Effect of Sm Volatilization on Magnetic Microstructures of Sintered Sm(Co,Fe,Cu,Zr)_z Magnets at High Temperatures

We present a magnetic force microscopy study of surface magnetic microstructure changes at high temperatures in 2:17-type Sm(Co,Fe,Cu,Zr)_z (z～7.4) magnets. Surface magnetic microstructures are found to change greatly in parallel and vertical specimens after heat-treatment at 400°C for one hour in vacuum of 10^-5Torr with Ar gas as protecting atmosphere. Changes of microstructures are attributed to the formation of a soft-magnetic surfaces layer in the specimens, resulting from Sm volatilization due to high temperature. This hypothesis is further confirmed by the heat-treatment experiments at 400°C for 0.5h and 2h. Finally, the existence of the soft-magnetic layers, which consist primarily of Fe--Co compounds, is verified by the results of both XRD and XPS of the vertical specimens before and after heat-treatment.