Monopole-limited nucleation of magnetism in Eu2Ir2O7

We present an in-depth analysis of muon-spin spectroscopy measurements of Eu2Ir2O7 under the effect of the Eu1-xBix isovalent and diamagnetic substitution as well as of external pressure. Our results evidence an anomalous correlation between the magnetic volume fraction and the order parameter only for stoichiometric Eu2Ir2O7, pointing towards highly unconventional properties of the magnetic phase developing therein. We argue that magnetism in Eu2Ir2O7 develops based on the nucleation of magnetic droplets at TN, whose successive growth is limited by the need of a continuous generation of magnetic hedgehog monopoles.