Determination of L21 vs. B2 phase content in Heusler alloys Ni2MnGa and Co2FeGe0.5Ga0.5 with x-ray diffraction
Abstrakt
Ni2MnGa and Co2FeGe0.5Ga0.5 are promising members of Heusler alloys because of their application potential. Ni2MnGa is a shape-memory alloy which can be used for micropumps [1] or actuators [2] and Co2FeGe0.5Ga0.5 has a potential in spintronics [3].
Important properties used for applications are conected to the low-temperature L21 phase. The high-temperature B2 phase (assuming the formula X 2 YZ, the elements Y and Z are mixed together at their positions in unit cell) is for these purposes parasitic and we need to get rid of it.
The presence of B2 phase can be observed for example in as cast samples without further heat treatment. This work presents the determination of L21 phase content by x-ray diffraction.
According to symmetry, we should observe L21 phase only diffraction with all indices odd or all even (fcc symmetry). Higher symmetry of B2 phase discribed with the equally big unit cell leads to the extinction of diffractions which have all indices odd.
Proper computation of structure factors shows that diffractions with all even indices remain the same in both phases. Therefore, the ratio of integrated intensities corresponding to diffractions with all indices odd and all indices even should reveal the content of L2 1 phase in the sample.
However, the whole problematics is more complicated, because it is neccessary to apply all angularly dependend corrections such as polarisation, Lorentz correction, absorption, irradiated volume and primary and secondary extinction. The results show that this procedure works fine with thin layers and with bulk single-crystals with small mosaicity.
If the mosaicity is large enough, it complicates the application of the corrections - especially the irradiated volume and extinction - because it is unclear how big volume of which mosaic block was irradiated. Extinction correction is connected to the size and misorientation of these blocks.
Nevertheless, it is clearly visible that good temperature treatment can increase the L2 1 content, because overall intensity of sensitive diffractions (all indices odd) increases after annealing procedure (diffraction measured in the same experimental arangement for all cases). [1] A. R.
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