Effect of Si Doping on the Electron Spin Resonance Properties of Ferrihydrite Nanoparticles

Electron spin resonance investigations at 9.24 GHz in Si-doped two-line ferrihydrite (Si/FeOOH [multiplied by] n[H.sub.2]O) nanoparticles of size [nearly equals to] 4 nm are reported for x = Si/(Si + Fe) = 0, 0.02, 0.05, 0.10 and 0.15. For all samples, a single broad resonance near g [nearly equals to] 2 is observed; however with increase in x, both the resonance field H, and linewidth [Delta]H increase but integrated intensity I of the line decreases, similar to the observed decrease in magnetization with x. These observations suggest that the observed resonance is due to uncompensated [Fe.sup.3+] surface spins. Temperature variation (300 K to 80 K) of H, and [Delta]H for x = 0.05 shows a downward shift ([Delta][H.sub.r]) of the line with a rapid increase in [Delta]H so that [Delta][H.sub.r] [varies] [([Delta]H).sup.3] predicted for randomly oriented nanoparticles is observed. Index Terms--Doped ferrihydrites, electron spin resonance, magnetometry, nanoparticles.