Conditions favoring the polar weak ferromagnetic state in BiFeO3-type multiferroics

The crystal structure and magnetic properties of Bi$_{1-x}$A$_x$FeO$_{3-x/2}$ (A = Ca, Sr, Pb, Ba), Bi$_{1-x}$A$_x$(Fe$_{1-x}$Ti$_x$)O$_3$, and Bi$_{1-x}$A$_x$(Fe$_{1-x/2}$Nb$_{x/2}$)O$_3$ solid solutions have been studied. It is shown that the homogeneous polar weak ferromagnetic state occurs in the vicinity of a morphotropic phase boundary in the systems where dopant ions lead to the reduction of the unit cell volume in the polar phase. In the case of A$\,=\,$Ca, the non-polar phase also exhibits weak ferromagnetism and the spontaneous magnetizations in the polar and nonpolar phases differ only slightly.