Evolution of Singular Anisotropic Magnetoresistance in Magnetic Weyl Metals*

The confluence between magnetism and topology is an opportunity to explore the interaction between local and non-local orders within the same material. Amongst candidate materials that may host both orders, Holmium (Ho), shows a distinct magnetic phase change from conical ferromagnet phase to helical antiferromagnetic phase as temperature increases and is thus of great interest as a potential topological magnet. In this talk, we describe theoretical and experimental quantum transport results in epitaxial Holmium films under a variety of conditions. Calculations show that Ho, when in the helical spin antiferromagnetic phase, is a Weyl semimetal. We find that the anisotropic magnetoresistance (AMR) under the high magnetic field applied in the basal plane possesses sharp delta function like six-fold symmetric peaks in the AMR in the spin-saturated ferromagnetic phase when the field is along the a-axis. These features are superimposed on a two-fold symmetry contribution.  At higher temperatures, when the magnetic phase of Holmium changes from conical spin to helical spin, there is a symmetry transition within the AMR where the 6-fold contribution dominates the response. Furthermore, we compare the conditions under which we find singular AMR in Ho to the well-known AMR of magnetic Weyl semimetal CeAlGe to show that the resultant AMR is unlikely to arise from the formation of magnetic domains.