Bismuth ferrite

Bismuth ferrite (BiFeO₃, also commonly referred to as BFO in Materials Science) is an inorganic chemical compound with a perovskite structure. It is one of the most promising lead-free piezoelectric materials by exhibiting multiferroic properties at room temperature. Multiferroic materials exhibit ferroelectric or anti-ferroelectric properties in combination with ferromagnetic (or antiferromagnetic) properties in the same phase. As a result, an electric field can induce change in magnetization and an external magnetic field can induce electric polarization. This phenomenon is known as the magnetoelectric effect (ME) effect and materials exhibiting this effect are called magnetoelectrics or seignetto magnets. Further proof of it being ferromagnetic is that it produces a hysteresis loop during ferroelectric characterization. The ability to couple to either the electric or the magnetic polarisation allows an additional degree of freedom in device designs.

However one of the major drawbacks of the material is its high current leakage. Therefore it allows current to pass through when a high voltage is applied. Attempts to improve the electrical properties have been made by doping it with rare earth elements such as lanthanum (La), samarium (Sm), gadolinium (Gd), terbium (Tb) and dysprosium (Dy) etc. The dopant can be at the A site or the B site. A site being the edges of the perovskite cell and the B site being the centre of the perovskite cell.

Sample Preparation

Solid-state reaction

The most common method is the solid state reaction method. Starting powders of Bi₂O₃ and Fe₂O₃ were weighed according to the formulae of BiFeO₃.The powders were mixed using zirconia balls and ethanol as the milling solvent. The mixture is then ball-milled for 24hrs. The mixture is then calcined in the range of 600 to 700 degree celsius then sintered at around 800 to 900 degree celsius. Other precursor materials can be acetates or hydroxides.

Thin film deposition

Epitaxial BiFeO₃ films grown on SrTiO₃ with SrRuO₃ as the epitaxial electrode using pulsed laser deposition (PLD) revealed an enhancement of the ferroelectric polarization and related properties by more than an order of magnitude in comparison to bulk BiFeO₃ [http://www.sciencemag.org/cgi/content/abstract/299/5613/1719] . The origin of the enhancement is the in-plane compressive stress imposed by the SrRuO3 electrode, which leads to a tetragonal distortion of the perovskite lattice as opposed to the rhombohedral distortion of the bulk crystal.

References

[1] Wang J et al 2003 Science 299 1719