  The realization of four-fold anisotropic magnetoresistance (AMR) in novel
3d-5d heterostructures has boosted major efforts in antiferromagnetic
spintronics. However, despite the potential of incorporating strong spin-orbit
coupling, only small AMR signals have been detected thus far, prompting a
search for new mechanisms to enhance the signal. In this study on CaMnO3/CaIrO3
heterostructures, we report a unique dual-four-fold symmetric 70% AMR; a signal
two orders of magnitude larger than previously observed in similar systems. We
find that one order is enhanced by tuning a large biaxial anisotropy through
octahedral tilts of similar sense in the constituent layers, while the second
order is triggered by a spin-flop transition in a nearly Mott-type phase.
Dynamics between these two phenomena as evidenced by the step-like AMR and a
superimposed biaxial-anisotropy-induced AMR capture a subtle interplay of
pseudospin coupling with the lattice and external magnetic field. Our study
shows that a combination of charge-transfer, interlayer coupling, and a
spin-flop transition can yield a giant AMR relevant for sensing and
antiferromagnetic memory applications.
