Altermagnetism in Folded Iron Chain Systems

Altermagnetism—moment-compensated magnetism with momentum-dependent spin splitting—has so far been realized primarily in N´eel-type antiferromagnets. Here, we show that folded

iron-chain systems with block-type magnetic order (e.g., ↑↑↓↓) naturally host altermagnetic behavior. Using symmetry analysis and first-principles calculations, we reveal that block order in

K₃Fe₂Se₄ breaks combined spacetime inversion symmetry, producing alternating spin polarization in momentum space despite zero net magnetization. Our findings unify real-space block magnetism and reciprocal-space altermagnetism, establishing folded iron chains as a tunable platform for zeromoment spintronic applications.