Synthesis of metal alloy particles with high magnetic surface anisotropy and development of new functions

铁磁材料由于超顺磁性的出现，当其尺寸减小到几个纳米时，就会失去磁性功能。高分辨率的磁畴观察一直是我们的原始目的，如果合成纳米尺寸的高磁能铁磁颗粒，克服超顺磁性，可以实现。本研究主要研究了通过合金化制备高能粒子并产生高表面能的方法。结果表明：在稀有气体中冷凝生成的Co粒子具有稳定的fcc相，其尺寸小于10 nm，而hcp相在该尺寸下是半稳定的。控制这一过程的机制是来自于颗粒的表面自由能，颗粒具有多种晶体形状，为了稳定颗粒中的hcp相，Co-Pt颗粒也以同样的方式产生。证实了hcp相颗粒的高度凝聚。用直接测量法得到的各向异性能 关于我们 这些颗粒的体积比散装材料的体积高得多。这强烈地表明与颗粒表面相关的表面各向异性起主导作用。制备了稀土-过渡金属颗粒并研究了其磁性。然而，由于氧化，颗粒不是化学稳定的。不稳定的速率可以通过包覆颗粒而明显降低，但是没有获得高的结晶度。这种特殊现象的可能原因是氧侵入颗粒体中，破坏了高表面能不可避免的表面光滑度。由于上述困难，研究在此阶段终止。合成了由过渡金属和氧化物组成的多层膜和颗粒膜，并开发了评估真正表面各向异性的方法。其中一个有价值的结果是Co/MgO多层膜的界面具有很高的表面各向异性，综合这些结果，在Co-Pt体系中获得了从一开始就作为研究目的的高能粒子，其尺寸小到10 nm，证实了其具有高的表面能。这为我们提供了一个有价值的建议，即具有较高表面能的颗粒可以通过颗粒表面的化学重构来合成。少

Ferromagnetic materials lose their magnetic functions if their sizes decrease down to several nanometers because of emerging of the superparamagnetism. High resolution observation of magnetic domains which has been our original purpose can be achieved if synthesis of nm size ferromagnetic particles with high magnetic energy which overcomes superparamagnetism is developed. In this research developing of high energy particles by alloying and producing a high surface energy was studied. The following are the results.Co particles were produced by condensation in rare gas with stable fcc phase with the size smaller than 10nm, but hcp phase is semi-stable for these sizes. The mechanism governing this process is from the surface free energy of the particles which takes a variety of crystal shapes.To stabilize hcp phase in the particles Co-Pt particles were produced in the same way. Higher condensation of hcp phase particles were confirmed. The anisotropy energy obtained by the direct measurem … More ent of these particles is extra-ordinarily high in comparison with that of the bulk material. This strongly suggests that a surface anisotropy associated with the particle surface is taking a dominant role.Rare earth-transition metal particles were produced and the magnetism was studied. However, the particles were not chemically stable due to oxidation. The rate of instability could be reduced appreciably by overcoating the particles, but high coercivity was not attained. The possible reason for this peculiar phenomena is invasion of oxygen into the particles bodies destroying the surface smoothness which is inevitable for high surface energy. The investigation was terminated at this stage because of the difficulty mentioned above.Multilayers and granular films consisting of transition metal and oxides were synthesized and a method to evaluate the genuine surface anisotropy was developed. One of the valuable results is that the interface of Co/MgO multilayer has a very high surface anisotropy.Summarizing these results, the high energy particles which have been a purpose of the research from the start were obtained in Co-Pt system with the size down to 10nm and a high surface energy was confirmed. This provides us with a valuable suggestion that particles with higher surface energy can be synthesized by chemical reconstruction of the particle surface. Less