The magnetic properties of ferrofluids containing polydisperse chemically coprecipitated Mn-Zn ferrite nanoparticles of various composition, as well as the dispersion characteristics and structure of particles, are investigated. Results of the neutron-activation analysis of particles give a Mn, Zn and Fe ratio different from that of stoichiometrically predicted ones at precipitation. Magnetogranulometric analysis of ferrofluid samples performed by using the Langevin-type magnetization model of noninteracting subdomain particles shows that, instead of the usually observed log-normal particle size distribution curves, there exists a more complex distribution of magnetic moments of particles. Ultracentrifugal separation of samples allows the distribution of two fractions of particles having different magnetic moment values, which are not proportional to the particle mean volumes determined using X-ray diffraction spectra measurements.
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