Investigation of magnetic phase transitions in B-site disordered PbBxB'1-xO3 (B = Fe, Co and B' = Nb, Ta)

Materials such as PbFe$_{0.5}$Nb$_{0.5}$O$_{3}$ (PFN-0.5)/ PbFe$_{0.5}$Ta$_{0.5}$O$_{3}$ (PFT-0.5) and PbCo$_{0.33}$Nb$_{0.67}$O$_{3}$ (PCN-0.33) are relaxor ferroelectrics. PbFe$_{0.5}$Nb$_{0.5}$O$_{3}$ (PFN-0.5) also shows anti-ferromagnetic order below $\sim$ 143 K. Though multiferroicity is an important property of PFN-0.5, its uniqueness stems from coexisting anti-ferromagnetic and spinglass phases below $\sim$ 12 K. Presently, it is the only known such case in a Heisenberg 3D spin system. We report a first systematic study of the H-T phase diagram of PFN-0.5 and discuss the results in the context of existing theories. In addition, we study the magnetic properties of PFT-0.5 and PCN-0.33. While PFN-0.5 and PFT-0.5 demonstrate similar behavior, PCN-0.33 does not show any anomalies that could signify magnetic ordering. We propose that the observed dramatic differences may result from partial B-site ordering.