Neutron scattering and magnetic studies of ferrihydrite nanoparticles

Abstract

Magnetic properties of two-line ferrihydrite $(\mathrm{FeOOD}\ensuremath{\cdot}n{\mathrm{D}}_{2}\mathrm{O})$ nanoparticles with an average size \ensuremath{\simeq}4 nm are investigated using neutron scattering and magnetometry. Comparison of the neutron scattering and x-ray diffraction patterns identifies the (002) peak at $Q=1.3{\AA{}}^{\ensuremath{-}1}$ as predominantly magnetic. The intensity of this peak, measured from 10 to 450 K, decreases almost linearly with temperature until 350 K, becoming temperature independent above 350 K. From this, ${T}_{N}\ensuremath{\simeq}350\mathrm{K}$ is identified to be the ordering temperature of the core spins of the nanoparticles. The width of the line is temperature independent, yielding a magnetic coherence $\mathrm{length}\mathrm{\ensuremath{\simeq}}\mathrm{particle}$ size. The temperature variations (5--300 K) of the initial susceptibility \ensuremath{\chi} for the field-cooled (FC) and zero-field-cooled (ZFC) cases yield a peak at ${T}_{p}(m)\ensuremath{\simeq}65\mathrm{K},$ below which $\ensuremath{\chi}(\mathrm{FC})g\ensuremath{\chi}(\mathrm{ZFC}).$ For $Tg{T}_{p}(m),$ the variation of ${\ensuremath{\chi}}^{\ensuremath{-}1}$ vs T is analyzed in terms of the model of El-Hilo et al., involving particle-size distribution and interparticle interactions, and substantial interparticle interactions are inferred. Following the observations in ferritin, the field dependence of the magnetization M for $Tg{T}_{p}(m)$ is analyzed in terms of the modified Langevin variation: ${M=M}_{o}\mathcal{L}({\ensuremath{\mu}}_{p}H/kT)+{\ensuremath{\chi}}_{a}H,$ where ${\ensuremath{\mu}}_{p}$ is the magnetic moment/particle. The fit at 100 K yields ${\ensuremath{\mu}}_{p}\ensuremath{\simeq}250{\ensuremath{\mu}}_{B},$ consistent with the theoretical estimates based on uncompensated surface spins of ${\mathrm{Fe}}^{3+}.$