Fractional Quantum Multiferroics from Coupling of Fractional Quantum Ferroelectricity and Altermagnetism

Multiferroics, which combine ferroelectric and magnetic order, offer a transformative platform for next-generation electronic devices. However, the intrinsic competition between the mechanisms driving ferroelectricity and magnetism in single-phase materials severely limits their performance, typically resulting in weak magnetoelectric coupling at room temperature. Here, we propose a solution to this long-standing challenge through the novel concept of fractional quantum multiferroics (FQMF), where strong magnetoelectric coupling is naturally realized by coupling fractional quantum ferroelectricity with altermagnetism. Symmetry analysis shows that reversing the fractional quantum ferroelectricity polarization necessarily inverts the altermagnetic spin splitting under parity-time or time-reversal operations. A minimal tight-binding model reproduces this effect, demonstrating electrically driven spin control without rotating the Néel vector. First-principles calculations further identify a broad family of candidate materials in two and three dimensions, including bulk MnTe, Cr_{2}S_{3}, Mn_{4}Bi_{3}NO_{15} and two-dimensional AB_{2} bilayers such as MnX_{2} (X=Cl, Br, I), CoCl_{2}, CoBr_{2}, and FeI_{2}. Notably, MnTe exhibits a high Néel temperature (∼300 K) and a large electrically switchable spin splitting (∼0.8 eV), demonstrating room-temperature magnetoelectric performance that surpasses that of conventional multiferroics. To further showcase the technological potential, we propose an electric-field-controlled FQMF tunnel junction based on MnTe that achieves tunneling magnetoresistance exceeding 300%. This Letter establishes FQMF as a distinct and promising route to achieving room-temperature robust electrically controlled magnetism, opening a new avenue for voltage-controlled spintronics.