Magnetic properties of ferrihydrite nanoparticles doped with Ni, Mo, and Ir

In this work, changes in the magnetic properties of ferrihydrite (FHYD) nanoparticles (formula FeOOH·nH₂O; size5nm) on doping with 5 at. % each of Ni, Mo, and Ir by coprecipitation are reported. The variations of magnetization M as a function of magnetic field H (up to ±{}50 kOe) and temperature T (5--375 K) were investigated for the four samples, viz., FHYD, Ni/FHYD, Mo/FHYD, and Ir/FHYD, both for the zero-field-cooled (ZFC) and field-cooled (FC) conditions. The T variation of the low-field (H=100Oe) magnetic susceptibility {χ} (ZFC) peaks at temperature Tₚ70, 47, 43, and 34 K for FHYD, Ni/FHYD, Mo/FHYD, and Ir/FHYD, respectively. For T<Tₚ, {χ} (ZFC)<χ (FC), and {χ} (FC) shows broad minima at Tₛ=30, 27, 22, and 16 K for FHYD, Ni/FHYD, Mo/FHYD, and Ir/FHYD, respectively. The data are analyzed in terms of the modified Langevin function M=M₀L(μₚH/kT)+χₐH, where μₚ is the magnetic moment/particle and k is the Boltzmann constant. From the analysis of the data, temperature-independent μₚ=369, 375, 237, and 239μB are determined for FHYD, Ni/FHYD, Mo/FHYD, and Ir/FHYD, respectively. It is argued that the decrease in Tₚ and Tₛ noted above with doping results from shape anisotropy due to demagnetization fields. For T<~Tₛ, the presence of exchange anisotropy may indicate spin-glass-like ordering of the surface spins. In this temperature regime, a steplike magnetization reversal behavior is observed in the low-field region of the hysteresis loops, in qualitative agreement with the theoretical predictions by Fraerman et al. [Phys. Rev. B 65, 184433 (2002)] for magnetic nanoparticles with interparticle interaction. Finally, from the observed magnitude of μₚ, it is inferred that Ni substitutes for Fe throughout the nanoparticle, whereas doping with Mo and Ir occurs primarily at the surface.