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

由于超顺磁性的出现，如果铁磁材料的尺寸减小到几个纳米，铁磁材料就会失去磁功能。如果能合成克服超顺磁性的纳米级高磁能铁磁性粒子，就可以实现对磁畴的高分辨率观测，这一直是我们的初衷。本文研究了通过合金化和产生高表面能来开发高能粒子。结果表明：Co颗粒是在稀有气体中冷凝生成的，具有稳定的面心立方相，其尺寸小于10 nm，而hcp相在这些尺寸下是半稳定的。控制这一过程的机理来自于颗粒表面的自由能，颗粒具有不同的晶型。为了稳定颗粒中的hcp相，以相同的方式生成了Co-Pt颗粒。证实了HCP相粒子的高度凝聚。由直接测量…得到的各向异性能量与大块材料相比，这些颗粒的ENT值高得多。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。产生了稀有的地球过渡金属颗粒，并研究了其磁性。然而，由于氧化，这些颗粒在化学上并不稳定。包覆颗粒可显著降低不稳定速率，但不能获得较高的矫顽力。造成这种特殊现象的可能原因是氧气侵入颗粒体，破坏了表面的光滑度，这是高表面能不可避免的。由于上述困难，这一阶段的研究结束了。合成了由过渡金属和氧化物组成的多层膜和颗粒膜，并发展了一种评价真实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，总结这些结果，在Co-Pt系统中得到了尺寸为10 nm的高能粒子，并证实了高的表面能。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较高表面能的粒子。较少

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