120 CHAPTER 12. PERMANENT MAGNETSby means of ball milling, mortar grinding, or jet milling. In order to avoid oxidation,
all milling operations have to be performed in an inert-gas atmosphere. Organic liquids
frequently used for ball milling and attrition milling are freon, cyclohexane, or toluene.
In all these cases, one has to be aware of a considerable explosion hazard because of the
presence of the Nd-rich eutectic in the cast alloy (i.e., the presence of fine Nd particles in
the coarse powder). After the milling treatment, the powder is dried under vacuum or by
gentle heating in a flow of purified-argon gas.
The purpose of the milling process is to obtain fine particles that, in the most favorable
cases, can be regarded as small single crystals. Because grain boundaries are absent, one
may expect that the particles will have only one single axis of preferred magnetization. This