Temperature dependence of magnetic anisotropy and domain wall tuning in BaTiO₃(111)/CoFeB multiferroics

Artificial multiferroics consist of two types of ferroic materials, typically a ferroelectric and a ferromagnet, often coupled interfacially by magnetostriction induced by the lattice elongations in the ferroelectric. In BaTiO3, the magnitude of strain induced by these elongations is heavily temperature dependent, varying greatly between each of the polar crystal phases and exerting a huge influence over the properties of a coupled magnetic film. Here, we demonstrate that temperature and, thus, strain are effective means of controlling the magnetic anisotropy in BaTiO3(111)/CoFeB heterostructures. We investigate the three polar phases of BaTiO3: tetragonal (T) at room temperature, orthorhombic (O) below 280 K, and rhombohedral (R) below 190 K across a total range of 77–420 K. We find two distinct responses: a step-like change in the anisotropy across the low-temperature phase transitions and a sharp high-temperature reduction around the ferroelectric Curie temperature, measured from hard axis hysteresis loops. Using our measurements of this anisotropy strength, we are then able to show by micromagnetic simulation the behavior of all possible magnetic domain wall states and determine their scaling as a function of temperature. The most significant changes occur in the head-to-head domain wall states, with a maximum change of 210 nm predicted across the entire range, effectively doubling the size of the domain wall as compared to room temperature. Notably, similar changes are seen for both high and low temperatures, which suggests different routes for potential control of magnetic anisotropy and elastically pinned magnetic domain walls.