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 > approximate to 2-4 nm. The blocking temperatures T-B(m) extracted from magnetization data increased from approximate to 10 to 20 K for increasing particle size. Low-temperature Mossbauer measurements allowed us to observe the onset of differentiated contributions from the particle core and surface as the particle size increases. The magnetic properties measured in the liquid state of the original emulsion showed that the ferrihydrite phase is not present in the liquid precursor, but precipitates in the micelle cores after the free water is freeze-dried. Systematic susceptibility chi(ac)(f, T) measurements showed the dependence of the effective magnetic anisotropy energies E-a with particle volume, and yielded an effective anisotropy value of K-eff = 312 +/- 10 kJ m(-3).