Correlation of microchemistry of cell boundary phase and interface structure to the coercivity of Sm(Co0.784Fe0.100Cu0.088Zr0.028)7.19 sintered magnets

Abstract

We revisited the microstructure of Sm2Co17-type sintered magnets to understand the underlying mechanism of substantial decrease of coercivity from 2.56 T for the optimally annealed sample to 0.14 T for the sample re-annealed at aging temperature of 850 °C for just 1 min and quenched (quenched sample). This low coercivity can be recovered by annealing of the magnet at 850 °C, slow cooling to 400 °C followed by quenching (slowly cooled sample) and with this is fully reversible between the two states. Magnetic domain observations of thermally demagnetized samples with c-axis out-of-plane showed an ultra-fine maze-like domain pattern comparable to the cell size in the slowly cooled sample. However, annealing at 850 °C, even for a short time, followed by quenching resulted in a substantial increase in the size of maze-like domain pattern. Microchemistry of the SmCo5 cell boundary phase for two samples was analyzed using 3D atom probe. 8.6 at. % Cu and 7.7 at. % Fe was found in the cell boundary of the quenched sample while the SmCo5 cell boundary phase of the slowly cooled sample contains 15.4 at. % Cu and 3.0 at. % Fe. Unlike the slowly cooled sample, a broader distribution of Cu than Sm was found in the cell boundary of the quenched sample. High resolution STEM-HAADF results showed SmCo5/Sm2Co17 interface is sharp and defect free for the slowly cooled sample while it is rough and zigzag shaped for the quenched sample. Diffused Cu into the matrix phase from the cell boundary results in a gradual increase of magnetocrystalline anisotropy form matrix to the cell boundary phase. Large Fe concentration in the cell boundary phase reduces its magnetocrystalline anisotropy. Micromagnetic simulations showed that these microstructural features of the quenched sample cause a weak pinning strength of the cell boundary phase explaining its low coercivity.