Effect of powder oxidation degree on changes in Nd-Fe-B magnetic properties

The effect of oxygen content (varied by exposition of powder to air) on the magnetic properties of Nd-Fe-B magnets are presented. The atmospheric corrosion rate and the change of magnetic properties versus time for different oxygen content was monitored. The investigations were carried out in conditions of a 'catastrophic corrosion' characterised by an abnormally high rate of this process. It has been found that the exposition of powder to air for 3-4 days slightly deteriorates the magnetic properties of magnets and reduces the rate of corrosion (measured by mass reduction). However it also leads to a complete destruction of magnets which occur faster for higher oxygen contents. High oxygen content leads to instability of the magnetic properties and structure of the material. The coercivity and remanence decrease within a few weeks to a much lower level. It has been shown that powder oxidation does not substantially effect the magnetic properties measured immediately after processing. However, after an elapse of time under the conditions of the 'catastrophic corrosion', the elevated oxygen content results in a rapid destruction of magnets. (author)