Fractional Quantum Multiferroics from Coupling of Fractional Quantum Ferroelectricity and Altermagnetism

TL;DR

This work introduces fractional quantum multiferroics (FQMF) by coupling fractional quantum ferroelectricity (FQFE) to altermagnetism (AM), exploiting symmetry to make electric switching invert the spin texture without reorienting the Néel vector ($L_1\leftrightarrow L_2$) under $\mathcal{PT}$ or $\mathcal{T}\tau$. A minimal tight-binding model and first-principles calculations reveal a broad material set, including MnTe and 2D MnX$_2$ bilayers, with MnTe exhibiting room-temperature Néel order and spin-splitting up to $\sim 0.8$ eV. A MnTe-based FQMF tunnel junction is predicted to achieve tunneling magnetoresistance $>300\%$ by combining spin-texture switching and interfacial polarization changes, signaling strong room-temperature magnetoelectric performance. Overall, FQMF offers a symmetry-grounded pathway to voltage-controlled spintronics in both bulk and two-dimensional materials.

Abstract

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 (FQFE) with altermagnetism (AM). Symmetry analysis shows that reversing the FQFE polarization necessarily inverts the AM spin splitting under parity-time ($\mathcal{PT}$) or time-reversal ($\mathcal{T}τ$) 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 ($\sim$300 K) and a large electrically switchable spin splitting ($\sim$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 work establishes FQMF as a distinct and promising route to achieving room-temperature strong magnetoelectric coupling, opening a new avenue for voltage-controlled spintronics.

Figures (4)

• Figure 1: Schematic illustration of FQMF in monolayer AB$_2$. (a)–(c) Atomic structures for $L_2$, $L_1$, and $L_2^\prime$, respectively. The A atom (black sphere) is non-magnetic, while B atoms (red and blue spheres) carry opposite magnetic moments. The displacement of the A atom is indicated by the pink arrow, and the white arrow indicates the spin of the B atoms. In panel (b), the point O marks the origin of the inverse operation. It is important to note that although (b) exhibits inversion symmetry, this is not a requirement for our proposed FQMF. A specific example can be seen in MnBr$_2$ [Fig. (\ref{['fig:mnbr2']})]. (d) and (e) Band structures for $L_2$ and $L_1$, respectively. The coordinates of the high-symmetry points are provided in Sec. SIV of the Supplemental Material supplementary. (f) Polarization difference from $L_1$ to $L_2$. $\boldsymbol{Q}$ denotes the polarization quantum modernpolar; $P_{tot}$, $P_{ele}$, and $P_{ion}$ are the total, electronic, and ionic contributions, respectively.
• Figure 2: Schematic illustration, polarization, and band structures of bulk MnTe. (a) $L_1$ and (c) $L_2$ crystal structures of MnTe. The green (yellow) spheres denote Mn (Te) atoms, and arrows indicate spin direction. (b) Evolution of the polarization along the $L_1$--$L_2$ pathway. (d) and (f) Spin-polarized band structure of $L_1$, and $L_2$, respectively. (e) Schematic diagram of atomic motion during the transition from $L_1$ to $L_2$.The pink arrow traces the Te displacement from $L_1$ to $L_2$.
• Figure 3: Schematic illustration, polarization, and band structures of bilayer MnBr$_2$. (a) $L_1$ and (c) $L_2$ crystal structures of MnBr$_2$. The green (gray) spheres denote Mn (Br) atoms, and arrows indicate spin direction. (b) Evolution of the polarization along the $L_1$--$L_2$ pathway. (d) and (f) Spin-polarized band structure of $L_1$, and $L_2$, respectively. (e) Schematic diagram of atomic motion during the transition from $L_1$ to $L_2$. The pink arrow traces the Mn displacement from $L_1$ to $L_2$.
• Figure 4: Schematic illustration, Fermi surfaces, transmission spectra, and TMR of FQMFTJ junctions. (a) Schematic illustration of three tunnel junction configurations. In parallel FQMF (P-FQMF), both the fixed and free layers adopt the $L_1$ structure with the same Néel vector. In antiparallel FQMF (AP-FQMF), the fixed layer remains in $L_1$ structure while the free layer transitions to $L_2$ structure, with both layers retaining the same Néel vector. In the antiparallel AM (AP-AM) configuration, both layers are in the $L_1$ structure but possess opposite Néel vectors. (b) Fermi surface slice of MnTe at E= -0.5 eV, $k_z=0.3\pi$, with the red (cyan) representing spin-up (spin-sown) electrons. The top panel shows the P-FQMF ($L_1\rightarrow L_1$) configuration, where spin states are momentum-matched during tunneling. The bottom panel shows the AP-FQMF ($L_1\rightarrow L_2$) configuration, where spin states are momentum-mismatched during tunneling. (c) Energy-resolved transmission spectra for the P-FQMF, AP-FQMF, and AP-AM configurations. (d) Energy-resolved TMR spectra of the FQMFTJ (blue) and AMTJ (green).