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The structure, magnetic properties, and magnetostriction of arc-melted polycrystalline Sm0.88Dy0.12(Fe1-xMx)(2) (M = Mn, Al, 0 less than or equal to x less than or equal to 0.2) alloys were investigated by microscopy, x-ray diffraction, A. C. initial susceptibility, vibrating sample magnetometry, and standard strain gauge techniques. It was found that the matrix consists almost entirely of the MgCu2-type cubic Laves phase, with a small amount of rare-earth rich phase. The lattice parameter increases approximately linearly with increasing x from 0.740 to 0.750 nm for M = Al. For M = Mn the lattice parameter increases by 0.002 8 nm over the same increase in x. The Curie temperature decreases with increasing x from 670 to 410 and 550 K for M = Al and Mn, respectively. The spontaneous magnetostriction, lambda(111), and polycrystalline magnetostriction, lambda(s), both decrease with increasing Al or Mn substitution for Fe from their initial values of -1452 and -1010 ppm, respectively. Premature disappearance of the splitting of the (440) line and changes in the shape of the magnetostriction curves suggest the substitution of Mn for Fe may produce a change in the anisotropy. No such effects were observed for M = Al. The saturation magnetization, M-S, decreases approximately linearly with increasing substitution of Al or Mn. The changes in the magnetization and magnetostriction can be understood on the basis of a decreasing 3d sublattice moment. (C) 1999 American Institute of Physics. [S0021-8979(99)02409-3].