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Theoretical saturation magnetization and magnetocrystalline anisotropy energy ͑MAE͒ of -phase ͑face-centered tetragonal͒ Mn 50 Al 50 alloy were obtained by first principles calculations, and the alloy was fabricated to compare the experimental values with the theoretical predictions. The calculated magnetic moment and MAE for -phase Mn 50 Al 50 were 2.37 B / f.u. and 0.259 meV/f.u. ͑1.525ϫ 10 6 J / m 3 ͒, respectively, which result in the maximum energy product ͑BH͒ max of 12.64 MG Oe and the magnetocrystalline anisotropy field of 38 kOe. The saturation magnetization for -phase Mn 54 Al 46 alloy was measured to be 98.3 emu/g, which gives 4.7 MG Oe of ͑BH͒ max . The magnetization is about 70% of the theoretical value of 144 emu/g.

We report unconventional exchange bias in oxide-coated manganese nanoparticles, in which the Curie temperature of Mn3O4 shell is lower than the Néel temperature of the antiferromagnetic core. The coercivity (873kA∕m) of the nanoparticles, which is more than four times greater than that of bulk Mn3O4, has been enhanced significantly. A considerable enhancement in Curie temperature compared to the bulk was also observed for Mn3O4 in nanoscale. An exchange bias field as large as 400kA∕m was observed due to the strong interfacial exchange coupling. A simple phenomenological model is given to understand these phenomena in this ferri/antiferromagnetic system.

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