Development of Spin-Tunneling Junctions with Large Magnetoresistance Using Half-Metallic Oxide Ferromagnets

为了利用半金属性质的过渡金属氧化物在隧道结中实现大的磁阻效应，研究了铁磁氧化物的表面磁性对隧道磁阻的影响，并对结的最佳制作条件进行了研究。研究结果总结如下：1。采用脉冲激光沉积（PLD）技术，在低温下制备了高质量的Fe_3O_4和MgO薄膜。为了防止Fe_3O_4表面氧化，在结的制作过程中，即使在1 × 10^<-5> Torr的氧压下，也需要在低于200℃的温度下生长MgO绝缘层。Sr_2FeMoO_6具有半金属性质，是一种很有前途的铁磁电极候选材料，在低于报道的生长温度500℃下成功生长。确定了MgO绝缘层生长pld的最佳条件。对最佳条件下制备的Fe_3O_4/MgO/Au结结构的电流电压特性分析表明，MgO绝缘层薄至1 nm的结具有良好的隧道穿通质量。在Co_<0.4>Fe_<2.6>O_4/Fe_3O_4/MgO/Fe_3O_4结中，由于Fe_3O_4电极反平行排列而产生了隧道磁电阻（TMR），但TMR变化低至- 0.5%。TMR的磁场依赖性表明，低TMR可能与Fe_3O_4电极层表面磁化的超顺磁行为有关。制备了Fe_3O_4/NiO/MgO/NiO/Fe_3O_4非磁性MgO -磁性NiO双绝缘层结结构。然而，由于结面积大和NiO层薄至1nm造成的一些问题，未能获得预期的结性能。为了克服这些问题，需要采用光刻等先进工艺来制造精细结构。

Optimum fabrication conditions of the junctions and influence of surface magnetic of the ferromagnetic oxides on tunneling magnetoresistance have been investigated in order to achieve large magnetoresistance effect in tunneling junctions using transition metal oxides with half metallic nature. The results are summarized as follows :1.Thin films of Fe_3O_4 and MgO with high quality were grown by pulsed laser deposition (PLD) at low temperature. The insulating MgO layer was required to be grown at lower temperature than 200℃ even at oxigen pressure of 1 x 10^<-5> Torr in order to prevent the Fe_3O_4 surface from oxidizing in a fabrication process of junction. Thin films of Sr_2FeMoO_6 with half metallic nature which is a promising candidate of ferromagnetic electrode were successfully grown at 500℃, lower than the reported growth temperature.2.Optimum PLD-growth conditions for MgO insulating layers were determined. Analysis of current-voltage characteristics of Fe_3O_4/MgO/Au junction structures fabricated under the optimum conditions showed that the junctions with MgO insulating layer as thin as 1 nm have good tunneling quality.3.Tunneling magnetoresistane (TMR) due to the magnetization of the Fe_3O_4 electrodes aligned in anti-parallel was observed in a Co_<0.4>Fe_<2.6>O_4/Fe_3O_4/MgO/Fe_3O_4 junction, though the TMR change was as low as -0.5 %. The magnetic field dependence of TMR showed that the low TMR may be associated with the superparamagnetic behavior of the surface magnetization of Fe_3O_4 electrode layers.4.The Fe_3O_4/NiO/MgO/NiO/Fe_3O_4 junction structures with double insulating layers of non-magnetic MgO - magnetic NiO were fabricated. However, the expected junction properties were not obtained due to some issues caused by large junction area and NiO layers as thin as 1 nm. The advanced process fabricating the fine structure such as photolithography will be required to overcome these problems.