Epitaxial magnetic tunnel junctions (MTJs) have the potential to be used as low-energy non-volatile random access memory. The use of half metallic ferromagnets as electrodes has drawn great interest due to the predicted 100% spin polarization at the Fermi level. Co2MnSi is predicted to be half-metallic[1], with a Curie temperature of 985K [2], and is a strong candidate for use in magnetic random access memory devices. These devices, however, are highly sensitive to interfacial and bulk disorder, which may result in the loss of a minority spin gap [3]. Therefore it is critical to understand the fundamental properties of the Heusler alloy films and correlate these properties with device performance. We have successfully grown highly-ordered, epitaxial Co2MnSi(001) films by molecular beam epitaxy (MBE) using a “seeded growth” technique to form a crystalline seed layer on various substrates, including MgO (001) and a Sc0.3Er0.7As lattice-matched diffusion barrier layer on GaAs (001). The lattice mismatch between GaAs and Co2MnSi is only -0.06% and x-ray diffraction of the MBE grown thin films show the two are nearly lattice matched. These films also have relatively smooth surfaces and coercivities down to 4 oersteds for stoichiometric films. Composition is also found to play a large role in the magnetic properties of the films. Off-stoichiometry films display lower saturation magnetization as well as higher coercivities. We have subsequently been able to grow epitaxial MTJ heterostructures of Co2MnSi/MgO/Co2MnSi with different switching fields for the top and bottom electrodes. We have characterized the structure of each layer and interfaces with reflection high-energy electron diffraction (RHEED), low-energy electron diffraction (LEED), scanning tunneling microscopy (STM) and cross-sectional transmission electron microscopy (XTEM) and the corresponding magnetic properties using vibrating sample magnetometry (VSM) and a superconducting quantum interference device (SQUID) to better understand the fundamental properties of these epitaxial heterostructures. This work was supported in part by the Semiconductor Research Corporation under award number 2011-IN-2153.

References

[1] S. Picozzi, A. Continenza, and A. J. Freeman. Phys. Rev. B 69 (9), 094423 (2004)

[2] PJ Webster and KRA Ziebeck. J. Phys. Chem. Solids 32, 1221 (1971)

[3] B. Hülsen, M. Scheffler, and P. Kratzer. Physical Review B 79 (9), 094407 (2000)