Significantly improved near-field communication antennas based on novel Ho³⁺ and Co²⁺ ions co-doped Ni–Zn ferrites

In near-field communication (NFC) antennas, soft magnetic ferrites are usually applied as a substrate to reduce eddy current loss and increase magnetic field coupling. For this purpose, the applied ferrites are required to have high permeability and saturation magnetization together with low magnetic loss and dielectric loss. However, for most soft magnetic ferrites, it is difficult to meet all the requirements. Herein novel Ni–Zn ferrite ceramics co-doped by Ho³⁺ and Co²⁺ ions with chemical formula Ni_0.5−xZn_0.5Ho_0.02Co_xFe_1.98O₄ (x = 0–0.2) were designed and prepared to balance these needs on the basis of molten salt synthesis with metal nitrates as raw materials and potassium hydroxide (KOH) as the precipitation agent and molten salt precursor. After the substitution of Ho³⁺, the saturation magnetization and initial permeability decrease, but with further doping of Co²⁺, the saturation magnetization gradually increases, while the initial permeability continues to decrease. When x = 0.1, the sample will have the lowest dielectric constant, magnetic and dielectric loss, as well as the highest Curie temperature (305 ℃). Moreover, the acquired Ni–Zn ferrites have been applied simulatively in NFC antennas, revealing that the device manufactured with the optimal Ni_0.4Zn_0.5Ho_0.02Co_0.1Fe_1.98O₄ ferrite ceramics would have significantly improved performance at 13.56 MHz with low leakage and long transmit distance of magnetic field. Therefore, the Ni_0.4Zn_0.5Ho_0.02Co_0.1Fe_1.98O₄ ferrite ceramics would be a good candidate for NFC antenna substrates.