Microstructural changes during the slow-cooling annealing of nanocrystalline SmCo 2:17 type magnets

The microstructure and magnetic properties of 2:17 type isotropic magnets were investigated. The slow cooling heat treatment (cooling at 1 °C/min from 820 to 400 °C, and isothermal treatment during 24 h) was interrupted after the temperatures of 820, 700, 600 and 500 °C and their hysteresis were measured with fields up to 9 T. The fully heat treated sample presented coercivity (μ0H) of 3.32 T, after 24 h at 400 °C. The microstructure was investigated with SEM–FEG (Scanning Electron Microscope with Field Emission Gun) and X-ray Diffraction Rietveld analysis. The application of the Stoner–Wohlfarth–Callen–Liu–Cullen (SW–CLC) model points out exchange coupling between ferromagnetic Sm2(CoFe)17 nanocells and ferromagnetic Sm(CoCu)5 present at the cell boundary phase. The results are interpreted with the double shell model: first-a cobalt-rich ferromagnetic Sm(CoCu)5 shell originates exchange coupling and second-a copper-rich paramagnetic Sm(CuCo)5 shell produces magnetic decoupling. This double shell helps to maximize coercivity and remanence. The anisotropy field of the Sm2(CoFe)17 cell phase was estimated in 7 T with the SW–CLC model.

► Use of the Stoner-Wohlfarth Callen Liu Cullen model in 2:17 type magnets.
► Data suggest exchange coupling between Sm2(CoFe)17 and Sm(CoCu)5 phases.
► It is given structural data for phase Sm0.33Zr0.67TM3, with TM=Co,Fe,Cu.
► The beneficial effect of the slow cooling treatment is explained.