Magnetite nanoparticles: Synthesis, thin film properties and inkjet printing of magnetic cores for inductor applications

Magnetic thin films of preferred thickness, patterns, and characteristics were produced using digital printing at room temperature and under ambient conditions to realize magnetic cores for Radio Frequency Identification resonators. The magnetite nanoparticles (Fe3O4) covered with oleic acid were synthesized for that purpose and inkjet printed from chlorobenzene solution on flexible polyimide (PI) substrate and on paper. The obtained nanoparticles have a homogenous morphology, approximately round shape and a size distribution of 7-10 nm. The crystallite size in the films remains the same as in the powder, although aggregation takes place to various extents in the films providing different magnetic properties in each of them. The inkjet printed magnetic cores were investigated in the frequency range from 10 kHz to 11 MHz. Only the magnetic cores printed on PI and annealed at 300 °C for 1 h and 2 h exhibit ferromagnetism (μr > 1) at low frequencies, and at higher frequencies all films saturate to μr < 1. For the 300 °C-2 h annealed films the μr saturation value is distinctly lower than for other films. μr of the core realized on paper is lower than unity in the entire investigated range of frequencies and very uniform, especially at high frequencies. The presented results put forward the possibility of exploitation of the inkjet printed thin magnetic films in well-established manufacturing industries, such as the watch making, banknote watermarking, and the smart tag production.