LAUSR

Disentangling the relationship between the insulating state with a charge gap and the magnetic order in an antiferromagnetic Mott insulator remains difficult due to inherent phase separation as the Mott state is perturbed1–7. Measuring magnetic and electronic properties at atomic length scales would provide crucial insight, but this is yet to be experimentally achieved. Here, we use spin-polarized scanning tunnelling microscopy (SP-STM) to visualize the periodic spin-resolved modulations originating from the antiferromagnetic order in a relativistic Mott insulator Sr2IrO4 (refs. 8,9), and how they change as a function of doping. We find that near the insulator-to-metal transition (IMT), the long-range antiferromagnetic order melts into a fragmented state with short-range correlations. Crucially, we discover 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. Our work establishes SP-STM as a powerful tool for revealing atomic-scale magnetic information in complex oxides.Spin-polarized tunnelling data show that the breakdown of antiferromagnetic order and the collapse of the spectral gap are not correlated in Sr2IrO4. This indicates that short-range magnetic correlations are not behind the emergence of the pseudogap.