  Atomically sharp oxide heterostructures often exhibit unusual physical
properties that are absent in the constituent bulk materials. The interplay
between electrostatic boundary conditions, strain and dimensionality in
ultrathin epitaxial films can result in monolayer-scale transitions in
electronic or magnetic properties. Here we report an atomically sharp
antiferromagnetic-to-ferromagnetic phase transition when atomically growing
polar antiferromagnetic LaMnO3 (001) films on SrTiO3 substrates. For a
thickness of five unit cells or less, the films are antiferromagnetic, but for
six unit cells or more, the LaMnO3 film undergoes a phase transition to a
ferromagnetic state over its entire area, which is visualized by scanning
superconducting quantum interference device microscopy. The transition is
explained in terms of electronic reconstruction originating from the polar
nature of the LaMnO3 (001) films. Our results demonstrate how new emergent
functionalities can be visualized and engineered in atomically thick oxide
films at the atomic level.
