Nickel ferrite nanoparticles were synthesized by a self-combustion method over nanocrystalline powders of Nd-Co substituted strontium hexaferrite with nominal composition Sr0.5Nd0.5Co0.5Fe10.5O19, at different mass relations. The samples were structurally characterized by X-ray diffractometry (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray analysis (EDX). The M vs. H loops of the composites were determined with a vibrating sample magnetometer (VSM) and the interaction with the X-band microwave radiation of the nanocomposites dispersed in epoxy resin was measured with a vector network analyzer (VNA). The hysteresis loops showed strong exchange-coupling between the two magnetic phases for the 30:70 and 50:50 Sr0.5Co0.5Nd0.5Fe10.5O19/NiFe2O4 nanocomposites, while a weak interaction was observed for the 70:30 mass ratio. The nanocomposite with an equal amount of hard and soft phase shows the highest performance both in reflectivity and in bandwidth, reaching a maximum in reflectivity of -34.4 dB at 11.1 GHz while the bandwidth below -10 dB is 3.5 GHz. (C) 2015 Elsevier B.V. All rights reserved.