Magnetoelectric Effect in Antiferromagnets Made of Square Cupolas

Abstract

The magnetoelectric (ME) effect is a cross correlation between the magnetic and electric properties. We observe an anomalous antiferroelectric behavior induced by non-collinear magnetic quadrupole type order, i.e., two-in, two-out arrangement of spins, in an antiferromagnet Ba (TiO) Cu₄(PO₄)₄ consisting of convex spin clusters Cu₄O₁₂ called square cupolas. We construct a quantum spin model with parameters determined based on full magnetization curves, which also well reproduces all the observed antiferroelectric behaviors. Our results elucidate how the asymmetric square cupola units activate the ME responses through antisymmetric interactions arising from the spin-orbit coupling.