Anomalous symmetry breaking in the Weyl semimetal CeAlGe

CeAlGe, a proposed type-II Weyl semimetal, orders antiferromagnetically below 5 K. At 2 K, spin-flop and spin-flip transitions to less than 1 ${\ensuremath{\mu}}_B$/Ce are observed in the $\mathit{M}(\mathit{H})$ data below 30 kOe, H$\ensuremath{\parallel}$a and b, and 4.3 kOe, H$\ensuremath{\parallel}\phantom{\rule{4pt}{0ex}}\ensuremath{\langle}110\ensuremath{\rangle}$, respectively, indicating a fourfold symmetry of the $M(H)$ data along the principal directions in the tetragonal ab plane with $\ensuremath{\langle}110\ensuremath{\rangle}$ set of easy directions. However, anomalously robust and complex twofold symmetry is observed in the angular dependence of resistivity and magnetic torque data in the magnetically ordered state once the field is swept in the ab plane. This twofold symmetry is independent of temperature and field hystereses and suggests a magnetic phase transition that separates two different magnetic structures in the ab plane. The boundary of this magnetic phase transition and possibly the type of low-field magnetic structure can be tuned by an Al deficiency.