Highly tunable magnetocrystalline anisotropy energy of magnetic dopants in ferroelectric oxides

Controlling the spin degree of freedom in materials with electric fields is of fundamental scientific interest and is appealing for potential spintronics applications. This has driven substantial research into magnetoelectric coupling mechanisms and materials that exhibit and enhance this property. Most work has focused on the control of long-range magnetic states in multiferroic materials, although recent studies have pushed towards the atomic-scale limit, considering electric field control of molecular and even single-atom magnets. In this talk, I will discuss our recent research on a complementary platform for electric field control of isolated spins: ferroelectric oxides hosting dilute concentrations of magnetic dopants. Here, the local crystal field of a magnetic dopant, via spin-orbit coupling, determines the dopant’s magnetocrystalline anisotropy energy (MCAE), and hence its preferred spin orientation. In a ferroelectric, this local crystal field environment and spin directionality in principle can be controlled by an electric field, which naturally couples to polar atomic displacements. Using density functional theory calculations, we investigate this scenario using Fe³⁺ dopant atoms in the tetragonal, orthorhombic, and rhombohedral phases of the prototypical ferroelectric BaTiO₃ as a model system. Our calculations reveal a highly tunable MCAE, which changes by an order of magnitude as BaTiO₃ traverses these structural phases of different crystalline symmetries. We show how to construct simple models based on crystal field theory that provide physical insight and reproduce trends in the first-principles MCAE results. Using these models, we formulate design principles for achieving large and tunable MCAEs, and discuss extensions to other ferroelectric hosts and magnetic dopant atoms.