Unusual Behavior of Magnetic Coercive Fields with Temperature and Applied Field in La-Doped BiFeO₃ Ceramics

We report on the temperature-dependent magnetic properties of Bi1–xLaxFeO3 (x = 0.0–0.3) bulk polycrystalline materials. Unexpectedly, the La-doped BiFeO3 samples show an anomalous enhancement of the coercive field as the temperature increases. This anomaly can be interpreted by the competition between magnetoelectric coupling and magnetic anisotropy. Whatever the mechanism such as coherent rotation or domain wall movement and the associated pinning effect involved in the magnetization reversal, on a large scale, the strength of the magnetic anisotropy will act on the coercivity. Similar unanticipated results were also found when we varied the applied magnetic field at room temperature. The magnetization vs temperature (M–T) curve was measured at a temperature range of 4–300 K, and it demonstrates a large bifurcation between field-cooled (FC) and zero-field-cooled (ZFC) data. A spin glass-like behavior was found at 50 K for a 20% La-doped sample. Another M–T curve, taken at higher temperatures, reveals that the FC and ZFC curves assimilate to each other at a point, and the Néel temperature is found to be 660 K