The phenomena of odd-parity magnetoresistance and the planar Hall effect are deeply entwined with ferromagnetism. The intrinsic magnetization of the ordered state permits these unusual and rarely observed manifestations of Onsagers theorem when time reversal symmetry is broken at zero applied field. Here we study two classes of ferromagnetic materials, rare-earth magnets with high intrinsic coercivity and antiferromagnetic pyrochlores with strongly-pinned ferromagnetic layers at domain walls, which both exhibit odd-parity magnetoresistive behavior. The peculiar angular variation of the response with respect to the relative alignments of the magnetization, magnetic field, and current reveal the two underlying microscopic mechanisms: spin-polarization-dependent scattering of a Zeeman-shifted Fermi surface and magnetoresistance driven by the anomalous velocity physics usually associated with the anomalous Hall effect.
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