Exchange Bias in Polycrystalline BiFe_1-xMn_xO₃/Ni₈₁Fe₁₉ Bilayers

Polycrystalline BiFe_1-xMn_xO₃ films with x up to 0.50 are prepared on LaNiO₃ buffered surface oxidized Si substrates. The doped Mn is confirmed to be partially in a +4 valence state. A clear exchange bias effect is observed with a 3.6 nm Ni₈₁Fe₁₉ layer deposited on the top BiFe_1-xMn_xO₃ layer, which decreases drastically with increasing Mn doping concentration and finally to zero when x is above 0.20. These results clearly demonstrate that the exchange bias field comes from the net spins due to the canted antiferromagnetic spin structure in polycrystalline BiFe_1-xMn_xO₃ films, which transforms to a collinear antiferromagnetic spin structure when the Mn doping concentration is larger than 0.20.