Exchange bias in magnetic nanoparticles

Exchange bias and related magnetic properties have been studied on CoO coated Co nanoparticles, gamma-Fe2O3 coated Fe nanoparticles, alpha-Cr 2O3 coated CrO2 nanoparticles, and Mn doped ZnO tetrapods. Among them, CoO and alpha-Cr2O3 are antiferromagnetic, but gamma-Fe2O3 and (Zn,Mn)Mn2O4 (or Mn3O4) are ferrimagnetic. It has been recognized that antiferromagnets can pin ferromagnets and lead to exchange bias. Large exchange bias field up to 9 kOe observed at 5 K in the Co-CoO system can be well interpreted by Meiklejohn-Bean model. However, exchange bias field of 224 Oe is observed at 5 K in the CrO2-Cr2O3 system, which is explained by random field model. This reveals that, if anisotropy energy is larger than interface exchange energy, exchange bias is described by Meiklejohn-Bean model; otherwise, random field model can usually be applied. Exchange bias in ferrimagnets-ferromagnetic systems, gamma-Fe2O 3 coated Fe nanoparticles and Mn doped ZnO tetrapods, is attributed to the pining of the spin-glass-like phase in ferrimagnets. The large exchange bias field of 6.3 kOe at 2 K in Fe-gamma-Fe2O3 system is partially due to the small size of the Fe cores. Significant training effect is also observed in the Fe-gamma-Fe2O3 system. Both the exchange bias and training effect can be well explained in a modified Stoner-Wohlfarth model. The shifts of field cooled hysteresis loops in the horizontal and vertical directions can be associated with the frozen spins, whose configurations change with the field during the hysteresis loop measurements. It seems that exchange bias is essentially determined by atom and spin configurations in the interface. Exchange bias observed in the magnetic semiconductor of Mn-doped ZnO tetrapods reveals the complexity of magnetic phase in the system, which indicates that exchange bias is a useful method in magnetic studies.