  Six-line ferrihydrite(FH) nanoparticles have been synthesized in the core of
reverse micelles, used as nanoreactors to obtain average particle sizes $<d>$
$\approx$ 2 to 4 nm. The blocking temperatures $T_B^m$ extracted from
magnetization data increased from $\approx 10$ to 20 K for increasing particle
size. Low-temperature \MOS 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 \FH phase is not present in the liquid
precursor, but precipitates in the micelle cores after the free water is
freeze-dried. Systematic susceptibility $\chi_{ac}(\emph{f},T)$ measurements
showed the dependence of the effective magnetic anisotropy energies $E_{a}$
with particle volume, and yielded an effective anisotropy value of $K_{eff} =
312\pm10$ kJ/m$^3$.
