Improved Magnetic Softness for NiCuZn Ferrite by Two-Step Sintering Method

The Ni_0.2Cu_0.3Zn_0.5Fe₂O₄ ferrite powders with calcining temperatures T_a of 500°C, 600°C, 650°C, 700°C and 800°C were prepared by sol-gel auto-combustion method. The 650°C-calcined sample has the highest saturation magnetic induction Bs and initial permeability μ_i. Then the 650°C-calcined powder was made into bulk samples and sintered by the two-step sintering method. The samples were first heated to a higher temperature,T_H = 1000 ~1100°C, then immediately cooled to a lower temperature,T_L = 950°C, for isothermal sintering with a dwelling time of 2 h. It was found that the two-step sintering method could significantly enhance the μ_i of the bulk Ni_0.2Cu_0.3Zn_0.5Fe₂O₄ samples, and keep an excellent temperature stability μ_i of in the range of 70 ~ 230°C. The ΔT(T_H - T_L) has a great influence on μ_i, and the larger ΔT is required for improving magnetic softness of the bulk samples.