Dielectric and magnetic properties of (Bi1-xLaxFeO3)0.5(PbTiO3)0.5 ceramics prepared by high energy mechanochemical technique

Multiferroic (Bi1-x La x FeO3)0.5(PbTiO3)0.5 ceramics with x = 0, 0.1, 0.2, 0.3 and 0.5 was prepared from mechanochemically synthesized nanopowders. The XRD studies revealed the tetragonal structure for x ≤ 0.2 and rhombohedral for x = 0.5 with mean grain sizes from 15 to 28 nm. Dielectric response of the compounds in the temperature range 125–525 K was found to contain two anomalies: at ~400 K attributed to the presence of oxygen vacancies and that below 300 K characteristic of ferroelectric relaxors; the Curie point anomaly was apparent at 500 K only for samples with x = 0.5. Analysis of the low-temperature response enabled us to characterize the relaxor state of the solid solution and to yield information on the decrease in the activation energy of dipole moment flipping in polar nanoregions. Magnetic studies revealed two contributions to the magnetization of the nanoceramics pointing to a coexistence of weak ferromagnetic and antiferromagnetic orderings in the range 4 ÷300 K. Low temperature variation of the magnetization in field-cooling and zero-field–cooling conditions revealed spin glass behavior which in (Bi1-xLaxFeO3)0.5(PbTiO3)0.5 is dependent on x. Moreover, a sharp step in magnetization at ~250 K was found to be dependent on the La content and related to the A-site distortion leading to a modification of Fe-O-Fe angles and antiferromagnetic coupling between magnetic Fe3+ moments.