Inverse Magnetocaloric Effect in Altermagnetic 2D Non‐van der Waals FeX (X = S and Se) Semiconductors

Abstract The concept of altermagnet (AM), which is neither ferromagnet nor antiferromagnet, has recently been proposed as compensated collinear magnet. Among 2D non‐van der Waals crystals, one of the thinnest altermagnets is identified and their interesting physical phenomenon at 2D limit is investigated. According to symmetry analysis and first‐principles calculations, 2D FeX (X = S, Se) sheets with one‐unit‐cell thickness are intrinsic altermagnetic semiconductors with observable magnitude of spin splitting (103−193 meV), high magnetic transition temperature (T_AM = 400 and 190 K), large out‐of‐plane magnetic anisotropy energy (0.24 and 0.74 meV per atom) and considerable metal‐like anomalous Hall conductivity (143 and 249 Ω⁻¹ cm⁻¹). Moreover, the spin lattices of these 2D FeX ultrathin films are formed by parallel diamond‐like chains with spin frustrated interaction through spin exchange along the b direction. Consequently, an inverse magnetocaloric effect with positive ∆S_m values of 1.7–8.2 mJ kg⁻¹ K⁻¹ near T_AM is observed. These results enrich the database of altermagnets and provide deep insights into the spin frustration effect in 2D magnets, which paves the avenue for the next‐generation spintronics.