Ferroelectricity in antiferromagnetic wurtzite nitrides

Wurtzite-type nitrides have recently emerged as promising candidates for ferroelectric applications, yet their magnetic counterparts remain largely unexplored. Here, we establish MnSiN$_2$ and MnGeN$_2$ as aristotypes of a new multiferroic wurtzite family that simultaneously exhibits ferroelectricity and antiferromagnetism. These Mn(II)-based nitrides crystallize in polar structures and display robust G-type antiferromagnetism at room temperature. First-principles calculations reveal that nonmagnetic analogs incorporating Zn and Mg possess high polarization reversal barriers (0.735 and 0.683 eV per formula unit) and wide band gaps (4.0 and 4.8 eV), making them ideal ferroelectric candidates. In contrast, MnSiN$_2$ and MnGeN$_2$ exhibit strong antiferromagnetic exchange interactions (5--9 meV per Mn site) and moderate band gaps (1.6 and 1.0 eV), with reversal barriers of 0.963 and 0.460 eV per formula unit, respectively. Despite their limited magnetoelectric coupling, we show this family of Type-1 multiferroics exhibits altermagnetic spin splitting which reverses sign upon polarization switching. By strategically substituting alkaline-earth metals, we engineer multiple materials with coexisting switchable polarization, spin texture, and magnetic order. These findings open new avenues for the design of nitride-based altermagnetic multiferroics, offering a platform for integrated antiferromagnetic spintronic devices.