Magnetic Memory Effects in ${\rm Fe}/\gamma \hbox {-}{\rm Fe}_{{2}}{\rm O}_{{\bf 3}}$ Nanostructures

We report an unusual consequence of magnetic memory effect in Fe/γ-Fe₂O₃ nanostructures, which is prepared using an electrochemical route. Chemical phases of Fe/γ-Fe₂O₃ nanostructures are characterized by X-ray diffraction, optical, and Mössbauer spectroscopic studies. The static and dynamic aspects of dc magnetization studies are carried out. Thermal variation of zero-field-cooled (ZFC) magnetization displays a signature of blocking temperature (T_B) at 215 K. Another broadened peak is also evident in the field-cooled (FC) magnetization at 40 K (T_G). Signature of memory effect is observed below TG after cooling the sample both in ZFC and FC protocols. Appearance of memory effect in the ZFC protocol confirms spin-glass-like state below T_G. Interestingly, memory effect is absent below T_B even in FC protocol, although a large particle size distribution is noticed in the Fe/γ-Fe₂O₃ nanostructures. The results address the fundamental question whether substantial particle size distribution is sufficient enough for the memory effect in the FC protocol as found in the literatures.