Structural and magnetic phase transitions in Ca-substituted bismuth ferromanganites

Abstract

Single-phase samples of the Bi0.85Ca0.15Fe1−xMnxO3 (0.1 ≤ x ≤ 0.5) perovskites have been prepared by a solid state reaction method and investigated using X-ray and neutron diffraction, scanning electron microscopy, piezoresponse force microscopy, magnetization, and ac susceptibility measurements. It has been found that the increase in Mn concentration induces a modification of crystal and magnetic structures giving rise to the canted antiferromagnetic polar phase → collinear antiferromagnetic polar phase → paramagnetic nonpolar phase transitions at room temperature. The appearance of spontaneous magnetization suggesting a transformation of the room-temperature antiferromagnetic structure characteristic of the ferroelectric Bi0.85Ca0.15Fe1−xMnxO3 compounds near the polar / nonpolar (R3c / Pnma) phase boundary is observed with decreasing temperature. The transformation of the magnetic arrangement can also be driven by a magnetic field application. Such metamagnetic transition can be accompanied by a large (up to ~4 emu/g for the sample with x = 0.4, when measured at T = 2 K) magnetization jump far exceeding that specific to a field-driven cycloidal order → canted order transformation in the pure BiFeO3. Nonpolar samples (x ≥ 0.45) acquire a large (a few emu/g) spontaneous magnetization at low temperature, a value that cannot be explained by a spin canting. It is shown that the low-temperature magnetic behavior of the polar and nonpolar Mn-enriched Bi0.85Ca0.15Fe1−xMnxO3 compounds can be successfully described in terms of an antiferromagnetic phase + glassy magnetic phase coexistence.