Development of Magnetic Nanopartides with High Surface Anisotropy for High Density Recording

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We have studied magnetic surface anisotropy (MSA) at 3d transition metals (TM)/Pd, Pt, SiO_2 interfaces, and investigated the surface effect on the effective magnetic anisotropy of nanopartocles.1. MSA at TM/Pd, Pt, interfaces The MSA strongly depends on the valence electron number of TM and reaches a maximum of 〜0.5 erg/cm^2 for TM = Co, which is consistent with the first principles calculation so far reportd. R is also found that addition of very small amount of rare earth elements (RE) into TM omsiderahly enhanoes the MSA due to the enhancement of orbital moment of Co.2. MSA at TM/nonmagnetic oxide (SiO_2) The MSA at TM/SiO_2 strongly depends on the valence electron number of TM. With increasing the valence electron number from TM = Fe, Co to Ni the easy axis changes from the film normal to the film plane. The maximum perpendicular anisotropy reaches 0.4〜0.8 erg/cm2 for TM = Fe, Co, which is one order of magnitude higher than the magnetocrystalline anisotropy of bulk bcp Co. The dep … More endence of the MSA on the valence electron number of TM has been successfully explained by considering the electronic states of TM combined with SiO_2 at the interface.3. MSA in TM/ nonmagnetic oxide granular films It is found that Fe/SiO_2 granular films (Fe particle size 5〜30 nm) exhibit very high coercibity of Hc〜500 Oe which is much higher than the anisotropy field (〜100 Oe) of bulk bccFe. By considering the measured MSA of 0.4 erg/cm^2 for Fe/SiO_2 we have quantitatively explained their large Hc and the particle size dependence. This result indicates that MSA effectively enhances the magnetic anisotropy of nanoparticles embedded in granular films.4. MSA in TM nanoparticles We have prepared isolated TM (Fe, Co, Ni) nanopartides with the diameter of D = 5〜30 nm and determined their magnetic anisotropy by the newly developed anisotropy measurement method. It is found that all the nanoparticles exhibit strong uniaxial magnetic anisotropy compared with that of bulk materials. Such anisotropy enhanoement is mainly due to the surface effect. From the micromagnetic calculations and experiments, we have confirmed that the magnetic behaviors of the nanopartivles perfectly follows the coherent rotation (Stoner Wohlfarth) model when the particle diameter Dis less than 15 nm.All the results mentioned above indicate that magnetic surface anisotropy significantly enhanoes the effective magnetic anisotropy of nanopartocles even when the bulk anisotropy is quite small. Such nanoparticles with large surface anisotropy would be very promising for development of ultra high density recording media. Less