Field induced changes in cycloidal spin ordering and coincidence between magnetic and electric anomalies in BiFeO3 multiferroic

The temperature dependences of ZFC and FC magnetization was measured for BiFeO3 ceramics in magnetic field up to μ0H=10 T and in temperatures from the range 2-1000 K. The antiferromagnetic order was detected from the hysteresis loops below the Néel temperature TN=646 K. The anomaly in M(H), which occurred in the low magnetic field range, was ascribed to the field-induced transition from circular cycloid to the anharmonic cycloid. At high field limit, we observed the field-induced transition to the homogeneous spin order. From the M(H) dependence, we deduced that the spin cycloid becomes anharmonic above the field Ha that caused nonlinear magnetization. The cycloid vanished above the field Hc and the system again exhibited linear magnetization M(H). The anomalies in the electric properties, ε′(T), tanδ(T), and σ(T), which are manifested within the 635-670 K range, coincide with the anomaly in the temperature dependence of magnetization M(T), which occurs in the vicinity of TN=646 K. We propose that this coincidence can be explained by the critical behavior of chemical potential µS, related to the magnetic phase transition.