Mössbauer Spectra of 57Fe in Superparamagnetic Nickel

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Abstract
The Mössbauer effect is used to study the superparamagnetic behavior of ultrafine particles of nickel dispersed on a high‐area substrate. In zero applied field, a characteristic hyperfine spectrum is observed at 78°K. As the sample is warmed to room temperature, the spectrum narrows to a single line as the result of rapid fluctuations in the magnetization direction. A fluctuation time of the order of 2 nsec is indicated from the room‐temperature data. With the application of a magnetic field the hyperfine spectrum is developed at room temperature. Although it is not possible to assign a single relaxation time to the magnetic‐field data, there is some indication that the fluctuations are slowed by the application of an external magnetic field.