(a) Ferritin particle magnetic moment vs. magnetic field as a function of different cluster numbers n of N/n exchange coupled spins in a particle of N spins. For the superparamagnetic spin arrangement of Figure 2c (n=1, black curve), the particle moment saturates at relatively low magnetic fields and the magnetic moment is three orders of magnitude higher than for the paramagnetic state of Figure 2a (n = 4500, light blue curve). The curves for the clusterparamagnetic configurations of Figure 2b (n = 5, 15, 45, 450) are also shown. The inset in (a) shows the attractive force configuration between two ferritins. (b) The interaction energy magnitude (E = m·B) of the iron loaded ferritin as a function of the external magnetic field. For modest clustering of iron spins into n clusters of N/n exchange coupled spins the interaction energy is above kBT in moderate magnetic fields. The maximum theoretically possible torque on an anisotropic ferritin particle , shown diagrammatically in the inset of (b), has interaction energy above kBT.