Platinum Metals Rev., 1967, 11, (4), 129
Cobalt-Platinum Alloy Magnets
Temperature Dependence of Magnetic Properties
Recent studies of cobalt-platinum alloys have shown the importance of crystal structure in relation to the exceptional magnetic properties which can be developed, particularly in the 50 atomic per cent alloy.
In work reported by Dr Hermann Dietrich of the Research Institute of Deutsche Edelstahlwerke, Krefeld, and reviewed in this journal (1), it was shown how the magnetic state of cobalt-platinum magnets was altered by heating to temperatures near the Curie point. More recently, the same author has reported the results of a further study of the temperature dependence of various cobalt-based permanent magnets, including cobalt-platinum, in which temperature coefficients of saturation magnetisation and demagnetisation curves were determined (2).
From the results of tests on small cylindrical cobalt-platinum magnets produced from melted and cast material, it was shown that variations in saturation magnetisation were small between −200°C and +200°C. Within this range the average saturation magnetisation was found to be 6750 gauss and this varied with temperature at the rate of −0.01 per cent per °C. A single Curie temperature was found at 530°C, despite the presence of a two-phase structure, indicating either that one phase only is ferro-magnetic or that the ordered and disordered phases have similar Curie temperatures.
Demagnetisation curves, shown as the relationship between intrinsic induction (4πI) and demagnetising field (H) at temperatures between −195° and 440°C, demonstrate the very high coercivity of the cobalt-platinum alloy. The “rectangular “ form of the curves in the lower temperature ranges indicates high stability under large demagnetising fields of at least 2000 oersteds, and while residual induction reduces more rapidly with increasing temperature, those magnets with a suitably high length: diameter ratio exhibit almost negligible temperature dependdence between −195° and +200°C. Above 200°C the magnetic properties reduce rather more rapidly, but can still be restored by remagnetising until changes in structure, which start to occur at about 500°C, produce permanent changes in the demagnetisation curve.
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- 1H. Dietrich, Cobalt, 1966, No. 30 (March), 3 ; L. A. Ford, Platinum Metals Rev., 1966, 10, 84
- 2H. Dietrich, Cobalt, 1967, No. 35 (June), 78