Effect of rare earth doping on structural, optical, dielectric, and humidity properties of Cu–Mg–Zn ferrites

Rare earth (RE) doped ferrites with the chemical formula Cu_0.3Zn_0.3Mg_0.4T_xFe_2–xO₄ (x = 0, 0.1; T = La, Ce, Sr) were synthesized by chemical co-precipitation method. The structural, optical, electrical and humidity sensing properties of Cu–Mg–Zn ferrites with rare earth element doping were investigated. Single-phase cubic spinel structure was confirmed via X-ray diffraction (XRD), and the crystal size ranges from 22.12 to 63.17 nm according to the Scherrer formula and from 25.66 to 67.46 nm according to the Williamson–Hall method. Porous structure and elemental characterization of the samples were investigated by scanning electron microscopy (SEM). The optic band gap varies between 2.21 and 2.49 eV. Electrical measurements were conducted in the frequency range of 1 Hz–20 MHz and temperature range of 25–400 °C. It has been determined that the dielectric results are consistent with the Maxwell–Wagner method and exhibit a non-Debye relaxation model, as observed from the Nyquist plots. At a minimum frequency value of 1 Hz, the dielectric constants for pure, Ce, Sr, and La samples are 9 × 10⁴, 5 × 10⁴, 1 × 10⁸, and 2 × 10⁵ at 25 °C, and 1.85 × 10⁸, 1.34 × 10⁸, 1.15 × 10¹⁰, and 4.4 × 10⁸ at 400 °C. In the same order, for the maximum frequency value of 20 MHz, the dielectric constants at 25 °C are 169, 166, 3799, and 60, while at 400 °C they are 734, 624, 12108, and 774. The La doped sample's low dielectric loss makes it suitable for high-frequency applications. Humidity measurements were performed at room temperature and in the 5%–95% relative humidity range. The humidity properties of the samples were investigated through humidity mapping, sensitivity, hysteresis, and long-term stability tests. Compared to other samples, the results indicate that Ce exhibits better humidity performance with 99% sensitivity and the highest repeatability (91.2%). These results show that Ce-doped ferrite can be used as a low-cost, high-performance humidity sensor.