Thermomagnetic analysis, annealing effects, and Cr distribution in CoCr sputtered films

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Abstract
Thermomagnetic analyses (TMA) of rf-sputtered CoCr films have shown unusual magnetization versus temperature behavior. On heating, magnetization persists to around 700 °C. After cooling from 800 °C, however, magnetization does not appear again until around 300 °C. On all subsequent heatings and coolings, such films retain a Curie temperature of around 300 °C. At the same time, room-temperature Ms values decrease considerably. TMA of bulk samples of the same composition show a Curie temperature around 300 °C and no subsequent change of Curie temperature upon heating and cooling. These data cannot be explained on the basis of Cr-rich oxide formation. Additional experiments have ruled out Cr segregation in a planar transition layer. X-ray and electron diffraction measurements have shown no evidence of long-range order-disorder phenomenon. However, the data are consistent with explanations involving either Cr grain-boundary segregation or atomic-scale redistribution of the Cr. A simple model of grain-boundary segregation has been developed which predicts the expected relationship of average composition and Cr-rich grain boundary width. Minimum widths and minimum composition differences can be calculated.