Highly tunable magnetocrystalline anisotropy energy in Fe3+-doped BaTiO3

Magnetic dopants in ferroelectric oxide host materials provide a platform for electric field control of isolated spins, facilitated by tuning of the magnetocrystalline anisotropy energy (MCAE). We present first-principles calculations of the MCAE experienced by isolated Fe3+ dopants in the tetragonal, orthorhombic, and rhombohedral phases of the prototypical ferroelectric BaTiO3. We identify an order-of-magnitude decrease in the MCAE in the rhombohedral phase relative to the tetragonal and orthorhombic phases. We explain this dramatic decrease, as well as the formation of a spin-easy plane in the tetragonal phase and spin-easy axes in the orthorhombic and rhombohedral phases, using crystal field theory arguments. Building a superposition model from crystal field theory, we show how a set of simple criteria based on crystalline environment can be used to estimate the MCAE. We suggest this as a route to rapidly screen candidate ferroelectric hosts and magnetic dopants that possess phases with spin-easy axes and maximal MCAE tunability.