核-壳型磁性纳米颗粒的可控合成与超大交换偏置效应

As always, the exchange bias (EB) effect and the novel physical properties of the core-shell magnetic nanoparticles (NPs) have been pursued intensively due to their great application potential in fields such as spin valve and tunnel junction. However, few works were reported on the formation mechanism of exchange bias and freak magnetic properties exhibited in the core-shell particles, according to the results of microscopic magnetic measurement and theoretical calculation/simulation. Meanwhile, the observed exchange bias effect in the present core-shell particle systems only occurs under room temperature, this will limit the practical applications of the EB-based device products. In this project, we synthesized the microstructure-controlled core-shell magnetic NPs through two-step chemical method. Based on the fact that the antiferromagnetic CoO and NiO phases has high Néel point (≥ room temperature), we focus on the room-temperature EB effect of CoO/Fe3O4 and γ-Fe2O3/NiO core-shell NPs. For the macroscopic magnetic analysis of core-shell NPs, we have performed exhaustive research on the correlation between magnetic parameters (such as exchange bias, coercivity, magnetization, Curie temperature, Néel temperature and blocking temperature) and interface microstructure. Further, the microscopic magnetic properties were investigated systemically with electron spin resonance, Mössbauer spectroscopy, nuclear magnetic resonance and neutron diffraction. Based on the results of first princinple calculation, Monte Carlo simulation and micromagnetism simulation/calculation, the micromechanism of exchange bias and the origin of anomalous magnetic properties were analyzed in depth. Finally, the objective of our study is expected to achieve the precise control of exchange bias, which provides powerful experimental support and theoretical basis for the practical use of EB effect.

由于在自旋阀、隧道结等方面有着巨大的应用潜能，核-壳磁性纳米颗粒中的交换偏置及其相关物性的研究备受关注。然而还是很少有工作结合微观磁性测试和理论计算模拟来研究交换偏置与奇异磁性的产生机理，并且实验上观察到的交换偏置效应均在室温以下，这严重制约了它的实际应用。本项目拟采用两步化学法可控合成出不同类型的核-壳磁性纳米颗粒，重点探究反铁磁相奈尔温度接近和高于室温的CoO/Fe3O4和γ-Fe2O3/NiO核-壳颗粒的室温交换偏置效应。在宏观磁性分析中着重研究交换偏置、矫顽力、磁化强度、居里温度、奈尔温度和截止温度等参数与界面微结构的关系。利用电子自旋共振、Mössbauer谱、核磁共振和中子衍射系统研究样品的微观磁性，通过第一性原理计算、Monte Carlo模拟及微磁学模拟与计算深入分析交换偏置的微观机制及异常磁性的来源，达到精确调控交换偏置的目的，为其实际应用提供强有力的实验支撑和理论依据。

在磁学领域，交换偏置(EB)效应是众所周知的现象，起源于铁磁性和反铁磁性成分在界面处的交换耦合相互作用。但是具体到实际纳米颗粒体系的研究中，其效应的微观机制却众说纷纭，且在临近室温时较少观察到理想的交换偏置。本项目以花状γ-Fe2O3/NiO核-壳纳米结构、Ni/NiO复合纳米颗粒、CoO/Fe3O4核-壳纳米颗粒和γ-Fe2O3/MnO二元纳米颗粒四种EB基体系为研究对象，通过简单、可重复的化学方法，凭借对实验条件的精确调控，合成出了不同微结构的磁性复合纳米材料，然后用全方位的磁性表征手段分析了样品的宏观、微观磁性及相关物性。重点关注：①室温、低温下的异常磁特性；②磁化强度、矫顽力和交换偏置随微结构参数、温度与冷却场的变化关系；③奈尔温度和阻截温度随外加磁场及微结构参数的变化规律；④实验上的微观磁性与理论计算结果的比较。.研究表明，前体物浓度、溶剂热/煅烧温度、保温时间和升温速率大大影响了γ-Fe2O3/NiO和Ni/NiO样品的微结构，如核-壳尺寸、组分含量比、结晶质量、界面特征(如接触方式、有效面积、粗糙度、致密性等)，而这又决定最终产物磁性能的优异，结果它们在5 K下都展现出一定的交换偏置(60 Oe/482 Oe)和增强的矫顽力(213 Oe/1335 Oe)。在CoO/Fe3O4和γ-Fe2O3/MnO体系中均观察到超大的交换偏置效应(8000 Oe和3500 Oe)，且前者的EB效应表现出良好的热稳定性，交换偏置的截止温度提升到290 K，接近于室温；后者ESR研究证实了磁性的微观物理机制。.本项目所用合成方法对其他核-壳纳米材料具有普适性；这种“实验上宏、微观磁性系统表征与理论上建模/计算相结合”的研究方法可推广到许多科研工作中；具有超大交换偏置效应的CoO/Fe3O4材料有望降低磁记录介质的超顺磁极限，提高永磁材料的磁能积，应用前景明朗。