Low temperature magnetization of Mn_xFe_1-xO-Fe₂o₃ ferrofluids

Magnetization of freezing Mn_xFe_1-xO-Fe₂o₃ (x=0,0.1.1) ferrofluids was investigated systematically. Kerosenebased ferrofluids were produced by co-precipitation method and we found that the average particle size increases from 118 Å to 178 Å with increasing of Mn concentration. It is also found that the blocking temperature TB increasing as Mn concentration decreasing. It shows that the increase of the Mn concentration resulted in increasing of the particles size would dismiss the blocking and hence decrease the relaxation time. The magnetic measurement shows the Curie law of superparamagnetic particles in the liquid region while it reveals the Curie-Weiss law in the unblocked solid region.