Imaging the impact of atomic-scale disorder and temperature fluctuations on antiferromagnetic ordering in iridates

Over the past decade, Ruddlesden-Popper iridates emerged as a versatile playground for investigations of exotic phenomena, such as a doped Mott insulator state, Fermi arcs, density wave instabilities and spin-related orders. We use spin-polarized scanning tunneling microscopy (SP-STM) to visualize spin-resolved modulations arising from the antiferromagnetic (AF) order in doped Mott insulators Sr₂IrO₄ and Sr₃Ir₂O₇. We find that near insulator-to-metal transition, the long-range antiferromagnetic order melts into a fragmented state with short-range correlations. We investigate the impact of chemical disorder on the distribution of these AF domains to pinpoint a defect most strongly correlated with the domain arrangement. Thermal erasure and re-entry into the AF state leads to a partial spatial reorganization of AF modulations, indicating multiple stable AF configurations at low temperature. Interestingly, regardless of this rearrangement, the AF puddles maintain scale-invariant fractal geometry. Our experiments shed light onto the sensitivity of the AF order to different types of atomic-scale disorder, and reveal its surprising fluidity with thermal cycling.