无偏置磁场、非挥发性磁化强度翻转的电场调控及其磁电耦合机理研究

Electric-field-controlled magnetization switching in the multiferroic materials is of significant importance for understanding magnetoelectric coupling mechanism from the viewpoint of fundamental science and also for developing novel storage techniques in the area of information technology. However, in the conventional magnetic/ferroelectric layered heterostructures, magnetoelectric coupling is often volatile and weak even under applied magnetic field at room temperature. This difficulty hinders the magnetoelectric heterostructures to realize the practical applications. To solve this problem, this proposal will use the inserting antiferromagnetic layer to the multiferroic heterostructures for introducing exchange bias field and utilize the periodic ferromagnetic/antiferromagnetic multilayers to accumulate this effect through the whole magnetic layers. Consequently, the enhanced magnetoelectric coupling without help of the external magnetic field bias will be expected. In addition, the magnetization 180o switching without volatility will be assisted partially by electric-field-induced domain switching and the resultant non-volatile strain in the relaxor ferroelectrics PMN-PT substrate. In detail, we perform normal magnetic measurements and micromagnetics simulations to artificially design and optimize the balance among unidirectional anisotropy energy (exchange bias), uniaxial anisotropy energy and electric-field-induced magnetoelastic anisotropy energy in the (FM/AFM)n/FE hybrid structures by choosing a desirable periodic of magnetic layers and proper strain states, and also study how the magnetization switching and non-volatile strain correlates to the ferroelectric polarization switching. We will know well the evolutions of the exchange bias effect, magnetic anisotropy and magnetization reversal with the stimuli of electric field. Synchrotron radiation advanced characterization tools (hard X-ray diffraction and soft X-ray absorption, XMCD and XLD) will be used to investigate the microstructural origination and underlying electronic mechanism of the electric-field control of magnetization 180o switching without magnetic field bias in this kind of heterostructure.

复合多铁性材料的电场调控磁性研究对于理解凝聚态物质电磁耦合规律和机理以及开发新型信息存储器件有重要意义。常规层状磁性/铁电多铁性结构的室温电场调控磁性效应大都较弱且是易失性的，并要外加偏置磁场。为解决该难题，本项目采用反铁磁插层技术、并用铁磁/反铁磁周期性多层膜结构的累积交换偏置场来增强无偏置磁场下的逆磁电耦合效应，结合电场调控弛豫铁电体电畴状态而诱导的非挥发性应变，实现磁化强度非易失性的180度翻转。具体利用测磁手段和微磁模拟研究该异质结单向磁各向异性(交换偏置)、单轴磁各向异性、磁弹各向异性的竞争关系及其随铁磁/反铁磁多层膜的周期和厚度、电场诱导应变的变化关系，考察磁化强度翻转、非挥发性应变与铁电极化翻转的关联，揭示交换偏置场、磁各向异性和磁化强度翻转的电场调控规律，进一步利用同步辐射衍射和吸收谱技术阐明无偏置磁场、非挥发性电场调控磁化强度翻转的微结构起源和电子结构机制。

IT技术突飞猛进的发展离不开信息存储技术的进步，同时实现高密度、低功耗、非挥发性以及高速存储是人们梦寐以求的目标。为此，科学家提出了使用电场来调控磁性状态，从而实现磁性存储单元的信息写入与擦除。电场调控磁性的优势在于：局域化的电场可大幅降低读写功耗、提高存储密度和信息存取速度。因此，电场调控磁性研究不仅仅对理解复合多铁性材料磁电耦合规律和机理有重要意义，也为设计和开发高密度、低功耗、非挥发性以及高速存储器件提供新途径。本项目针对常规层状磁性/铁电多铁性结构的室温电场调控磁性效应弱、易失性、需要外加偏置磁场等缺点，提出了利用铁磁/反铁磁多层膜结构诱导交换偏置场的途径，增强了无偏置磁场下的磁电耦合效应；结合电场调控弛豫铁电体电畴状态而诱导的非挥发性应变，实现磁化强度非易失性的180度翻转。本项目经过三年的努力取得了一系列研究成果，主要有：一、利用磁退火在CoFeB/PMN-PT多铁性异质结中诱导出单轴各向异性，在无偏置磁场下，发现了电场调控CoFeB磁化强度翻转的效应明显增强，还发现了非挥发性的磁化强度翻转；二、研究了以CoFeB/MgO/(CoFeB(t1)/IrMn(t2))n/PMN-PT为代表的多铁性异质结中的电场调控磁性，揭示了磁退火诱导的单轴各向异性能、电场诱导的磁弹各向异性能和Zeeman能之间的竞争规律，找到了利用PMN-PT铁弹翻转实现CoFeB非挥发性磁化强度180o旋转的途径；进一步，在上述研究工作的基础上，我们拓展研究了FePt/PMN-PT多铁性异质结奇异霍尔效应的电场调控行为，获得了非挥发性的高低霍尔电阻态，同时，在VO2/PMN-PT多铁性异质结中发现了电场动态调控的金属-绝缘体转变行为。本项目设计和制备了高质量的多层铁磁薄膜结构、关联电子体系薄膜构成的多铁性异质结，实现了多铁性异质结磁化强度的180度翻转、非挥发性的霍尔电阻态和动态的金属-绝缘体转变电场调控磁性，为开发新型磁电存储器件提供了新思路。