Exchange-bias and superparamagnetic behaviour of Fe nanoparticles embedded in a porous carbon matrix

Combining different experimental techniques, the physical properties of Fe nanoparticles (NPs) randomly dispersed in a porous carbon have been investigated. From the analysis of TEM images and the broad maximum observed in ZFC–FC magnetization measurements under an applied magnetic field of 2.5 mT, a broad-size distribution NPs (5–50 nm) is estimated. The nature of these NPs (α-Fe and γ-Fe) was determined from X-ray diffraction. Furthermore, magnetization measurements suggest that these Fe-NPs remain completely blocked below 60 K due to a magnetic exchange-bias coupling induced on the surface of these NPs. However, the true nature of the core–shell responsible for this exchange bias within the NPs, is not totally understood yet. In addition, the system begins to be magnetically unblocked above 60 K, becoming almost superparamagnetic above 200 K. Room temperature Mössbauer spectroscopy confirms the existence of both α- and γ-Fe phases; an additional quadrupole component is required to properly fit the spectrum, which could be associated with the NP’s shell or to an oxide phase probably located on the surface of the NPs.