Figure 5.

Effect of the Co²⁺ treatment on the exchange bias features of FeO/CoFe₂O₄ nanocrystals. (a) Magnetization after zero field cooling, measured with an applied field of 100 Oe, shows a disappearance of the antiferromagnetic to paramagnetic phase transition upon Co²⁺ treatment (note that the data for the initial material is magnified by a factor of 10). (b) Opening of the hysteresis loop, measured at their vertically shifted center, is greatly enhanced. The inset shows an open hysteresis loop at 300 K for the Co treated nanocrystals (blue line) whereas the initial material (red line) exhibits a completely closed hysteresis. (c) Vertical loop shifts before (red line) and after (blue line) Co²⁺ treatment. The inset shows the high temperature region on magnified scale.