Optimisation of Heusler Alloy Thin Films for Spintronic Devices

Heusler alloys are one of the leading candidate material classes for achieving high spin polarisation. A number of Co-based Heusler alloys are predicted to be half-metallic ferromagnets that would theoretically provide 100% spin polarisation at the Fermi energy. However, there are yet to be any reports of this 100% spin polarisation experimentally. To develop these materials as a viable spin source their magnetic and structural properties must be fully characterised and optimised.
In this study both epitaxial and polycrystalline thin films have been deposited and their structural and magnetic properties studied in detail using a wide variety of magnetometry and electron microscopy techniques. The polycrystalline films form an amorphous matrix in the as-deposited state which crystallises into ordered grains after annealing at 235°C. These films have a wide range of magnetic and structural properties due to the crystallisation processes. Films are found to exhibit magnetisation reversal by both single domain particle rotation and domain wall processes which lead to coercivities ranging from 100 Oe to 2000 Oe. The individual grains themselves are found to be highly ordered into the B2 or L21 crystal phases. In the single crystal films long range L21 ordering is observed, the extent of which increases monotonically with annealing temperature. These films also show extremely low coercivities <30 Oe. The magnetisation reversal is controlled by a series of misfit dislocations at the film substrate interface which could make these films potentially unsuitable for device applications.
To control the magnetic and structural properties a number of seed layers have been tested. Ag seed layers were found to reduce the coercivity of the polycrystalline films to similar values to those found for the singe crystal films (Hc <50 Oe). The Ag seed layer exhibits island growth resulting in Co2FeSi grain segregation which reduces the loop squareness. The island growth can be removed and the squareness restored through the use of Cr buffer layers before the Ag layers. This reduces film roughness to sub 1 nm therefore approaching the conditions that are required for device fabrication.