Alterferroicity*

Primary ferroicities like ferroelectricity and ferromagnetism are essential physical properties of matter. Multiferroics, with coexisting multiple ferroic orders in a single phase, provide a convenient route to magnetoelectricity. Even so, the general trade-off between magnetism and polarity remains inevitable, which prevents practicable magnetoelectric cross control in the multiferroic framework. Here an alternative strategy, i.e. the so-called alterferroicity, is proposed to circumvent the magnetoelectric exclusiveness, which exhibits multiple but non-coexisting ferroic orders. The natural exclusion between magnetism and polarity, as an insurmountable weakness of multiferroicity, becomes a distinct advantage in alterferroicity, making it an inborn rich ore for intrinsic strong magnetoelectricity. The general design rules for alterferroic materials rely on the competition between the instabilities of phononic and electronic structures in covalent systems. Based on primary density functional theory calculations, Ti-based trichalcogenides are predicted to be alterferroic candidates, which exhibit unique seesaw type magnetoelectricity. This alterferroicity, as an emerging branch of ferroic family, re-shapes the framework of magnetoelectricity, going beyond the established scenario based on multiferroicity.