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Structural, morphological, magnetic, and dielectric investigations are carried out in Nb-doped (viz., for x equal to 0.0-0.4 by wt%) nanocrystalline Ni-Zn ferrites synthesized by hydrothermal method. XRD, IR studies infer the growth of nano structures and Fe 2 O 3 phase. Nb ions segregate at grain boundaries. Crystallite size (D) varies from 62 to 18 nm with x. FTIR absorption exhibits split at higher x. Saturation magnetization witnesses an overall decrease with x. Trends of M s (x) are explained by nature of dopant, preferential replacement, surface canting (Y-K angles) and lattice contraction. Variations of coercive field (H c ) and D infer singledomain to multi-domain morphological transformation. Occurrence of superparamagnetism (SPM) is predicted for zeroed values of H c and for a critical concentration x crit equal to 0.321. Enhanced Nb 5? -Fe 2? binding with dopant (x up to 0.2) results for decrease in dielectric constant e 0 r , loss factor tand and increase in resistivity q. Lowered ac conductivity is attributed to blockade of path by Nb 5? ions in the vicinity of B-sites. Relatively higher q (*10 8 X cm) and lower loss (tand *10 -2 -10 -3 ) evinced for 10 kHz. Enhanced core loss is realized with x manifested as lowered H c and tand to usher their utility in high-frequency applications.