Coexistence of Antiferromagnetic Cubic and Ferromagnetic Tetragonal Polymorphs in Epitaxial CuMnSb

TL;DR

This study reveals that epitaxial CuMnSb films contain both antiferromagnetic cubic and ferromagnetic tetragonal phases at the nanoscale, with experimental and theoretical analysis explaining their coexistence and magnetic properties.

Contribution

It demonstrates the epitaxial stabilization of a ferromagnetic tetragonal polymorph in CuMnSb and provides a detailed theoretical understanding of its magnetic behavior and defect effects.

Findings

Coexistence of antiferromagnetic and ferromagnetic phases at nanometric scales.

The ferromagnetic phase is stabilized by epitaxial strain due to small energy difference.

Defects like Mn_Cu antisites influence magnetic properties but do not induce ferromagnetism.

Abstract

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 N\'{e}el 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 $\beta$--CuMnSb is higher than that of $\alpha$--CuMnSb only by 0.12~eV per formula unit, which allows for epitaxial stabilization of this phase, (ii) the metallic character of $\beta$--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.