Large magnetic anisotropy in ferrihydrite nanoparticles synthesized from reverse micelles

Six-line ferrihydrite(FH) nanoparticles have been synthesized in the core of reverse micelles, used as nanoreactors to obtain average particle sizes $<d>$ ≈ 2 to 4 nm. The blocking temperatures TBᵐ extracted from magnetization data increased from ≈ 10 to 20 K for increasing particle size. Low-temperature measurements allowed to observe the onset of differentiated contributions from particle core and surface as the particle size increases. The magnetic properties measured in the liquid state of the original emulsion showed that the phase is not present in the liquid precursor, but precipitates in the micelle cores after the free water is freeze-dried. Systematic susceptibility χac(f,T) measurements showed the dependence of the effective magnetic anisotropy energies Eₐ with particle volume, and yielded an effective anisotropy value of Keff = 312±10 kJ/m³.