[en] Ferrihydrite nanoparticles of the size 2-5 nm produced by environmental Klebsiella oxytoca bacteria through biomineralization of iron salt solutions possess unique magnetic properties. Namely, in these grains there co-exist the antiferromagnetic order inherent to bulk ferrihydrite and the permanent magnetic moment caused by the decompensation of the nanoparticle spin sublattices. The magnetic susceptibility of the particles enhanced by the superantiferromagnetism effect, together with the presence of magnetic moment independent of the external field, provides the possibility of magnetic manipulation of these natural objects. Thereby a way to their use in nanomedicine and biotechnologies opens. In the present work measurement results on magnetization of the two main crystallization phases of ferrihydrite produced by Klebsiella oxytoca are reported and theoretical analysis of these data is performed. This enables us to evaluate the magnetic parameters of real biomineral nanoparticles