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    SUPERPARAMAGNETISM AND MOESSBAUER SPECTROSCOPY: A REVIEW AND NEW RESULTS ON IRON DISPERSION.
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    Superparamagnetism
    Alpha iron phthalocyanines (α-FePc) oxygenated at low temperatures were investigated with the help of 57Fe Mössbauer spectroscopy, magnetization measurements (SQUID) and X-ray diffractometry (XRD). Mössbauer spectroscopy revealed that upon oxygenation of α-FePc, new species were formed which could be associated with FeIIIPc oxygen adducts. Unexpectedly, magnetically split spectrum of oxygenated α-FePc was observed below 20 K. In-field Mössbauer spectra in a 5 T external magnetic field at 5K and magnetization measurements indicate antiferromagnetic coupling in oxygenated α-FePc.
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    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.
    Superparamagnetism
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