Emerging Type III Multiferroics

Abstract

Type III multiferroics have recently been proposed as an emerging class of materials in which ferroelectricity and magnetism are linked through mechanisms that fall outside the conventional Type I/Type II classification. This perspective examines three representative theoretical routes. The first is symmetry locking in bilayer MnPSe₃, where sliding-ferroelectric switching reverses the altermagnetic spin polarization through a crystal-symmetry operation, without reversing the underlying Néel order. The second is ferroelectricity-driven Stoner magnetism in Cu-adsorbed monolayer InI₃, where an out-of-plane polar displacement narrows the bands near the Fermi level and drives a nonmagnetic-to-ferromagnetic transition. The third is the competing-electron-population mechanism proposed for monolayer TiCdO₄, in which an electron-deficient oxygen network produces both d° ferroelectricity and d⁰ magnetism from a common electronic origin. These mechanisms establish three conceptually distinct forms of coupling: symmetry-enforced reversal of spin polarization, polar-distortion-driven emergence of magnetism, and noncausal but strongly entangled electric and magnetic orders. Together, they broaden the design space of magnetoelectric materials beyond the conventional Type I/Type II dichotomy. The field nevertheless remains predominantly theoretical. Key challenges include establishing a consistent definition of Type III multiferroicity, expanding the candidate-material landscape, experimentally validating the predicted coupling, and translating the proposed mechanisms into device-relevant functionality.