Layer-dependent Magnetic Phase Diagram in FenGeTe2 (3≤n≤7) Ultrathin Films: Coexistence of Localized and Itinerant Electronic States of Fe with Variable Valence

Two-dimensional (2D) ferromagnets with high Curie temperature (TC) 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 TC values are below the room temperature. By first-principles calculations, herein we design a family of stable 2D FenGeTe2 (4≤n≤7) ultrathin films that are similar to the reported Fe3GeTe2, which exhibit coexistence of itinerant and localized magnetism. Among them, 2D Fe3GeTe2 and Fe4GeTe2 are ferromagnetic metals with TC of 138 K and 68 K, respectively; 2D Fe5GeTe2, Fe6GeTe2 and Fe7GeTe2 ultrathin films are Néel’s P-types, R-type, R-type ferrimagnetic metals with TC 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 Fe3+ and Fe2+ 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.