High temperature magnetic properties of mechanically-alloyed SmZr(CoFe) nanophase hard magnets

The usual approach to development of coercivity in bulk Sm-Co based materials for high temperature applications is to prepare a nanophase cellular microstructure with cell walls which act as pinning sites (precipitation hardening). An alternative approach is to mechanically alloy the materials, introducing planar crystalline defects that act as pinning sites. In this work, a systematic investigation of magnetic properties in the temperature range 20degreesC less than or equal to T less than or equal to 500degreesC is carried out on SmxZry(Co0.9Fe0.1)(100-x-y) nanocrystalline powders, where 13 less than or equal to x less than or equal to 15, and 0 less than or equal to y less than or equal to 5. The samples were prepared by mechanical alloying followed by an optimized annealing procedure. X-ray diffraction patterns showed that the as-milled powders were amorphous, Annealed samples showed a mixture Of Sm2Co17 and SmCo5 nanocrystalline phases, dependent on Zr content. A maximum room temperature coercivity of 1.74 T is obtained in the composition Sm14Zr3(Co0.9Fe0.1)(83). Enhanced remanence (sigma(r)/sigma(r) greater than or equal to 0.65) is achieved in all compositions, indicating intergrain exchange coupling among the fine grains. High temperature magnetic measurements show that the proper-ties degrade irreversibly at temperatures around 400degreesC. Loops taken at low temperature after measurement change from that of a single magnetic phase to that of a two phase mixture. This behavior is attributed to the growth of exchange-coupled nanocrystallites. Possible methods to inhibit crystallite growth are discussed.