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

Choong H. Kim

paper

Strain-driven spin-flop transition and collapse of the giant magnon gap in the bilayer iridate Sr $_{3}$ Ir $_{2}$ O $_{7}$

arXiv:2608.22473

Abstract

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 ( $ε_{c} \approx - 2.4%$ ) 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 $J / U \approx 0.15$ , removes the collinear state altogether. The flop is not a rigid rotation---the ordered moments of the two states cross at $ε_{c}$ ---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 spin--orbit-entangled order.

Strain-driven spin-flop transition and collapse of the giant magnon gap in the bilayer iridate Sr3​Ir2​O7​

Abstract

Strain-driven spin-flop transition and collapse of the giant magnon gap in the bilayer iridate Sr $_{3}$ Ir $_{2}$ O $_{7}$