  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.
