Demagnetization curves and coercivity mechanism in Sm(CoFeCuZr) z magnets

Abstract

The hysteresis behavior of Sm(CoFeCuZr) z magnets aged at 850°C and slowly cooled to temperatures down to 400°C has been examined. All the samples show two-step demagnetization curves. Field dependence of irreversible susceptibility χ irr shows two peaks, namely H ci1 and H ci2 . H ci1 increases from almost zero to 21 kOe when the end-ageing-temperature ( T end) decreases from 600 to 400°C, while H ci2 remains virtually unchanged. This behavior is attributed to the inhomogeneous distribution of Cu in the magnet. The change of H ci1 is believed to be related to the segregation of Cu into the cell boundary phase during the slow cooling process. H ci2 does not change with T end, which may suggest that the Cu content of the cell boundary in the Cu-rich region may be high enough to make it non-ferromagnetic at room temperature. Further Cu segregation during slow cooling to a lower temperature would not increase H ci2 because the difference of anisotropy constant between the matrix and cell boundary phase Δ K 1 has already reached its maximum. Temperature dependence of χ irr shows that the higher the temperature, the smaller the difference between H ci1 and H ci2. When the temperature reaches 320°C, H ci1= H ci2, which might suggest that 320°C is higher than the Curie temperature of the cell boundary phase in the Cu-poor region. Then the contribution of the cell boundary phase to domain wall pinning becomes almost the same in both the Cu-rich and -poor regions.