Date from:Direct-exchange-driven Room-temperature Ferromagnetism in Two-dimensional Semiconductors

二维 (2D) 铁磁 (FM) 半导体为下一代自旋电子学应用带来了巨大前景。然而,在室温下维持 FM 有序仍然具有挑战性,因为导致 FM 耦合的超级交换相互作用通常相当弱。在这里,我们提出半导体中的 FM 耦合可以起源于直接交换相互作用。利用紧密结合模型,我们表明,当一个离子的轨道小于半填充(例如 d<5)而另一个离子的轨道超过一半填充(例如 d>5)时,两个磁离子之间的 FM 耦合会通过直接交换增强,但会受到超级交换相互作用的抑制。然后,通过第一性原理计算,我们预测双过渡金属化合物半导体中的 d<5-d>5 直接交换相互作用 [例如 CrCoCF2 单层中的 d3(Cr3+)-d8(Co+) 相互作用] 可以将 FM 级稳定在室温以上。这些发现揭示了实现室温 FM 半导体的一般机制,为自旋电子学开辟了一条新途径。Two-dimensional (2D) ferromagnetic (FM) semiconductors hold great promise for next-generation spintronic applications. However, maintaining FM order at room temperature remains challenging, as the superexchange interactions that induce FM coupling are usually rather weak. Here, we propose that FM coupling in semiconductors can originate from direct exchange interactions. Using tight-binding models, we demonstrate that when the orbital of one ion is less than half-filled (e.g., d<5) while that of the other ion is more than half-filled (e.g., d>5), the FM coupling between two magnetic ions is enhanced via direct exchange but suppressed by superexchange interactions. Subsequently, through first-principles calculations, we predict that the d<5-d>5 direct exchange interactions in double transition metal compound semiconductors [e.g., the d³(Cr³⁺)-d⁸(Co⁺) interaction in CrCoCF₂ monolayers] can stabilize FM order above room temperature. These findings reveal a general mechanism for realizing room-temperature FM semiconductors, opening a new avenue for spintronics.