Fe/ferrite composite magnetic cores far exceeding Snoek´s limit fabricated by simplified ferrite plating in open air

Iron spheres 2.0 μm in diameter in distilled water was successfully encapsulated in dense Ni-Zn ferrite layer 15-20 nm thick by simplified ferrite plating method. In this method, a reaction solution of FeCl₂+NiCl₂+ZnCl₂ and a pH adjusting solution of KOH were added to the water. The solution was stirred at 25°C in open air. The dissolved oxygen worked to oxidize a part of Fe²⁺ to Fe³⁺. The deposited Ni-Zn ferrite layer on Fe microsphere surface was composed of fine grains 10-15 nm in diameter. Since the Ni-Zn ferrite layer was very thin and dense, a large saturation magnetization M_s=210 emu/g, almost the same as that of bare Fe, was obtained. The compacted cores of these Fe/ferrite microspheres exhibited excellent frequency characteristics exceeding Snoek´s limit for Ni-Zn ferrite cores by factor 4.