Preparation of granular-tunneling magnetoresistive materials by a multi-step ion implantation method

对于粒状隧穿磁阻材料，期望中等尺寸的超顺磁性细颗粒尽可能致密地分散在绝缘体基质中。本研究通过重复一组“小剂量注入”和“适度退火”来证明中等尺寸粒子的选择性生长是可能的，并优化了注入、退火和重复条件。1.对剂量为1.1E17 ions/cm 2的样品在150-600℃的真空中进行等时退火。结果表明，在300℃以下，1h退火，小于3 nm的小颗粒被整合到3-5 nm的中等颗粒中。2.在总剂量为1.1E17ions/cm ~ 2的两步注入样品中，证实了中等尺寸粒子的选择性生长。与一次注入的样品相比，样品的MR比增加了1.4倍。3.在上述样品中以1.5E17离子/cm 2的总剂量进行额外的第三次注入，导致与一次注入的样品中观察到的颗粒尺寸分布相同的颗粒尺寸分布。临界剂量略微增加，在该临界剂量以上形成大颗粒变得占主导地位，并且具有最大MR比的样品的零场电阻急剧降低，但MR比无明显改善。5.不同初始剂量无明显差异。6.从零-最佳退火温度（或时间）取决于在该步骤样品中的累积剂量，因此它是优化过程中最重要的参数。

For granular tunneling magnetoresistive materials, it is desirable that medium-sized superpararnagnetic fine particles are dispersed in an insulator matrix as densely as possible. The present study was done to demonstrate that selective growth of the medium-sized particles is possible by repeating a set of "small dose implantation" and "moderate annealing", and to optimize implantation, annealing and repetition conditions.1.A sample with a dose of 1.1E17 ions/cm2 was annealed isochronally in vacuo at temperatures in a range of 150-600℃. It was found that small particles less than 〜3nm are integrated into medium particles of 3-5 nm by a moderate annealing (below 300℃, 〜1h). As a result, rater-particle spacings are enlarged so that the zero-field resistance of the sample is increased.2.Medium-sized particles are confirmed to be grown selectively in a sample with a total dose of 1.1E17 ions/cm2 prepared by a 2-step implantation. The MR ratio of the sample is increased by a factor of 1.4 as compared with that of a sample implanted at once.3.An additional third implantation to a total dose of 1.5E17 ions/cm2 into the above sample resulted in a particle size distribution identical to that observed in a sample implanted at once.4.When a dose for each step is decreased, the critical dose above which formation of large particles becomes dominant is slightly increased, and the zero-field resistance of the sample with the maximum MR ratio is drastically reduced, though no significant improvement is observed in the MR ratio.5.No clear difference was observed for different initial doses.6.It was suggested from the change of the zero-field resistance that the optimum annealing temperature (or time) depends on the accumulated dose in a sample at that step and therefore it is most important parameter in the optimization process.