Abstract:
Layered transition-metal fluorides provide an ideal platform for exploring the interplay among lattice, orbital, and spin degrees of freedom in low-dimensional magnets. Here, we investigate the structural and magnetic properties of quasitwo-dimensional CsMnF₄ through a combined first-principles and experimental study. Density functional theory calculations reveal that the high-symmetry prototype structure is dynamically unstable and undergoes a structural distortion driven by Jahn–Teller-active MnN3⁺ ions. Phonon analysis identifies the unstable modes responsible for stabilizing a tetragonal P4/n ground state, characterized by cooperative octahedral rotations and orbital ordering. The resulting electronic and magnetic structure corresponds to a ferromagnetic insulating ground state with strong in-plane exchange interactions and weak interlayer coupling, leading to quasi-two-dimensional ferromagnetism and an estimated Curie temperature of 22 K. These predictions are corroborated by synchrotron X-ray and neutron powder diffraction measurements, which confirm the P4/n structure and the presence of Jahn–Teller-distorted MnF₆ octahedra. Magnetization measurements reveal a magnetic transition at 18.8 K, in close agreement with the theoretical prediction. Neutron diffraction further establishes ferromagnetic spin alignment within the ab plane, with an ordered moment of 3.76 μB per Mn. The excellent agreement between theory and experiment demonstrates that Jahn–Teller-driven orbital ordering is central to stabilizing quasi-two-dimensional ferromagnetism in CsMnF₄. More broadly, this work establishes a direct connection between local structural distortions, orbital order, and anisotropic magnetic interactions, providing microscopic insights into the design of low-dimensional magnetic materials.