Coexistence of relaxor behavior and ferromagnetic order in multiferroic Pb(Fe_0.5Nb_0.5)O₃-BiFeO₃solid solution

The coexistence of relaxor ferroelectric behaviour and ferromagnetic ordering in a single-phase material is of both fundamental interest and practical potential for applications. To study this rather unusual phenomenon, a series of multiferroic solid solutions of (1 - x)Pb(Fe0.5Nb0.5)O3-xBiFeO3 (PFN-BFO, with 0 ≤ x ≤ 0.6) were synthesized in the form of ceramics using the solid-state reaction technique and its relaxor and magnetic properties were systematically characterized in this work. Structural refinements based on X-ray diffraction data at room temperature reveal the phase evolution from a monoclinic phase with Cm symmetry to a pseudo-cubic phase with Pm3m symmetry with increasing BFO content. The ferroelectric phase transition and relaxor behaviour were investigated via variable-temperature dielectric spectroscopy. A temperature-composition phase diagram was constructed in terms of TC, Tm, the Burns temperature (TB) and freezing temperature (Tf), which delimits a ferroelectric phase (FE) for x < 0.025 at T < TC, a non-ergotic relaxor state (NR) below Tf and an ergotic relaxor state (ER) at Tf < T < TB for 0.025 ≤ x ≤ 0.3, and a paraelectric state (PE) above TB for all the compositions. The differences in the microstructures and electrical properties between this work and those reported in the literature are carefully compared and discussed, which are closely related to the preparation conditions. In addition, the evolution of magnetic ordering with composition and temperature was investigated. A ferromagnetic order is induced by the substitution of a moderate amount of BFO (0.1 ≤ x ≤ 0.2), which exists up to room temperature. The complex magnetic phase diagram is established, which delimits an antiferromagnetic state (AFM1) for x = 0, two weakly ferromagnetic states, WFM1 and WFM2 for 0.1 ≤ x ≤ 0.2, another antiferromagnetic state (AFM2) for the compositions with x ≥ 0.25 at T ≤ TN, and a paramagnetic phase (PM) for all the compositions at T ≥ TN. The coexistence of relaxor behaviour and ferromagnetic ordering at room temperature makes the PFN-BFO solid solution a particularly interesting multiferroic material.