Heusler alloy

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A Heusler alloy is a ferromagnetic metal alloy based on a Heusler phase. Heusler phases are intermetallics with particular composition and face-centered cubic crystal structure. They are ferromagnetic even though the constituting elements are not as a result of the double-exchange mechanism between neighboring magnetic ions, usually manganese which sit at the body centers in a Heusler alloy. The magnetic moment usually resides almost solely on the manganese atom in these alloys. See the Bethe-Slater curve for more info on why this happens.

The term is named after a German mining engineer and chemist Friedrich Heusler, who studied such an alloy in 1903. It contained two parts copper, one part manganese, and one part tin. The Heusler alloy Cu₂MnAl has been the subject of a considerable number of studies and the stoichiometric alloy (i.e. one in which the proportion of elements is exactly as in the formula above) has the following properties. It has a room temperature saturation induction of around 8,000 gauss (Bouchard 1970) which is in excess of that of the element nickel (around 6100 gauss) although less than that of iron (around 21500 gauss). Early studies (Heusler 1903, Knowlton and Clifford 1912, review Bozorth 1951) showed that the magnetic properties varied considerably with heat treatment and composition. Bradley and Rogers (1934) first showed that the room-temperature ferromagnetic phase was a fully ordered structure of the L2₁ type. This has a primitive cubic lattice of copper atoms which has alternate cells body-centred by manganese and aluminium . The lattice parameter is 5.95 angstrom units. The molten alloy has a solidus temperature around 910^oC. As it is cooled below this temperature the fully disordered solid body-centred cubic beta phase forms. Below 750^oC a B2 ordered lattice forms (Nesterenko 1969, Bouchard 1970) with a primitive cubic copper lattice body-centred by a disordered manganese aluminium sublattice. Cooling below 610^oC causes further ordering of the manganese and aluminium sub-lattice to the L2₁ form (Bouchard 1970, Ohoyama et al 1968). Studies of off-stoichiometric alloys have been made by West and Lloyd-Thomas (1956), Johnston and Hall (1968) and Bouchard (1970). In general the ordering temperatures decrease for these compositions and the range of temperatures within which the alloy can be annealed without forming microprecipitates becomes small.

Oxley et al (1963) found a value of 357^oC for the Curie temperature, below which the alloy becomes ferromagnetic. A variety of investigators using neutron diffraction and other techniques (e.g. Endo et al 1963, Bouchard 1970) 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 Angstrom units apart, it seems likely that the exchange interaction aligning the spins must be indirect through conduction electrons or the aluminium and copper atoms. Theoretical studies of the interaction have been made by Oxley et al (1963) and Geldart and Ganguly (1970).

Electron microscope studies (Nesterenko 1969, Bouchard 1970) have shown that thermal antiphase boundaries (APBs) form during cooling through the ordering temperatures as ordered domains nucleate at different centres within the crystal lattice and are often out of step with each other where they meet. The anti-phase domains grow as the alloy is annealed. There are two types of APB corresponding to the B2 and L2₁ types of ordering. APBs also form between dislocations if the alloy is deformed. At the APB the manganese atoms will be closer than in the bulk of the alloy and electron microscope studies (Lapworth and Jakubovics 1974) showed that for non-stoichiometric alloys with an excess of copper (e.g. Cu_2.2MnAl_0.8) an antiferromagnetic layer forms on every thermal APB. 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.

In recent times, the importance of Heusler alloys for spintronics has been increasing.

Another useful Heusler alloy is the class of materials known as ferromagnetic shape memory alloys which can change their length by up to 10% on application of a magnetic field. These are generally an alloy of nickel-manganese-gallium.

[edit] List of Heusler alloys

Cu₂MnAl, Cu₂MnIn, Cu₂MnSn,
Ni₂MnAl, Ni₂MnIn, Ni₂MnSn, Ni₂MnSb
Co₂MnAl, Co₂MnSi, Co₂MnGa, Co₂MnGe
Pd₂MnAl, Pd₂MnIn, Pd₂MnSn, Pd₂MnSb

[edit] References

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