Magnetic relaxation in nano-phase chromium substituted goethite

The magnetic and crystalline properties of chromium substituted goethite, alpha-Fe(1-x)Cr(x) OOH ( where x = 0.00, 1.44, 3.00, 7.00, 10.14 wt.%) were studied using Mossbauer spectroscopy and X-ray diffraction. Transmission electron microscopy measurements showed that as the chromium concentration increased from 0 to 10.14 wt.%, the particle size decreased rapidly from 200 to 10 nm. X-ray diffraction analysis indicated that no lattice strain was present, even for chromium concentrations as high as 10.14 wt.%. Mossbauer spectra recorded at 300 K showed the presence of superparamagnetic relaxation in each sample. The relative area of the doublet component decreased, while that of the sextet component increased as the temperature decreased from 300 to 77 K. The mean hyperfine field measured at 77 K decreased from 49.6 T for the pure goethite to 44.9 T for the 10.14 wt.% chromium substitution. This decrease in the hyperfine field was attributed to both decreasing particle size and increasing chromium concentration.