Spin disorder versus Exchange bias coupling in magnetic nanoparticles with complex architecture (Hollow, shell/shell,…)

An important relevant features of the size effect in magnetic nanoparticles (NPs) is the occurrence of non-collinear spin structures (spin canting, magnetic frustration, spin disorder). Indeed, the non-collinear spin structures could strongly modify the magnetic properties of the magnetic NPs, for example as it was shown recently by us which found a strong correlation between complex spin disorder and exchange bias (EB) features in iron oxide hollow NPs. Although the macroscopic model of EB effect existed for nearly six decades, the microscopic origin of this phenomenon is still requiring further investigation in some specific systems at nanoscales. Even in the case of very well-studied and investigated nanomaterials such as spinel ferrites, a full understanding of the correlation between spin structure, surface/interface effect and EB coupling is still lacking. In this context, we propose to investigate this phenomenon in systems presenting high spin disorder like Hollow NPs with different sizes and shell thickness and extended this study to a system of shell/shell that consists of two magnetic phases: a ferrimagnetic phase and an antiferromagnetic one. The idea is to discriminate the EB resulting from surface spin disorder from that resulting from spins interface coupling between two different magnetic phases. Figure below show the Mössbauer spectra for hollow (g-Fe2O4) and shell/shell (g-Fe2O4/NiO) NPs with same external diameter and thickness (15 nm and 7.5 nm). As a preliminary result, we observe that the spin disorder is bigger in the shell/shell than in the hollow one. On the other hand the EB observed in shell/shell NPs is bigger than that observed in hollow NPs. This raises the question about the effect of the antiferromagnetic phase on the spin disorder and the EB coupling in this kind of nanoscale systems ? Finally, In order to avoid the effect of dipolar interaction and study its effects on spin disorder and EB we propose to disperse these systems in non magnetic matrix.

In-field Mössbauer spectra of magnetic NPs with different architectures: γ-Fe2O3, (hollow) and γ-Fe2O3/NiO (Shell/shell)