Effect of Mn and Ba Codoping on a Magnetic Spin Cycloid of Multiferroic Bismuth Ferrite Nanoparticles
Résumé
Bismuth ferrite (BFO) is the drosophila of research in multiferroic materials due to its simultaneous magnetic and electric ordering at room temperature. The unfortunate detail is its antiferromagnetic ordering, which practically cancels magnetization and magnetoelectric coupling of the crystals. To induce finite coupling, dopants have been introduced with a certain success so far. Nanoparticles (NPs) can additionally constrain the formation of the magnetic cycloid in BFO due to size confinement. Doping nanoparticles can thus potentially provide a sizeable magnetization of BFO, making applications in computer memories and hyperthermia cancer treatment feasible. We show that the codoping of BFO NPs by Ba and Mn balances the electrochemical equilibrium, reduces the particle size, and shifts the magnetic phase transition to lower temperatures. The ferroelectric properties are retained and the remanent magnetization is increased by 1 order of magnitude: Bi0.95Ba0.05Fe0.95Mn0.05O3 possesses a remanent magnetization of 0.277 Am2/kg. Our Mössbauer studies reveal that two effects drive this increase: partial destruction of the spin cycloid due to Mn and increased spin canting due to Ba doping inducing local stress fields. This dopant combination and particular concentration improve the effective magnetization value exceptionally well.