Abstract
Mössbauer spectra of bulk NiO doped with 57Co show that the 57Fe exists in both the divalent and the trivalent state and that the ratio of their concentrations varies reversibly with temperature. The Fe 3+ hyperfine field at 0°K is (548 ± 3) kOe and decreases in a normal manner with increasing temperature. The Fe 2+ hyperfine field shows an anomalous maximum of 210 kOe at 250°K, and decreases to 90 kOe at 0°K. In contrast, ultrafine NiO particles with an average particle diameter of 103 Å show only an Fe 3+ spectrum. The transition from the antiferromagnetic to the superparamagnetic state for the small NiO particles as the temperature is lowered is an example of a slow collapse of the hyperfine field due to relaxation effects. From the observed transition the particle size distribution and the crystalline anisotropy constant are estimated.
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