Emergent super-antiferromagnetic correlations in monolayers of Fe3O4 nanoparticles throughout the superparamagnetic blocking transition

We report nanoscale inter-particle magnetic orderings in self-assemblies of Fe3O4 nanoparticles (NPs), and the emergence of inter-particle antiferromagnetic (AF) (super-antiferromagnetic) correlations near the coercive field at low temperature. The magnetic ordering is probed via x-ray resonant magnetic scattering (XRMS), with the x-ray energy tuned to the Fe-L3 edge and using circular polarized light. By exploiting dichroic effects, a magnetic scattering signal is isolated from the charge scattering signal. The magnetic signal informs about nanoscale spatial orderings at various stages throughout the magnetization process and at various temperatures throughout the superparamagnetic blocking transition, for two different sizes of NPs, 5 and 11 nm, with blocking temperatures TB of 28 K and 170 K, respectively. At 300 K, while the magnetometry data essentially shows superparamagnetism and absence of hysteresis for both particle sizes, the XRMS data reveals the presence of non-zero (up to 9/100) inter-particle AF couplings when the applied field is released to zero for the 11 nm NPs. These AF couplings are drastically amplified when the NPs are cooled down below TB and reach up to 12/100 for the 5 nm NPs and 48/100 for the 11 nm NPs, near the coercive point. The data suggests that the particle size affects the prevalence of the AF couplings: compared to ferromagnetic (F) couplings, the relative prevalence of AF couplings at the coercive point increases from a factor ~ 1.6 to 3.8 when the NP size increases from 5 to 11 nm.