Effect of heat treatment on magnetic properties of iron-based soft magnetic composites with Al2O3 insulation coating produced by sol–gel method

•New soft magnetic composite with alumina insulation was prepared by sol–gel method.•The EDX analysis confirmed that the alumina coating had a high thermal stability.•Different parts of the core loss factor before and after annealing were calculated.•Preservation of the insulation after annealing highly reduced the eddy current loss. In this study, influences of the annealing process on the magnetic properties of new soft magnetic composite (SMC) materials with alumina insulator coating were investigated. Iron powders were coated with alumina by the sol–gel process at room temperature. The results of energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and density measurements showed that the iron powders were uniformly coated by a thin layer of alumina coating with high thermal stability. Magnetic measurements indicated that the annealing treatment increased the permeability of the composites at low and medium frequency ranges. It was shown that the annealed composites exhibited noticeably higher frequency stability of the magnetic permeability compared to the heat treated pure iron compacts at the same annealing temperature. The results of the loss separation implied that the heat treatment suppressed the hysteresis loss coefficient while it increased the eddy current loss coefficient of the SMCs. The annealed SMCs showed a lower eddy current loss and higher hysteresis loss coefficients compared to the relaxed pure iron compacts due to the preservation of the alumina coating after heat treatment.