Anomalously High Remanence in (γ−Fe2O3)1−x· (α−Fe2O3)x Particles

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Abstract
The remanent magnetization of acicular γ−Fe2O3 particles partially converted to hematite by heating in air was measured after saturation and ac demagnetization using a vibrating‐sample magnetometer. It is possible that commercial γ−Fe2O3 recording tapes could undergo this partial conversion which may result in a deterioration of their magnetic properties. The complete composition range from pure γ Fe2O3 to hematite was studied. The composition was determined by interpolation of the saturation magnetization of the sample and of the values for pure γ Fe2O3 and hematite. The remanent magnetization increases as a function of hematite composition. At 90% hematite there is a peak in the remanent magnetization which is a factor of 2 greater than the remanence of the two components. The coercivity was approximately constant, 320 Oe, as a function of composition except that pure hematite had a value of 635 Oe. Partially converted particles were examined using dark‐field electron microscopy to indicate the method of conversion.

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