Tuning Single-Spin Magnetic Anisotropy in Ferroelectric Oxides: The Role of Strain and Defect Complexes

Manipulating isolated spins with electric fields is of fundamental interest in condensed matter physics and has potential applications in low-power spintronic devices. Ferroelectric materials are a promising platform for controlling the magnetic dopant’s spin via changes to its local structural environment. An external electric field can switch the ferroelectric polarization, which changes the local crystal electric field of the dopant. Through spin-orbit coupling, this change in the crystal field directly reorients the magnetic spin. In this work, we use density functional theory (DFT) to study Fe³⁺ dopants in the incipient ferroelectric SrTiO₃. Although non-polar in bulk, SrTiO₃ transitions to a ferroelectric state under biaxial strain. We investigate the magnetocrystalline anisotropy energy (MCAE) surfaces of Fe³⁺ in these polar phases under compressive and tensile strain. We also analyze the MCAE in the non-polar tetragonal bulk phase of SrTiO₃ and explore how octahedral rotations in this phase influence the MCAE. Furthermore, to address charge compensation, we systematically study the oxygen vacancy and the stability of Fe³⁺–Oᵥ defect complexes at symmetrically distinct sites around the Fe dopant. The presence of such a vacancy dramatically alters the local symmetry and the crystal field at the Fe site, therefore significantly impacting the MCAE. These findings shed light on fundamental design principles for optimizing electric-field control of single-spin sites in ferroelectric hosts, advancing the development of novel magnetoelectric technologies.