Strain-driven spin-flop transition and collapse of the giant magnon gap in the bilayer iridate Sr${}_3$Ir${}_2$O${}_7$

The bilayer iridate Sr${}_3$Ir${}_2$O${}_7$ is a $c$-axis collinear antiferromagnet, held there by a giant interlayer pseudodipolar anisotropy, whereas single-layer Sr${}_2$IrO${}_4$ cants in the $ab$ plane. We show from first principles that biaxial compression of a few percent (${\epsilon }_c\approx -2.4\mathrm{\%}$) flops the easy axis of Sr${}_3$Ir${}_2$O${}_7$ into the plane. A magnetic model Hamiltonian built from Wannier functions with no fitted parameter—reproducing the giant magnon gap of the bulk, so far known only from fits to experiment—identifies the mechanism. Compression collapses the interlayer exchange channel, whose straight Ir–O–Ir path weakens as the bent in-plane path strengthens. Hund’s exchange sets the scale of the anisotropy and, beyond $J\mathrm{/}U\approx 0.15$, removes the collinear state altogether. The flop is not a rigid rotation—the ordered moments of the two states cross at ${\epsilon }_c$—and it carries a stark fingerprint, in that the giant easy-axis magnon gap collapses to a gapless, Goldstone-like spectrum. Compressively strained films thus sit on a metamagnetic phase boundary ending in a zero-temperature bicritical point, a charge-neutral handle on the moment direction.