Symmetry-Driven Unconventional Magnetoelectric Coupling in Perovskite Altermagnets: From Bulk to the Two-Dimensional Limit

Abstract

The emergence of altermagnets establishes a new paradigm for multiferroics. Unlike conventional multiferroics relying on direct magnetoelectric coupling, multiferroic altermagnets host a crystal-symmetry-mediated magnetoelectric interaction that is intrinsically more efficient and robust. Among candidate material platforms, layered perovskites are particularly appealing owing to their structural diversity and synthetic versatility. However, magnetoelectric properties at the two-dimensional scale remain largely unexplored, hindering their applicability in miniaturized, highly integrated devices. Here, we systematically investigate the dimensional evolution of ferroelectric polarization and magnetism in perovskite systems through symmetry analysis. We demonstrate that altermagnetism can persist in the two-dimensional limit, yet is strongly constrained by the magnetic configuration-with only C-type antiferromagnetic order supporting it. Based on mode-decomposition calculations, we further reveal that symmetry-restricted multimode couplings simultaneously govern ferroelectric polarization and altermagnetic spin splitting. Finally, combined with first-principles calculations, we propose several strategies to lift the magnetic-configuration constraint, extending the range of viable altermagnetic systems. These results underscore the critical role of dimensionality in symmetry-driven magnetoelectric coupling in perovskite altermagnets and pave the way toward next-generation electrically controlled spintronic and multiferroic devices.

Figures (4)

• Figure 1: (a) Crystal structures of bulk FEAM Ruddlesden–Popper perovskites [space group: $\textit{Cmc2}_1$, labeled as Bulk$_{(327)}$] and (b) AFEAM GdFeO$_3$-type perovskites [space group: $Pnma$, labeled as Bulk$_{(113)}$]. Black arrows indicate the direction of ferroelectric polarization, and black solid lines delineate the crystallographic unit cell. (c) Two-dimensional slabs derived from the bulk structures via exfoliation or surface-induced dimensional reduction. (d) Schematic illustration of the symmetry dependence of magnetic octahedra on the $m_z$ mirror symmetry under different magnetic configurations in 2D perovskites. Red and blue colors denote opposite spin channels. (e) Summary of ferroelectric and magnetic orders in bulk and 2D perovskites. (f) Schematic illustration of symmetry-driven unconventional magnetoelectric coupling in 2D perovskite altermagnets.
• Figure 2: Spin-resolved band structures of the Ca-Mn-O multiferroic altermagnets for (a) Bulk$_{(327)}$, (b) Bulk$_{(113)}$, and (c) 2D structure. A-, G-, and C-type AFM orders are considered. Red (blue) lines correspond to spin-up (spin-down) channels. Gray shaded regions highlight momentum paths exhibiting spin splitting. The corresponding first Brillouin zones are provided in Fig.S3.
• Figure 3: (a) Major structural distortions of the ground state in the studied 2D perovskites, including in-plane ferroelectric polarization $P$, in-plane octahedral rotation $\Theta_z$, octahedral tilting $\Phi_{xy}$, and Jahn–Teller distortion $Q$. (b) Energy surface as a function of the amplitude of each mode for the 2D high symmetry parent $P4/mmm$ phase in the Ca-Mn-O system. (c) Energy gain associated with different lattice distortion modes acting on the $P4/mmm$ phase. (d) Altermagnetic spin splitting as a function of the amplitude of different distortion modes; dashed and solid lines correspond to mode amplitudes of 0.5 and 1 (DFT-relaxed structures), respectively.
• Figure 4: (a) Several strategies to break $m_z$ symmetry in real space, including superlattice engineering, shear strain, electric field, and substrate engineering. (b) Spin-resolved band structures of different magnetic orders in the 2D Ca-Mn/Cr-O superlattice (chemical formula Ca$_6$Mn$_2$Cr$_2$O$_{14}$).