Bismuth ferrite
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Bismuth ferrite (BiFeO3) is an inorganic chemical compound. It is one of the most promising lead-free piezoelectricity by exhibiting multiferroic properties at room temperature and has a perovskite structure. Multiferroic materials exhibit ferroelectric or anti-ferroelectric properties in combination with ferromagnetic (or anti-ferromagnetic) 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.
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[edit] Sample Preparation
[edit] 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.
[edit] 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 [1]. 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.
[edit] References
[1] Wang J et al 2003 Science 299 1719