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

In this work, changes in the magnetic properties of ferrihydrite (FHYD) nanoparticles (formula $\mathrm{FeOOH}\cdot n{\mathrm{H}}_2\mathrm{O};$ $\mathrm{size}\simeq 5\mathrm{nm})$ 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 $(H=100\mathrm{}\mathrm{Oe})$ magnetic susceptibility χ (ZFC) peaks at temperature $T_p\simeq 70,$ 47, 43, and 34 K for FHYD, Ni/FHYD, Mo/FHYD, and Ir/FHYD, respectively. For $T<\mathrm{}{T}_p,$ χ $\left(\mathrm{ZFC}\right)<\mathrm{}\chi$ (FC), and χ (FC) shows broad minima at $T_s=30,$ 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 ${M=M}_0\mathcal{L}({\mu }_{p}H/kT)+{\chi }_{a}H,$ where ${\mu }_p$ is the magnetic moment/particle and k is the Boltzmann constant. From the analysis of the data, temperature-independent ${\mu }_p=369,$ 375, 237, and $239{\mu }_B$ are determined for FHYD, Ni/FHYD, Mo/FHYD, and Ir/FHYD, respectively. It is argued that the decrease in $T_p$ and $T_s$ noted above with doping results from shape anisotropy due to demagnetization fields. For $T<~T_s,$ 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 ${\mu }_p,$ it is inferred that Ni substitutes for Fe throughout the nanoparticle, whereas doping with Mo and Ir occurs primarily at the surface.