Novel facets of magnetic transitions in multiferroics

The coexistence of mutually exclusive phenomena of magnetic and ferroelectric orderings with a coupling between the corresponding order parameters in multiferroics has evoked enormous interest because of the interesting underlying physics and also the possibility of designing new generation sensor, actuator and memory devices. Amongst the magnetoelectric multiferroics, BiFeO₃ and its solid solutions have attracted maximum attention as they exhibit multiferroicity at room temperature required for any device application. In this talk, I shall outline the consequences of the interplay of magnetic and ferroelectric degrees of freedom taking three different phenomena. It is well known that with decreasing particle size, the magnetic and ferroelectric transition temperatures decrease. The scaling theories of phase transitions in finite size systems also predict a decrease in transition temperature with particle size. Contrary to these observations/predictions, I shall show how Neel transition temperature (TN) in the tetragonal phase of solid solution of BiFeO₃ with PbTiO₃ can increase with decreasing particle size. In the second example, I shall show how an isostructural phase transition accompanying the Neel transition temperature can mediate linear magnetoelectric coupling in solid solutions of BiFeO₃. The third example will deal with the discovery of magnetoelectric spin glasses in solid solutions of BiFeO₃