Spin reorientation effects in DyFe 12-x Nb x compounds with x=0.6, 0.7 and 0.8 have been investigated by ac magnetic susceptibility and 57 Fe Mössbauer effect measurements over the temperature range 4.2-300 K. Refinements of the X-ray diffraction patterns show that DyFe 12-x Nb x compounds crystallize in the ThMn 12 -type structure and that the Nb atoms occupy the 8i sites. A spin reorientation from easy axis to easy cone at T sr1 , then to easy plane at T sr2 with decreasing temperature has been detected by ac susceptibility. Both T sr1 and T sr2 decrease monotonically with increasing Nb content (from T sr1 ~ 236 K for x=0.6 to T sr1 ~ 204 K for x=0.8 and from T sr2 ~ 154 K for x=0.6 to T sr2 ~ 94 K for x=0.8). Analyses of the Mössbauer spectra (4.2-300 K) reveal a discontinuity in the temperature dependences of the hyperfine parameters at T sr2 for the 8i Fe.

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We have systematically studied the flux pinning behavior of MgB 2 bulks synthesized by direct diffusion of Mg into pressed pellets of high purity crystalline B powder, with and without mixing with C and SiC nanoparticles, at a reaction temperature of 850 C for 10 hrs. All of the samples showed very high purity and high density, but their microstructure and flux pinning behavior showed significant differences. It was found that the pure MgB 2 agrees with the T c pinning model, nano-C doped MgB 2 agrees with the pinning model, while the SiC+MgB 2 composite agrees with the pinning model (stress/strain field pinning), since the dominant micro-defects that influence the flux pinning in these three samples are different.Index Terms-Critical current density, flux pinning, MgB 2 , strain field.