Ferromagnetic resonance study of biogenic ferrihydrite nanoparticles: spin-glass state of surface spins

Ferrihydrite nanoparticles ($2$–$3$ nm in size), which are products of the vital activity of microorganisms, are studied by the ferromagnetic resonance method. The “core” of ferrihydrite particles is ordered antiferromagnetically, and the presence of defects leads to the appearance of an uncompensated magnetic moment in nanoparticles and the characteristic superparamagnetic behavior. It is established from the ferromagnetic resonance data that the field dependence of the frequency is described by the expression $2\pi\nu/\gamma = H_{\text{R}} + H^{\text{A}}_{(T = 0)}(1-T/T^*)$, where $\gamma$ is the gyromagnetic ratio, $H_{\text{R}}$ is the resonance field, $H_{\text{A}}\approx7\,$kOe, and $T^*\approx50\,$K. The induced anisotropy $H^{\text{A}}$ is due to the spin-glass state of the near-surface regions.