Increased Low‐Temperature Magnetization and Spin‐Reorientational Transition in the Polar Phase of (Ca, Mn)‐Doped Bismuth Ferrites

Solid‐state synthesis and an investigation of the crystal/magnetic structure and magnetic properties of the Bi1−xCaxFe0.6Mn0.4O3+δ [x = 0.1, 0.15; δ = 0.02(2)] multiferroics have been conducted to explore an approach to designing ferroelectric materials with the enhanced magnetization attributed to the ferromagnetic superexchange involving the Mn 3d states. It is shown that the (Ca, Mn)‐doped samples maintain the polar R3c structure characteristic of the pure bismuth ferrite. For both these compounds, an antiferromagnetic (AFM) G‐type ordering of the Fe/Mn magnetic moments along the hexagonal c‐axis is revealed at room temperature. The reorientation of the magnetic moments from the c‐ to a‐axis occurs as temperature decreases. Being consistent with the competing character of the superexchange between Mn3+, Mn4+, and Fe3+ ions, the coexistence of AFM long‐range‐ordered and superparamagnetic phases underlying the appearance of a significant magnetization in the low‐temperature range is observed. Herein, a room‐ and low‐temperature neutron diffraction study of the Bi1−xCaxFe0.6Mn0.4O3+δ (x = 0.1, 0.15) multiferroics is reported. The temperature‐driven spin‐reorientational transition taking place in the ferroelectric R3c phase is found. Evidence is presented for the formation of a superparamagnetic phase significantly affecting the magnetic properties of the Mn‐enriched compounds at low temperatures.