| Summary: | We have studied the densification, phase formation, microstructure, and permeability of stoichiometric and Fe-deficient Ni-Cu-Zn ferrites of composition Ni<sub>0.30</sub>Cu<sub>0.20</sub>Zn<sub>0.50+z</sub>Fe<sub>2-z</sub>O<sub>4-(z/2)</sub> with 0 ≤ z ≤ 0.06 sintered at temperatures from 900 °C to 1150 °C. The shrinkage is shifted from 1000 °C for z = 0 towards lower temperatures and reaches its maximum rate at 900 °C for z = 0.02. Stoichiometric ferrites show regular growth of single-phase ferrite grains if sintered at <i>T</i><sub>s</sub> ≤ 1100 °C. Sintering at 1150 °C leads to the formation of a small amount of Cu<sub>2</sub>O, triggering exaggerated grain growth. Fe-deficient compositions (z > 0) form Cu-poor stoichiometric ferrites coexisting with a minority CuO phase after sintering at 900–1000 °C. At <i>T</i><sub>s</sub> ≥ 1050 °C, CuO transforms into Cu<sub>2</sub>O, and exaggerated grain growth is observed. The formation of Cu oxide second phases is investigated using XRD, SEM, and EDX. The permeability of the ferrites increases with sintering temperature up to a maximum permeability of <i>µ</i> = 230 for z = 0 or <i>µ</i> = 580 for z = 0.02, respectively, at <i>T</i><sub>s</sub> = 1000 °C. At higher sintering temperatures, the permeability decreases, which is due to the formation of a microstructure with intra-crystalline porosity in large grains, and a non-magnetic Cu oxide grain boundary phase.
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