L21compounds
are referred to as full-Heusler alloys. Extensiveexperimental studies showed
that the majority of Heusler compounds order ferromagnetically in
stoichiometric composition. Crystal structure, composition and heat treatment
were found to be important parameters for determining magnetic properties.
With the discovery of half-metallic
ferromagnetism in NiMnSb and the observation ofshape memory effect in Ni2MnGa
compound, Heusler alloys received tremendous experimen-tal and theoretical
interest. In this chapter we will briefly present the previous experimentaland
theoretical studies on structural and magnetic properties of Heusler alloys.
Also, anoverview of the experimental and theoretical studies on exchange coupling
will be given.
2- Structural properties
Heusler alloys are defined as the ternary intermetallic compounds. At the stoichiometric composition, full Heusler alloys (X2YZ) and semi Heusler alloys (XYZ) crystallize in L21 and C1b structures (see figure4.1), respectively. The elements normally associated with the X, Yand Z are indicated in table1. The unit cell consists of four interpenetrating fcc sublatticeswith the positions (000) and (1/2,1/2,1/2) for X, (1/4,1/4,1/4) for Y and (3/4,3/4,3/4) for Z atom. The site (1/2,1/2,1/2) is vacant in semi Heusler compounds. The two structures are closely related with vacant cite. C1b structure can be obtained from L21 one by replacing the half of the X sites in an ordered manner. Consequently, the structure no longer centro-symmetric. Inmajority of the Heusler alloys Mn element enters as the Y element. The compounds where Mn assumes the X positions are very rare. Up to now, only two systems of this type were studied experimentally: Mn2VAl [40] and Mn2VGa [41].
At the stoichiometric composition,
disorder can exist in the form of partial interchange of atoms in different
sublattices. Johnston and Hall [42] proposed a single disordering parameter α
to describe the effects of certain types of preferential disorder on the
structure amplitudes of alloys of the type X2YZ. For alloys ordered in
L21structureαis defined as the fraction ofeither Y or Z atoms being not on
their correct sites. Partial occupation of Y and Z atoms on each others
sublattices leads to L21-B2 type disorder. B2-type structure can be obtainedby
allowing half of the Y and Z atoms interchange their positions. The ratio of
the L21/B2depends on the heat treatments. Due to smaller interatomic distances
in B2-type structure,an antiferromagnetic ordering becomes energetically
favorable.
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