Giant anomalous Hall effect in epitaxial Mn3.2Ge films with a cubic kagome structure

We report on the first example of epitaxial ${\mathrm{Mn}}_{3+\ensuremath{\delta}}\mathrm{Ge}$ thin films with a cubic $L{1}_{2}$ structure. The films are found to exhibit frustrated ferromagnetism with an average magnetization corresponding to $0.98\ifmmode\pm\else\textpm\fi{}0.06\phantom{\rule{0.16em}{0ex}}{\textmu{}}_{B}$/Mn, far larger than the parasitic ferromagnetism in hexagonal ${\mathrm{Mn}}_{3}\mathrm{Ge}$ and the partially compensated ferrimagnetism in tetragonal ${\mathrm{Mn}}_{3}\mathrm{Ge}$. The Hall conductivity is the largest reported for the kagome magnets with a low-temperature value of ${\ensuremath{\sigma}}_{xy}=1587$ S/cm. Density functional calculations predict that a chiral antiferromagnetic structure is lower in energy than a ferromagnetic configuration in an ordered stoichiometric crystal. However, chemical disorder driven by the excess Mn in our films explains why a frustrated ${120}^{\ensuremath{\circ}}$ spin structure is not observed. Comparisons between the magnetization and the Hall resistivity indicate that a noncoplanar spin structure contributes the Hall signal. Anisotropic magnetoresistance and planar Hall effect with hysteresis up to 14 T provides further insights into this material.