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

我们研究了3d过渡金属（TM）/Pd，Pt，SiO_2界面的磁表面各向异性（MSA），研究了表面效应对纳米粒子有效磁各向异性的影响.在TM/Pd，Pt界面上的MSA强烈地依赖于TM的价电子数，当TM = Co时，MSA达到最大值~ 0.5 erg/cm ^2，这与第一性原理计算结果一致。研究还发现，在TM中加入极少量的稀土元素（RE），由于Co的轨道矩增加，使MSA显著增加。TM/SiO_2体系的MSA与TM的价电子数密切相关。从TM = Fe、Co到Ni，随着价电子数的增加，易磁化轴从垂直于膜面的方向发生变化。当TM = Fe，Co时，最大垂直各向异性可达0.4 ~ 0.8erg/cm ~ 2，比体相bcp Co的磁晶各向异性高一个数量级 ...更多信息 通过考虑TM与SiO_2界面结合的电子态，成功地解释了MSA对TM价电子数的影响. TM/SiO_2氧化物颗粒膜中的MSA发现Fe/SiO_2颗粒膜（Fe颗粒尺寸为5 ~ 30 nm）具有很高的各向异性场（Hc ~ 500 Oe），远高于体相bccFe的各向异性场（Hc ~ 100 Oe）。考虑到Fe/SiO_2的测量MSA为0.4 erg/cm ^2，我们定量地解释了它们的大Hc和粒度依赖性。这一结果表明MSA有效地增强了嵌入颗粒膜中的纳米颗粒的磁各向异性.我们制备了直径为5 ~ 30 nm的TM（Fe，Co，Ni）纳米颗粒，并利用新发展的磁各向异性测量方法测定了它们的磁各向异性。结果表明，与块体材料相比，所有纳米颗粒都表现出较强的单轴磁各向异性。这种各向异性增强主要是由于表面效应。微磁学计算和实验结果表明，当粒径D小于15 nm时，纳米颗粒的磁行为完全符合Stoner Wohlfarth模型，表明即使在体相各向异性很小的情况下，磁性表面各向异性也能显著增强纳米颗粒的有效磁各向异性.这种具有大的表面各向异性的纳米颗粒将非常有希望用于超高密度记录介质的开发。少

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