High-Temperature Stability of Magnetic Materials

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Abstract
Magnetic materials able to perform in environmental extremes are a prerequisite to many military applications such as space vehicles, ballistic missiles, and other advanced systems. Industry, in cooperation with various government agencies, is in constant search for better magnetic materials to meet these requirements. Of the environments to be considered for these applications, temperature has the most deleterious effect on magnetic materials. Most high Curie temperature magnetic materials will show a significant irreversible change in magnetic properties as a result of exposure to 500°C and return to room temperature. This paper will present a possible explanation to the cause of some of these changes especially in the high Curie temperature alloys. Our investigations showed that cobalt-iron alloys containing from 27 to 43% cobalt can be stable magnetically up to at least 600°C. The 49% cobalt alloy can be stable up to at least 500°C. However, this temperature stability is dependent upon several important considerations. One of these is the geometry of the test system. For example, a tape-wound toroidal core tested at elevated temperatures in air becomes severely oxidized—this oxidation or the strains induced by it adversely affect the magnetic properties. However, when the same core is tested in a neutral atmosphere, the strains due to oxidation are minimized and the magnetic properties remain stable. Data will be presented comparing tests on toroids with and without neutral atmospheres, both oriented and nonoriented alloys containing 27%, 35%, 43%, and 49% cobalt. Finally all results will be compared with square punchings relating the differences in high-temperature stability of the magnetic properties due to geometric configurations.

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