Neutron scattering and magnetic studies of ferrihydrite nanoparticles

Magnetic properties of two-line ferrihydrite (FeOOD . nD(2)O) nanoparticles with an average size similar or equal to 4 nm are investigated using neutr on scattering and magnetometry. Comparison of the neutron scattering and x- ray diffraction patterns identifies the (002) peak at Q = 1.3 Angstrom (- 1 ) as predominantly magnetic. The intensity of this peak, measured from 10 t o 450 K, decreases almost linearly with temperature until 350 K, becoming t emperature independent above 350 K. From this, T(N)similar or equal to 350 K is identified to be the ordering temperature of the core spins of the nan oparticles. The width of the line is temperature independent, yielding a ma gnetic coherence length similar or equal to particle size. The temperature variations (5-300 K) of the initial susceptibility chi for the field-cooled (FC) and zero-field-cooled (ZFC) cases yield a peak at T-p(m)similar or eq ual to 65 K, below which chi(FC) >chi(ZFC). For T>T-p(m), the variation of chi(-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 fer ritin, the field dependence of the magnetization M for T> T-p(m) is analyze d in terms of the modified Langevin variation: M = M0L(mu(p)H/kT) + chi(alp ha)H, where mu(p) is the magnetic moment/particle. The fit at 100 K yields mu(p)similar or equal to 250 mu(B), consistent with the theoretical estimat es based on uncompensated surface spins of Fe3+.