A process has been developed for preparing dense nickel ferrite bodies composed of grains small enough to remain single domain. The magnetic and crystallographic properties of these bodies have been determined. This was accomplished by combining two techniques, flame-spraying and hot-pressing, and investigating the effect of varying the parameters in each of these. Powder as fine as 0.02 micron was prepared of single phase nickel ferrite, which was subsequently densified by hot-pressing while maintaining the crystallite size as small as 0.06 micron (below the critical size for multidomains). The effect on the dispersion of the complex permeability as the ferrite is annealed through the critical size is shown. Properties measured included μ′and μ′′ (to 3800 Mc), Bmax, Brem, and Hc, as well as microwave properties. Of practical interest were an improvement in the temperature coefficient of initial permeability over conventionally prepared ferrite, and good linearity in the main resonance loss at high power levels at X-band. X-ray diffraction determinations of crystalline phases and crystallite sizes are given, along with measurements of apparent density. Theoretical considerations indicate domain rotation to be the mechanism responsible for the resonances observed at high frequencies.