Effects of calcination temperature on structure and magnetic properties of pure FeCo₂O₄ powders

A series of FeCo2O4 powders was initially synthesized using a hydrothermal method and subsequently calcined at various temperatures to produce the final product. Pure phase FeCo2O4 powders can only be formed in the temperature range of 950-1050 degrees C. In this work, we study the cation occupation, cation valence, bond length and bond angle changes of the pure phase FeCo2O4 powders formed in such a narrow temperature range. Octahedral lattice distortion in the pure phase FeCo2O4 samples has been observed. More tetrahedral Fe3+ and octahedral Co2+ are excited and exchanged their sites as the calcination temperature increases from 950 degrees C to 1000 degrees C, and part of Co3+ ions are reduced to Co2+ in the sample calcined at 1050 degrees C. The structure of the sample calcined at 1000 degrees C is close to that of the ideal FeCo2O4 spinel. Magnetic measurements show that ferrimagnetism and anti-ferromagnetism coexist in the pure phase FeCo2O4 samples. Interaction changes between ferrimagnetism and anti-ferromagnetism caused by the structural changes of the samples have been studied. Due to the pinning of the local anti-ferromagnetism to ferrimagnetism in the sample, the sample shows a Barkhausen jump below 150 K. As the measurement temperature increases further, the system enters into a reentrant spin glass state.