Ferromagnetic Phase Diagram for Nanoscale Epitaxial (Co,Mn) Alloys on GaAs(001)

A important goal of device and materials research is directed at discovering ways to combine the properties of ferromagnets and semiconductors. Our current work is aimed at synthesizing ferromagnetic materials which are compatible with GaAs. In a previous study, it was shown that the Heusler ferromagnet, Co$_2$MnAl can be grown epitaxially on GaAs and has a Curie temperature T$_C$=800-1000~K.$^{[1]}$ In the present work, nanoscale alloys of Co$_{1-x}$Mn$_x$ were grown epitaxially on GaAs(001) substrates. Nanometer thick layers of (Co,Mn) are found to be ferromagnetic over a wider range of x-values than for bulk materials --- x$\leq$0.8 for 5-10~nm thick films, compared to only x$\leq$0.32 for bulk materials. The Curie temperature of (Co,Mn) nanometer films decreases linearly for increasing x, from T$_C$=800~K at x=0.35 to T$_C$=0 at x=0.8. The extended range of x for ferromagnetism is attributed to the strained lattice which is in registration with the GaAs lattice. Work supported by NSF-DMR-0305360. [1] Y. Chen, D. Basiaga, J.R. O'Brien, and D. Heiman, \emph{Anomalous magnetic properties and Hall effect in ferromagnetic Co$_2$MnAl epilayers}, Appl. Phys. Lett. \textbf{84}, 4301 (2004).