Interaction of Frustrated Magnetic Sublattices inErMnO3

A spontaneous or field induced “hidden” phase transition with antiferromagnetic-to-ferromagnetic reordering is disclosed in multiply frustrated hexagonal ${\mathrm{ErMnO}}_3$. It is revealed by Faraday rotation and second harmonic generation as sublattice-sensitive probes to the Er and Mn systems. The acquired phase diagram in the magnetic-field–temperature plane is shown to be a consequence of a broken geometric frustration between the Er and Mn lattices, giving way to anisotropic superexchange between the $3d$ and $4f$ ions.