Structural and Compositional Analysis of Co₂MnSi Ultra-thin Films with Perpendicular Magnetic Anisotropy for Spintronics Applications

Spintronics exploits the electron spin as an additional degree of freedom in solid state devices, thus achieving additional functionalities. Amongst these are novel opto-electronic devices in which injection of perpendicularly spin polarized electrons from a ferromagnetic electrode into a light emitting diode is expected to yield circularly polarized photons [1]. Butler et al. [2] propose that such high polarization electron injection can be achieved in an ultrathin (2-3nm) chemically ordered Co₂MnSi (CMS) full Heusler due to a combination of perpendicular magnetic anisotropy (PMA) and half-metallicity. PMA is expected due to tetragonal distortion of cubic CMS on epitaxial MgO(001).

Here, we correlate between the structure and magnetic properties of magnetron sputter-deposited CMS electrodes with varying thicknesses and annealing temperatures using transmission electron microscopy (TEM: phase contrast, energy-loss spectroscopy, scanning TEM, HAADF) and magnetometry.

Magnetometry measurements show clear PMA in the ultra-thin layers. Structural characterization using aberration corrected high resolution phase contrast imaging indicates almost complete absence of ordered phases in the CMS ultra-thin layer, consistent with the relatively low magnetic saturation moment. Compositional analysis using atomic column resolution STEM-EELS indicates chemical intermixing of the Heusler layer with both the MgO and Pd adjacent layers. Consequently, PMA is attributed in this case to interface intermixing anisotropy comparable to Co/Pd multilayers [3] and/or to Co-Pd alloying [4].

We also note, that in several cases, the reported structural and compositional analyses of these Heusler alloys are undertaken on thin layers (20-50nm) and this structure/composition is assumed to be comparable in ultra-thin layers (2-3nm). According to our results, this assumption does not appear to be valid, and hence these type of challenging ultra-thin film requires direct TEM analysis.

[1] Farshchi, Rouin, et al. , Applied Physics Letters16 (2011).

[2] Munira, Kamaram, Jonathon Romero, and William H. Butler. , Journal of Applied Physics17 (2014).

[3] Den Broeder, F. J. A., et al. , Journal of Applied Physics8 (1987).

[4] Shunichi Hashimoto et al 1989 Jpn. J. Appl. Phys. 28 1596