Pronounced Spin-Lattice Coupling in the Ferroelastic Altermagnet Ca₂Fe₂O₅

Altermagnets are a new family of magnetic materials where spin-split bands are dictated by lattice symmetries even in the absence of net magnetization. Perovskite-related lattices are especially appealing for realizing such states because their flexible octahedral distortion modes can tune magnetization when coupled with altermagnetism. In addition, coherent ferroelastic domains in these lattices can act as building blocks for manipulating local magnetic states, offering new routes toward domain-based functionalities. Here we present Ca₂Fe₂O₅, a high-T_N brownmillerite oxide that naturally combines altermagnetic order with ferroelastic twinning. Symmetry analysis and DFT calculations indicate the existence of d-wave-like altermagnetism in its magnetically ordered state. Polarized optical and magnetic force microscopy reveal well-defined twin domains carrying different local magnetization locked to crystallographic orientations, showing strong spin–lattice coupling. Dilatometry measurements further reveal clear magnetostriction, highlighting significant magnetoelastic interactions governed by altermagnetism. Together, these results establish Ca₂Fe₂O₅ as a platform for exploring ferroelastic-coupled altermagnetism and its potential for strain-mediated control of magnetic properties.