Intrinsic Altermagnetic-Ferroelectric Multiferroicity and Magnetoelectric Coupling in Reversed Transition-Metal Bromides.

Multiferroic materials, which exhibit the coexistence of magnetic and ferroelectric orders, offer opportunities for the exploration of new physical phenomena. A recent study proposed that van der Waals altermagnetism can combine with sliding ferroelectricity within the same crystal, constituting a novel type of multiferroic material [Wu et al., Sci. China-Phys. Mech. Astron. 2025, 68, 297511]. This represents significant advancement in the research of novel multiferroics, Nevertheless, the diverse magnetoelectric couplings and their underlying mechanisms warrant further investigation. Here, through targeted interlayer engineering informed by first-principles calculations and spin space group analysis, we identify NiBr2 and MnBr2 as compounds simultaneously hosting robust altermagnetism and sliding ferroelectricity. Crucially, altermagnetism of MnBr2 becomes spontaneously locked to the ferroelectric state at maximal polarization via symmetry constraints, enabling deterministic magnetoelectric coupling and nonvolatile electric control of magnetization. This breakthrough not only presents the first material realization of such a coupled phenomenon but also establishes a generic symmetry-engineering paradigm for developing advanced multiferroics in transition metal bromides.