Prediction of huge magnetic anisotropies in 5d transition metallocenes

The stability, electronic structure and non-collinear magnetic properties of a series of 5d metallocenes, namely, two cyclopentadienyl (Cp) rings sandwiched with a single 5d transition metal atom, are investigated. Our first-principles calculations reveal that Cp rings not only provide a suitable ligand environment for metal atoms, but also result in tunable magnetism depending on the transition metal element. Among them, HfCp₂ and WCp₂ show a high preference for the magnetization axis perpendicular to the Cp plane, with large magnetic anisotropy energies (MAEs) around 10 meV. We further consider triple decker metallocenes (M₂Cp₃), and find a huge MAE of above 60 meV in Ta₂Cp₃. The orbital energy split and shifts induced by composition change in metallocenes is mainly responsible for the significant MAE enhancement. By choosing a suitable crystal field for transition metal atoms, we pave a feasible pathway for designing promising building blocks of future magnetic storage devices.