Coexistence of antiferromagnetic cubic and ferromagnetic tetragonal polymorphs in epitaxial CuMnSb films

High-resolution transmission electron microscopy and superconducting quantum interference device magnetometry shows that epitaxial CuMnSb films exhibit a coexistence of two magnetic phases, coherently intertwined in nanometric scales. The dominant $\alpha$ phase is half-Heusler cubic antiferromagnet with the Néel temperature of 62 K, the equilibrium structure of bulk CuMnSb. The secondary phase is its ferromagnetic tetragonal $\beta$ polymorph with the Curie temperature of about 100 K. First principles calculations provide a consistent interpretation of experiment, since (i) total energy of $\beta –\mathrm{CuMnSb}$ is higher than that of $\alpha –\mathrm{CuMnSb}$ only by 0.12 eV per formula unit, which allows for epitaxial stabilization of this phase, (ii) the metallic character of $\beta –\mathrm{CuMnSb}$ favors the Ruderman-Kittel-Kasuya-Yoshida ferromagnetic coupling, and (iii) the calculated effective Curie-Weiss magnetic moment of Mn ions in both phases is about $5.5{\mu }_{\mathrm{B}}$, favorably close to the measured value. Calculated properties of all point native defects indicate that the most likely to occur are ${\mathrm{Mn}}_{\mathrm{Cu}}$ antisites. They affect magnetic properties of epilayers, but they cannot induce the ferromagnetic order in CuMnSb. Combined, the findings highlight a practical route towards fabrication of functional materials in which coexisting polymorphs provide complementing functionalities in one host.