Magnetic correlations in in multilayers and monolayers of iron oxide nanoparticles

The fitting of PNR data also provides insight into the microscopic magnetic structure of multilayers of iron oxide nanoparticles. The splitting of the reflectivities for up and down polarised neutrons at high fields confirms a finite magnetisation of the multilayer, which is expressed in a finite magnetic SLD (middle left panel). At remanence the splitting disappears, which implies that the sample has zero net magnetisation. Surprisingly, the magnetic SLD value remains finite and identical to the value at high fields. This can be explained if the system is assumed to have a domain- like structure as shown in the sketch (bottom right pane). At the applied field of 5 kOe, the NPs’ magnetic moments are aligned mainly along the field axis and a finite magnetisation is observed. When the field is removed, the net magnetisation falls to zero. However, due to strong dipolar interaction between the NPs, their magnetic moments are heavily correlated over microscopic regions. These regions are much smaller than the sample area, but greater than the lateral projection of the neutron coherence length. Within such regions, the NPs’ magnetic moments are aligned ferromagnetically forming quasi-domains. Arising purely due to dipolar interaction, they are different from usual ferromagnetic domains and have no well - expressed domain walls.

D. Mishra et al., Nanotechnology 23 (2012) 055707