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

由于超顺磁性的出现，当铁磁材料的尺寸减小到几纳米时，其磁性功能就丧失了。研制出纳米级高磁能铁磁粒子，克服超顺磁性，可以实现对磁畴的高分辨率观测。本文研究了利用合金化和产生高表面能的方法制备高能粒子。结果如下：用稳定的fcc相在稀有气体中缩合得到的Co颗粒尺寸小于10nm，而hcp相在这些尺寸下是半稳定的。控制这一过程的机制是由各种晶体形状的粒子的表面自由能决定的。为了稳定颗粒中的hcp相，用同样的方法制备了Co-Pt颗粒。证实了hcp相粒子的高缩聚。通过直接测量获得的各向异性能量与块状材料相比，这些粒子的能量异常高。这强烈表明，与颗粒表面相关的表面各向异性起着主导作用。制备了稀土过渡金属颗粒，并对其磁性进行了研究。然而，由于氧化，颗粒的化学性质不稳定。涂覆可明显降低颗粒的不稳定性，但不能获得高矫顽力。产生这种特殊现象的可能原因是氧气侵入粒子体，破坏了表面的平整度，这是高表面能不可避免的。由于上述困难，调查在此阶段终止。合成了由过渡金属和氧化物组成的多层和粒状薄膜，并提出了一种评价表面各向异性的方法。其中一个有价值的结果是，Co/MgO多层界面具有非常高的表面各向异性。综上所述，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，其尺寸降至10nm，并具有较高的表面能。这为我们提供了一个有价值的建议，即可以通过粒子表面的化学重构来合成具有更高表面能的粒子。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