We report the observation of an extreme magnetoresistance (XMR) in HoBi with a large magnetic moment from Ho $f$ electrons. Neutron scattering is used to determine the magnetic wave vectors across several metamagnetic transitions on the phase diagram of HoBi. Unlike other magnetic rare-earth monopnictides, the field dependence of resistivity in HoBi is nonmonotonic and reveals clear signatures of every metamagnetic transition in the low-temperature and low-field regime, at $T<2$ K and $H<2.3$ T. The XMR appears at $H>2.3$ T after all the metamagnetic transitions are complete and the system is spin polarized by the external magnetic field. The existence of an onset field for XMR and the intimate connection between magnetism and transport in HoBi are unprecedented among the magnetic rare-earth monopnictides. Therefore, HoBi provides a unique opportunity to understand the electrical transport in magnetic XMR semimetals.

• Received 30 May 2018
• Revised 10 July 2018

DOI:https://doi.org/10.1103/PhysRevB.98.045136