Power capacity enhancement and loss reduction induced by limited solid solubility of Ho ³⁺ rare-earth substitution in NiCuZn spinel ferrites

The application of NiCuZn ferrites (NCZFs) in high-power communication systems is constrained by their nonlinear excitation. To reduce nonlinear effects, it is essential for ferrite materials to possess a relatively high spin-wave linewidth (Δ<i>H</i>_k). Doping with ions such as cobalt and rare-earth (RE) ions with fast relaxation has proven effective in increasing Δ<i>H</i>_k of ferrites. However, the regulatory mechanism of doping NCZFs with RE ions with larger ionic radii remains unclear. In this study, Ho³⁺-substituted NCZFs were synthesized via a solid-state reaction route. The spatial distribution and substitution amount of the Ho³⁺ ions were carefully investigated via elemental and phase composition analysis, revealing the limited solid solubility of the Ho³⁺ ions in NCZFs. Some of the Ho³⁺ ions enter the lattice and occupy the octahedral sites, accelerating relaxation and increasing Δ<i>H</i>_k to a maximum value of 2.63 kA·m⁻¹. Insoluble Ho³⁺ ions combine with Fe³⁺ ions to form a HoFeO₃ heterogeneous phase with Fe³⁺ ions at the grain boundaries, leading to iron deficiency within NCZF crystals and significantly reducing the dielectric loss tangent at microwave frequencies. These results reveal the great potential of Ho³⁺-substituted NCZFs for high-power, low-loss microwave applications.