Neutron scattering and magnetic studies of ferrihydrite nanoparticles

Magnetic properties of two-line ferrihydrite (FeOOD·nD₂O) nanoparticles with an average size {}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^-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, TN350K 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 lengthparticle size. The temperature variations (5--300 K) of the initial susceptibility {χ} for the field-cooled (FC) and zero-field-cooled (ZFC) cases yield a peak at Tₚ(m)65K, below which χ(FC)>χ(ZFC). For T>Tₚ(m), the variation of χ^-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 T>Tₚ(m) is analyzed in terms of the modified Langevin variation: M=MₒL(μₚH/kT)+χₐH, where μₚ is the magnetic moment/particle. The fit at 100 K yields μₚ250μB, consistent with the theoretical estimates based on uncompensated surface spins of Fe³⁺.