Effect of cobalt substitution on the structural, ferroelectric, and magnetic properties of bismuth ferrite thin films

Epitaxial films of multiferroic BiFe1−xCoxO3 (BFCO) with 0≤x≤0.35 are grown on (001)-oriented SrTiO3 and SrRuO3 buffered SrTiO3 substrates using the pulsed laser deposition technique. The effect of structural transformation from rhombohedral to tetragonal phase with increasing cobalt substitution on the magnetic, electrical, and piezo-/ferroelectric properties is investigated. Piezoresponse force microscopy is used to quantify the coercive voltage from the phase hysteresis loops for different thickness films to investigate the semi-empirical Kay–Dunn scaling law with varying cobalt concentrations. For the rhombohedral structure, a reduction of the coercive voltage is observed with increasing substitution of Fe by Co. The coercive voltage of a 10 nm BFCO (x=0.35) film is found to be 0.63 V, which is 67% lower than that of a pure BiFeO3 (BFO) (1.9 V) film of the same thickness. Cobalt substitution also leads to changes in the magnetic and electrical properties due to modification of spin ordering and reduction of the bandgap, respectively. Further, to validate the experimental results, we have performed theoretical calculations using density functional theory. The theoretical results indicate a reduction in unit cell volume and enhancement in net magnetization can be achieved with cobalt substitution, in agreement with experimental results. Partial Co substitution can, thus, provide a pathway to realize BFO-based nonvolatile magnetoelectric devices with reduced operating voltage.