Piezoelectric tuning of exchange bias from negative to positive bias fields

Tuning of the exchange bias has been attempted using magnetoelectric and multiferroic systems. Alternatively, we propose tuning of the exchange bias via the piezoelectric property of ferroelectric material. A ferromagnetic Co thin film is deposited on top of a ferroelectric tetragonal BaTiO$_{3}$ (001) by using MBE at a base pressure of 1.5$\times $10$^{10 }$m bar. An ex-situ antiferromagnetic CoO film is naturally formed on top of the Co Hereby, the piezoelectric BaTiO$_{3}$ induced electrically tunable stress in the adjacent Co film. The stress induced strain alters the magnetic anisotropy of the Co film and by that the magnetization at the Co/CoO-interface modifying the exchange bias field. This includes sign change of the exchange bias from negative to positive bias fields by increasing electric field applied on BaTiO$_{3}$. The observed complex electric field dependence of the exchange bias is interpreted through competition between ferromagnetic and antiferromagnetic exchange at the rough Co/CoO interface. The competition involves weakening of negative exchange bias through deviations from collineraity of the Co and CoO interface magnetization and simultaneous activation of antiferromagnetic exchange giving rise to a crossover into positive exchange bias.