On the enhanced dielectric and magnetic properties of BiFeO3 ceramics sintered under meta-stable conditions

Multiferroic BiFeO3 (BFO) ceramics are synthesized by non-thermodynamically stable methods, such as fast firing, cryomilling and Spark Plasma Sintering (SPS) with applied pressure of 60 MPa and high heating rate of 150 K/min. We obtained BFO ceramics and powders that present enhanced magnetic and electrical properties. The observed enhancements are caused by the chosen routes, such as cryomilling, which causes a significant reduction of crystallite size, reaching 23 nm, and a high micro-strain level, reaching 1.2%. This is reflected in the magnetic curve which is changed from antiferromagnetic-like to weak-ferromagnetic-like, with an increased maximum magnetization from 0.085 emu/g to 0.369 emu/g. The dielectric constant reached a high value, close to epsilon'= 102 coupled with low dielectric loss (tan delta similar to 0.019) over a wide range of frequencies (1 kHz to 2 MHz), reaching values close to 0.005 near 2 MHz. Such dielectric constant (dielectric loss) values are generally present for highly resistive ceramics. By equivalent circuit fitting we managed to separate the grain and grain boundary contributions. These results and those from XRD diffraction and Mossbauer spectroscopy indicate a migration of defects to the surface of the grains. In summary, we managed to consolidate this sintering method as a powerful method to obtain BFO ceramics with enhanced electrical and magnetic properties. Such improvements, especially the high resistivity and low tangent losses, can open new avenues for future technological applications of BiFeO3 nanostructured ceramics, such as ferroelectric memories and the magnetic enhancements for spintronic based devices.