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

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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.