“单倒置”型反铁磁/亚铁磁核壳纳米结构对交换偏置效应调控的研究

Exchange bias (EB) effect in magnetic nanoparticles systems has its potentially technological application in the spintronic devices, and its mechanism has been an attractive issue among topics of interest. The exchange interaction between both constituents in core-shell magnetic nanoparticles brings about an extra degree of freedom to tailor its overall properties. However, less attention has been paid to “single inverted” antiferromagnetic/ferrimagnetic (AFM/FiM) core shell nanostructures. This project will take AFM CoO or NiO with higher Neel temperature as core, and FiM CoFe2O4, NiFe2O4 or Ni0.5Co0.5Fe2O4 with higher Curie temperature as shell to construct “singly inverted” AFM/FiM core shell nanoparticles, nanowires and nanosheets, and then investigate the influence of dimension of nanostructures on the EB effect. Furthermore, the effects of the ratio of core to shell, crystal size, and crystallinity on the EB effect will also be revealed. The core and shell size dependence of the magnetic properties of the “single inverted” AFM/FiM core/shell nanoparticles is studied using Monte Carlo (MC) simulations. The objective of this study is to increase the thermal stability of EB effect and find some structures presenting EB phenomenon at room temperature, and then disclose its physical origin, and provide some instructions for its practical applications. This topic has the distinctive combining features on experimental research, numerical simulation and theoretical analysis. This project not only enriches the category of EB effect, but also promotes the physical, chemical and materials multidisciplinary fusion, which has important academic significances.

交换偏置效应不仅在自旋电子器件上具有潜在的应有价值，其内在的物理机制也是研究人员感兴趣的热点。不同磁相之间的界面耦合作用为核壳纳米颗粒磁性的调控提供了一个额外的自由度。本项目提出以具有较高相变温度的CoO或NiO为核、以CoFe2O4、NiFe2O4或Ni0.5Co0.5Fe2O4为壳来构建“单倒置”型反铁磁/亚铁磁核壳纳米颗粒、纳米线和纳米片；探索材料维度对核壳纳米结构中交换偏置效应的调控作用；探究核与壳的厚度比、颗粒尺寸以及结晶性等因素对核壳纳米结构中交换偏置效应的影响；采用蒙特卡罗模拟进行“单倒置”型核壳纳米颗粒中交换偏置效应的研究。本项目以提高交换偏置效应的热稳定性、获得室温交换偏置效应为研究目标，并且揭示其物理起源，为实际应用提供实验及理论指导。此外，本课题具有实验研究、数值模拟和理论分析相结合的特色。它不仅丰富了交换偏置效应的体系，也促进了物理、化学和材料等多学科的交叉融合。

在国家自然科学基金的资助下，本项目基于交换偏置效应产生的机理，探索室温或接近室温的交换偏置效应。选择居里温度和奈尔温度均接近或者高于室温的铁磁和反铁磁材料来构筑核壳磁性复合纳米结构，例如CoFe2O4和NiFe2O4的居里温度分别为790 K和850 K；CoO和NiO的奈尔温度分别为291 K和523 K。课题组按照研究计划，成功制备了研究所需的NiO/NiFe2O4 、Co3O4/CoFe2O4 以及CoO/CoFe2O4纳米复合物系统，研究了退火温度对所制备样品磁性能的影响。实验结果表明，构建的反铁磁/铁磁复合系统均具有交换偏置效应；自旋动力学研究表明复合物系统存在自旋玻璃行为；磁锻炼效应的研究指出交换偏置效应起源于界面处铁磁与反铁磁的耦合作用。研究也进一步指出具有交换偏置效应的体系往往表现出自旋玻璃行为。然而，所制备材料交换偏置效应的截止温度却低于室温，这表明界面耦合作用的热稳定性还需要进一步提高。总体来说，基本达到了该项目的既定目标。此外，在项目执行过程中，也开展了一些原计划没有列入的工作，主要包括稀土铁酸盐LaFeO3微米多面体的制备及室温自发交换偏置效应的研究，自发交换偏置效应由于去除了外加磁场冷却或外加磁场制备过程的限制，使其在实际器件中的应用会更加广泛和便捷；MOF衍生Co/Co3O4/CNTs和La2O3/Co@CNTs复合材料的制备及微波吸收性能研究。