 The collective behavior of magnetic nanoparticles due to interparticle interactions, such as strong magnetic dipole and exchange interactions, can lead to properties similar to spin glasses and superparamagnetic suppression, while the temperature dependence of order parameters in hematite and gufite nanoparticles can be described by a simple mean field model. Additionally, exchange interactions between nanoparticles with different easy axis orientations can cause sublattice magnetization rotations. This article was authored by Steen Moorup, Michael Faut Hansen and Catherine Fransen.