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The Samarium Depleted Zone in SmCo5 Magnets

After sintering, SmCo5 magnets may present a surface layer depleted in Samarium. This happens because Samarium is much more susceptible for oxidation than cobalt. Besides, samarium has high vapour pressure. This Sm depleted zone is of the order of ~ 1 mm (in a 10 mm diameter magnet), and present oxygen content 1000 ppm higher than the center of the magnet. This layer is composed by SmCo(5+x), which presents low coercivity (1-2 kOe). The occurrence of the Sm depleted layer is associated with kinks in the 2nd quadrant of the hysteresis curve. After the removal of this layer, the hysteresis kinks disappear. The efficacy of different chemical etchings (Nital and chromic acid) to reveal microstructural constituents was discussed in detail. Nital is recommended for observation of magnetic domains under polarized light (Kerr effect), whereas chromic acid is more suitable to identify Sm2Co7 and other high Sm phases.