2D Ruddlesden‐Popper Phase Hydride Double Perovskite A ₄ NiVH ₈ (A = Na, K, Rb): Ferromagnetic Semiconductors With High Curie Temperatures

2D ferromagnetic semiconductors are recognized as the cornerstone of next-generation spintronics devices. However, their practical applications are severely hindered by the low Curie temperature, which originates from the weak d-p-d ferromagnetic superexchange interaction. H^- anion with short ionic radius can effectively shorten the distance between magnetic centers and simultaneously induce a perfect 180° superexchange angle to strengthen the magnetic coupling, thus achieving high-temperature magnetic ordering. Here, by first-principles calculations, such a case in 2D Ruddlesden-Popper phase hydride double perovskite A₄NiVH₈ (A = Na, K, Rb) is demonstrated. These hydride monolayers possess quite good thermodynamic stability and can retain their structures under normal pressure at least at 500 K. Magnetic and electronic properties calculations reveal that they are all ferromagnetic semiconductors with high Curie temperatures (up to 429 K) and superior electron mobilities (up to 5522 cm² V^-1 s^-1, based on the deformation potential theory). In addition, monolayer Na₄NiVH₈ exhibits the characteristics of a bipolar magnetic semiconductor with gate-tunable spin polarization.