  The interplay between electronic transport and antiferromagnetic order has
attracted a surge of interest as recent studies have shown that a moderate
change in the spin orientation of a collinear antiferromagnet may have a
significant effect on the electronic band structure. Among numerous electrical
probes to read out such magnetic order, unidirectional magnetoresistance (UMR),
where the resistance changes under the reversal of the current direction, can
provide rich insights into the transport properties of spin-orbit coupled
systems. However, UMR has never been observed in antiferromagnets before, given
the absence of intrinsic spin-dependent scattering. Here, we report a UMR in
the antiferromagnetic phase of a FeRh$|$Pt bilayer, which undergoes a sign
change and then increases strongly with an increasing external magnetic field,
in contrast to UMRs in ferromagnetic and nonmagnetic systems. We show that
Rashba spin-orbit coupling alone cannot explain the sizable UMR in the
antiferromagnetic bilayer and that field-induced spin canting distorts the
Fermi contours to greatly enhance the UMR by two orders of magnitude. Our
results can motivate the growing field of antiferromagnetic spintronics, and
suggest a route to the development of tunable antiferromagnet-based spintronics
devices.
