La2O3Mn2Se2: A correlated insulating layered d-wave altermagnet

Altermagnets represent a new class of magnetic phases without net magnetization, invariant under a combination of rotation and time reversal. Unlike conventional collinear antiferromagnets (AFM), altermagnets could lead to new correlated states and important material properties deriving from their nonrelativistic spin-split band structure. Indeed, they serve as the magnetic analogue of unconventional superconductors and can yield spin-polarized electrical currents in the absence of external magnetic fields, making them promising candidates for next-generation spintronics. Here, we report altermagnetism in the correlated insulator, magnetically ordered tetragonal oxychalcogenide, $\mathrm{L}{\mathrm{a}}_{2}{\mathrm{O}}_{3}\mathrm{M}{\mathrm{n}}_{2}\mathrm{S}{\mathrm{e}}_{2}$. Symmetry analysis reveals a ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$-wave-like spin-momentum locking arising from the $\mathrm{M}{\mathrm{n}}_{2}\mathrm{O}$ Lieb lattice, supported by density functional theory (DFT) calculations. Magnetic measurements confirm the AFM transition below $\ensuremath{\sim}166\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ while neutron pair distribution function analysis reveals a 2D short-range magnetic order that persists above the N\'eel temperature. Single crystals are grown and characterized using x-ray diffraction, optical and electron microscopy, and micro-Raman spectroscopy to confirm the crystal structure, stoichiometry, and uniformity. Our findings establish $\mathrm{L}{\mathrm{a}}_{2}{\mathrm{O}}_{3}\mathrm{M}{\mathrm{n}}_{2}\mathrm{S}{\mathrm{e}}_{2}$ as a model altermagnetic system realized on a Lieb lattice.