  Artificial multiferroics consist of two types of ferroic materials, typically
a ferroelectric and 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, is an 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 K to 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 behaviour 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 suggest different
routes for potential control of magnetic anisotropy and elastically pinned
magnetic domain walls.
