  The large spin orbit coupling in topological insulators results in helical
spin-textured Dirac surface states that are attractive for topological
spintronics. These states generate an efficient spin-orbit torque on proximal
magnetic moments at room temperature. However, memory or logic spin devices
based upon such switching require a non-optimal three terminal geometry, with
two terminals for the writing current and one for reading the state of the
device. An alternative two terminal device geometry is now possible by
exploiting the recent discovery of a unidirectional spin Hall magnetoresistance
in heavy metal/ferromagnet bilayers and (at low temperature) in magnetically
doped topological insulator heterostructures. We report the observation of
unidirectional spin Hall magnetoresistance in a technologically relevant device
geometry that combines a topological insulator with a conventional
ferromagnetic metal. Our devices show a figure-of-merit (magnetoresistance per
current density per total resistance) that is comparable to the highest
reported values in all-metal Ta/Co bilayers.
