Probing spin polarization in Heusler alloy thin films via point-contact Andreev reflection

The utilization of the spins of electrons has led to many important device applications. In order to further improve the spin-dependent signals in spintronic devices, incorporating half metals (i.e. materials with a spin polarization of 100{\%}) into current device designs is highly desired. Besides several confirmed examples of half metals, such as CrO$_{\mathrm{2}}$, mixed-valence manganites, etc., Heusler alloys have also been predicted to exhibit a tunable spin polarization that can reach as high as 100{\%}. In this work, we use a combinatorial fabrication method together with point-contact Andreev reflection (PCAR) measurements to determine the spin polarization in Heusler alloys. Both single-composition Co$_{\mathrm{2}}$FeAl thin films and composition-spread thin films were fabricated on Si (001) or MgO (001) substrates in a DC magnetron co-sputtering system with a base pressure below 5$\times$ 10$^{\mathrm{-7}}$ Torr. X-ray diffraction measurements indicate that the films are primarily c-axis orientated. In order to suppress the current crowding effect and also increase the junction stability during the PCAR measurements, various buffer layers and/or capping layers were adapted. Further, by using a pre-sharpened Nb tip to complete a superconductor/ferromagnet junction for each sample, a spin polarization in a range of 55{\%}-65{\%} has been obtained at low temperatures.