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

Abstract

In this work, changes in the magnetic properties of ferrihydrite (FHYD) nanoparticles (formula $\mathrm{FeOOH}\ensuremath{\cdot}n{\mathrm{H}}_{2}\mathrm{O};$ $\mathrm{size}\ensuremath{\simeq}5\mathrm{nm})$ 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 \ifmmode\pm\else\textpm\fi{}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 $(H=100\mathrm{Oe})$ magnetic susceptibility \ensuremath{\chi} (ZFC) peaks at temperature ${T}_{p}\ensuremath{\simeq}70,$ 47, 43, and 34 K for FHYD, Ni/FHYD, Mo/FHYD, and Ir/FHYD, respectively. For $Tl{T}_{p},$ \ensuremath{\chi} $(\mathrm{ZFC})l\ensuremath{\chi}$ (FC), and \ensuremath{\chi} (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 ${M=M}_{0}\mathcal{L}({\ensuremath{\mu}}_{p}H/kT)+{\ensuremath{\chi}}_{a}H,$ where ${\ensuremath{\mu}}_{p}$ is the magnetic moment/particle and k is the Boltzmann constant. From the analysis of the data, temperature-independent ${\ensuremath{\mu}}_{p}=369,$ 375, 237, and $239{\ensuremath{\mu}}_{B}$ 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 $Tl~{T}_{s},$ 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 ${\ensuremath{\mu}}_{p},$ it is inferred that Ni substitutes for Fe throughout the nanoparticle, whereas doping with Mo and Ir occurs primarily at the surface.