Atomic-scale fragmentation and collapse of antiferromagnetic order in a doped Mott insulator

Measuring magnetic and electronic properties at atomic length scales would provide crucial insight into the physics of doped antiferromagnetic Mott insulators, but this has been difficult to achieve. We use spin-polarized scanning tunneling microscopy (SP-STM) to visualize the periodic spin-resolved modulations originating from the antiferromagnetic order in a J_eff = ½ strongly spin-orbit coupled Mott insulator Sr₂IrO₄. We discover that near insulator-to-metal transition (IMT), the long-range antiferromagnetic order melts into a spatially fragmented state with short-range correlations. Importantly, we find that the short-range antiferromagnetic order is locally uncorrelated with the observed spectral gap magnitude. This suggests that static short-range antiferromagnetic correlations are unlikely to be the cause of the inhomogeneous closing of the spectral gap and the emergence of pseudogap regions near the IMT in doped Sr2IrO4. Our work establishes SP-STM as a powerful tool for revealing atomic-scale magnetic information in complex oxides.