Nanocrystalline Sm_12.5(Co,Zr)_87.5 magnets: Synthesis and magnetic properties

Isotropic nanostructured Sm_12.5Co_87.5-xZr_x (0≤x≤5) magnets with the TbCu₇ structure have been synthesized by mechanical milling and subsequent annealing. The remanence is found much enhanced in the nanostructured Sm(Co,Zr)₇ magnets with a grain size of 10-20 nm. A suitable amount of Zr addition (x = 1, 2) is helpful to improve hard magnetic properties, including high remanence ratio of 0.71, coercivities up to 21 kOe, and energy products up to 13.0 MGOe, and hard magnetic properties are sensitive to the milling time and the annealing temperature. The dependence of hard magnetic properties on milling time may be correlated to the refinement of grain size and the intergranular structure induced during the milling and the subsequent annealing. The enhancement of remanence due to a small amount of Zr in this series of Sm_12.5Co_87.5-xZr_x powders results from the enhanced exchange coupling between the nanosized grains. The hard magnetic properties are discussed with correlation to the milling process and in terms of the nanostructure developed in these materials.