Layer-dependent magnetic phase diagram in FenGeTe2 (3≤n≤7) ultrathin films: coexistence of localized and itinerant electronic states of Fe with variable valence

Abstract Two-dimensional (2D) ferromagnets with high Curie temperature (T C ) are highly desirable due to their potential applications in spintronic devices. However, they are rarely obtained in experiments mainly due to the challenge of synthesizing high-quality 2D crystals, and their T C values are below the room temperature. By first-principles calculations, herein we design a family of stable 2D Fe n GeTe 2 (4≤ n ≤7) ultrathin films that are similar to the reported Fe 3 GeTe 2 , which exhibit coexistence of itinerant and localized magnetism. Among them, 2D Fe 3 GeTe 2 and Fe 4 GeTe 2 are ferromagnetic metals with T C of 138 K and 68 K, respectively; 2D Fe 5 GeTe 2 , Fe 6 GeTe 2 and Fe 7 GeTe 2 ultrathin films are Néel’s P-types, R-type, R-type ferrimagnetic metals with T C of 320 K, 450 K and 570 K, respectively. The thickness induced magnetic phase transition is mainly originated from the competition between itinerant and localized states, which is also correlate well with the content of Fe 3+ and Fe 2+ ions. A valence/orbital dependent magnetic exchange model is proposed to clarify such interesting thickness and composition effect. Our results not only endow 2D Fe-Ge-Te ultrathin films as promising candidates for spintronics at room temperature, but also propose a universal mechanism to understand the magnetic coupling in complex magnetic systems.