Magnetic structural unit with convex geometry: A building block hosting an exchange-striction-driven magnetoelectric coupling

Abstract

A magnetic structural unit with asymmetric geometry may be a source for symmetry-dependent unique phenomena such as the magnetoelectric effect. The authors report the discovery of ferroelectricity and a magnetic-field-induced sign reversal of ferroelectric polarization in Pb(TiO)Cu${}_{4}$(PO${}_{4}$)${}_{4}$, whose structure is characterized by a staggered array of Cu${}_{4}$O${}_{12}$ magnetic units with convex geometry known as square cupola. Their model and first-principles calculations reveal that the observed complex magnetoelectric behavior originates from an exchange striction, where leading exchange interactions between spins play a crucial role. This result demonstrates that materials with convex-shaped magnetic structural units deserve to be explored to achieve strong magnetoelectric couplings.