Abstract
Electron spin resonance investigations at 9.24 GHz in Si-doped two-line ferrihydrite (Si/FeOOH/spl middot/nH/sub 2/O) nanoparticles of size /spl sime/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/spl sime/2 is observed; however with increase in x, both the resonance field H/sub r/ and linewidth /spl 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/sub r/ and /spl Delta/H for x=0.05 shows a downward shift (/spl delta/H/sub r/) of the line with a rapid increase in /spl Delta/H so that /spl delta/H/sub r//spl prop/(/spl Delta/H)/sup 3/ predicted for randomly oriented nanoparticles is observed.
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