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Dynamics of magnetic vortex core switching in nanometer-scale permalloy disk, having a single vortex ground state, was investigated by micromagnetic modeling. When an in-plane magnetic field pulse with an appropriate strength and duration is applied to the vortex structure, additional two vortices, i.e., a circular-and an anti-vortex, are created near the original vortex core. Sequentially, the vortex-antivortex pair annihilates. A spin wave is created at the annihilation point and propagated through the entire element; the relaxed state for the system is the single vortex state with a switched vortex core.

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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.

The chemical coprecipitation process was used to synthesize about 7 nm, spherical magnetite nanoparticles to study magnetic properties and the aging effect. As-produced spherical magnetite nanoparticles have been aged in the atmosphere for 19 months. Magnetic properties and aging effect were studied by Mössbauer spectroscopy at a temperature ranging from 77 to 300 K, vibrating sample magnetometer, and x-ray diffraction. Saturation magnetization and coercivity were found to be 49 emu/g and nearly 0 Oe at room temperature, respectively. A singlet Mössbauer spectrum was observed at room temperature, implying superparamagnetic behavior of the particles, while a two-sextet spectrum was observed at 77 K. The particle size in this study is about 7 nm, which is smaller than the superparamagnetic size of 26 nm as calculated from Neel’s theory of single domain particles. After having aged these particles for 19 months, all magnetic properties and their original shapes were retained. Superparmagnetic magnetite nanoparticles synthesized in this study can be applied to microbead applications of a biosensor.