Role of Surface Iron Oxides in Coercivity Deterioration of Sm₂Fe₁₇N₃ Magnet Associated with Low Temperature Sintering

Abstract

Sm₂Fe₁₇N₃ powders were analyzed by X-ray photoelectron spectroscopy to elucidate the role of surface Fe oxides in the significant deterioration of coercivity observed when Sm₂Fe₁₇N₃ is sintered at relatively low temperatures. It was demonstrated that the Fe oxides on the powder surface completely disappear after heat treatment at 500°C in Ar. This suggests that the chemical reaction which causes the coercivity deterioration involves consumption of the Fe oxides. A reaction model that postulates a redox reaction between the Fe oxides and Sm₂Fe₁₇N₃ to produce soft-magnetic α-Fe consistently explains both the coercivity drop and the consumption of the Fe oxides. The conventional model of simple Sm₂Fe₁₇N₃ decomposition, on the other hand, cannot rationally explain the disappearance of the Fe oxides. It is therefore reasonable to consider the redox reaction to be the primary mechanism of coercivity deterioration, in which the Fe oxides play a role as one of the major reactants.