Peculiar Kinetics of Coercivity of Sintered Sm(Co_0.78Fe_0.10Cu_0.10Zr_0.02)₇ Magnet Upon Slow Cooling

The origin of coercivity of the sintered Sm(Co_0.78Fe_0.10Cu_0.10Zr_0.02)₇ magnets has been studied in the course of step annealing by X-ray diffraction and magnetic measurements. The Cu redistribution in the Sm(Co, Cu)₅ boundary phase of the cellular structure modifies the domain-wall energy and increases the coercivity upon step cooling. Interphase stresses are considered to be the main reason for copper redistribution upon slow cooling. It is shown that the coercivity abruptly increases in the course of annealing at a temperature of 500 °C, which is close to the Curie temperature of the Sm(Co,Cu)₅ phase. The effect may be caused by an increase in the interphase stresses at the boundary of the coherent 1:5 and 2:17 phases in the course of the annealing at the temperature close to the Curie temperature of the 1:5 phase. The accelerated Cu diffusion into the boundary compensates these stresses. The new scheme of the Cu redistribution over the cell-boundary phase is suggested. Contrary to the known schemes, it assumes that Cu is localized in the 1:5 phase near the interface between the phases, rather than in the center of the 1:5 phase.