Spin-canting driven Weyl physics in EuCd$_2$As$_2$.

Though rare, magnetic Weyl semi-metals stand as the best platform to study elusive Weyl physics as they can host the minimal allowable number of Weyl points. Here we present neutron diffraction and density functional theory work elucidating the magnetic structure realized in the candidate magnetic Weyl semi-metal EuCd$_2$As$_2$ . Our work shows an unanticipated magnetic structure (magnetic space group $C2'/m'$) with an in-plane [210] moment direction and a slight out-of-plane canting. This canted structure indicates that subtle tuning (rather than a phase transition) may be able to stabilize the sought c-polarized state. Our density functional theory work shows that though Weyl physics should exist for a purely in-plane [210] structure, even a slight canting drastically alters the relevant bands leading to well defined Weyl points. Furthermore, we find that relative to the \textit{c}-polarized state the [210] order with a small canting brings the Weyl points closer to the Fermi level and thus may lead to clearer signatures of the Weyl physics.