Altermagnetic type-II multiferroics with Néel-order-locked electric polarization

Altermagnetism, an emergent magnetic phase featuring compensated collinear magnetic moments and momentum-dependent spin splittings, has recently garnered widespread interest. A critical issue concerns whether the unconventional spin structures can generate spontaneous electric polarization in altermagnets, thereby achieving type-II multiferroicity. Here, with the combination of symmetry analysis and metal-ligand model, we explicitly demonstrate the generation of electric polarization by altermagnetic Néel order. We further uncover the locking behaviors between Néel order and electric polarization, which are classified into eight distinct categories for two-dimensional altermagnets governed by layer group symmetries. Then we take monolayer MgFe₂N₂ as a prototypical example of altermagnetic type-II multiferroics by first-principles calculations. We also propose to identify the Néel order and accompanying electric polarization in altermagnetic multiferroics by magneto-optical microscopy. Bridging type-II multiferroics and altermagnets, our work could pave the way for altermagnetic multifunctional spintronics.