Spin sprayed Ni(-Zn)-Co ferrite films with natural resonance frequency exceeding 3 GHz

Magnetic films having in the gigahertz range high real permeability μ′ and yet exhibiting weak imaginary permeability μ″ (thus low in tan δ= μ″ μ′) are in great need recently, for use in next-generation magnetic devices such as radio frequency identification (RFID) tags utilizing 900 MHz or 2.45 GHz band. In this study we successfully prepared films (composition: Ni0.23±0.01 Znx Coy Fe2.76±0.01-x-y, x=0-0.34 and y=0.23-0.27) which exhibit such highly capable complex permeability spectra. They were prepared from an aqueous solution at 90 °C by spin spray ferrite plating. The films are very low in magnetic losses, having tan δ=0.05-0.14 even at 900 MHz, because they have high natural resonance frequencies, fr =3.3-5.4 GHz. The tan δ is much lower than that (>0.4) observed for composite-type magnetic sheets at 900 MHz. Transmission characteristics of 900 MHz band RFID tags were prominently improved by placing the ferrite films of 3 μm thickness on the tags.