The helimagnet FeP is part of a family of binary pnictide materials with the MnP-type structure which share a nonsymmorphic crystal symmetry that preserves generic band structure characteristics through changes in elemental composition. It shows many similarities, including in its magnetic order, to isostructural CrAs and MnP, two compounds that are driven to superconductivity under applied pressure. Here we present a series of high magnetic field experiments on high quality single crystals of FeP, showing that the resistance not only increases without saturation by up to several hundred times its zero field value by 35 T, but that it also exhibits an anomalously linear field dependence over the entire field range when the field is aligned precisely along the crystallographic c-axis. A close comparison of quantum oscillation frequencies to electronic structure calculations links this orientation to a semi-Dirac point in the band structure which disperses linearly in a single direction in the plane perpendicular to field, a symmetry-protected feature of this entire material family. We show that the two striking features of MR-large amplitude and linear field dependence-arise separately in this system, with the latter likely due to a combination of ordered magnetism and topological band structure.
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