2D to 3D Magnetism in Synthetic Micas

Abstract Fe‐based mica minerals usually display two opposing magnetic ground states, either they behave as spin‐glasses or as layered ferrimagnets. No definite reason has been proposed as an explanation for this duality. This conundrum is unraveled by comparing the synthetic micas KFe₃[MGe₃]O₁₀X₂ with M═Fe and Ga, X═OH⁻ and F⁻. Neutron diffraction demonstrates a 2D to 3D magnetic transition in KFe₃[FeGe₃]O₁₀(OH)₂ while just hints or no order at all are observed for the fluorides with M═Fe and Ga respectively. The 3D transition is triggered by the presence of iron in the intralayer tetrahedra. DFT+U calculations show that the magnetic exchange couplings between the previously believed solely magnetic octahedral layers would otherwise be frustrated without this intralayer iron.