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Nanocrystalline Ni-Zn ferrite nanoparticles (NPs) Ni x Zn 1−x Fe 2 O 4 have been synthesized followed by detailed analysis of compositional effects on morphology, crystalline phase formation, lattice constant, crystallite size, magnetic, optical and dielectric properties describing the underlying physics as well. Cubic spinel structure with (311) preferred orientation and switching trend in crystallite size variation with gradual decrease in lattice parameter (from 8.377 to 8.323 Å) with the increase in nickel content has been observed through calculations. The magnetic behavior of NPs has been investigated at lower temperatures up to 5 K illustrating significantly improved magnetic parameters including M s = 36.8 emu/g, M r = 3.61 emu/g, SQ = M r /M s = 0.1028, H c = 115.33 Oe, μ B = 1.5664 emu, and K 1 = 2122.18 erg/g. This enhancement has been found possibly due to less thermal fluctuations and significant superparamagnetic contributions from the very small fine nanoparticles being in blocking state owing to such a cool environment. Optical band gap for both direct (1.74–1.39 eV) and indirect (1.25–1.16 eV) transitions shows decreasing trend with the increase in Ni contents probably due to the redistribution of Ni or Zn contents from octahedral to tetrahedral site. Dielectric constant (real and imaginary) and tangent loss decreased with the increase of frequency, this refers to the Maxwell-Wagner type of polarization in the material in accordance with Koop's theory, however, a switching is observed at the stoichiometric ratio of x = 0.25 and 0.75. The impedance (Z′) has maximum value 5539 kΩ at x = 0.75 and a minimum value 682 kΩ at x = 1.0. The Nyquist plot pointed out the contribution of grain boundaries, however the contribution varies with the stoichiometric ratio x. The Cole-Cole plot confirms the contribution of grains in the material for all value of x. • Superparamagnetic contributions enhanced the magnetic behavior at 5 K owing to small fine nanoparticles in blocking state. • Optical study of samples revealed that both direct and indirect band gaps decrease with the increase of Ni concentration. • The dielectric constant and loss decrease with the increase of applied frequency but switching for x = 0.25 & 0.75. • Nyquist plots pointed out major contribution of grain in conduction process, conduction mechanism follows non-Debye model. • The resistive effects of grain boundaries are dominant for all values of x according to Cole-Cole plots. [ABSTRACT FROM AUTHOR]