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Description
The arrangement of magnetic moments at the vertices of a pyrochlore lattice - composed of corner-sharing tetrahedra - leads to a great variety of electronic ground states for $R_{2}M_{2}$O$_{7}$ materials. Here, we present an in-depth analysis of muon-spin spectroscopy measurements of Eu$_{2}$Ir$_{2}$O$_{7}$ under the effect of the Eu$_{1–x}$Bi$_{x}$ isovalent and diamagnetic substitution [1] as well as of external pressure [2]. Below $T_{N}$, Eu$_{2}$Ir$_{2}$O$_{7}$ shows a topologically non-trivial $4$-in/$4$-out order where the Ir$^{4+}$ magnetic moments all point inwards or outwards the tetrahedron they are located at (magnetic hedgehog monopole). Our results evidence an anomalous correlation between the magnetic volume fraction and the order parameter only for stoichiometric Eu$_{2}$Ir$_{2}$O$_{7}$, pointing towards highly unconventional properties of the magnetic phase developing therein [3]. We argue that magnetism in Eu$_{2}$Ir$_{2}$O$_{7}$ develops based on the nucleation of magnetic droplets at $T_{N}$, whose successive growth is limited by the need of a continuous generation of magnetic hedgehog monopoles [3].
[1] P. Telang et al., Physical Review B 99 201112(R) (2019).
[2] G. Prando et al., Physical Review B 93 104422 (2016).
[3] G. Prando et al., Physical Review B 101 174435 (2020).
