[论文解读] Investigation of Room Temperature Ferroelectricity and Ferrimagnetism in Multiferroic AlxFe2-xO3 Epitaxial Thin Films

Badari Narayana Rao, Shintaro Yasui+5 more|arXiv (Cornell University)|Mar 27, 2019

Multiferroics and related materials参考文献 71被引用 5

一句话总结

本研究探讨了外延AlxFe2-xO3（x-AFO）薄膜在室温下的多铁性，通过在SrTiO3<111>衬底上采用脉冲激光沉积法获得低漏电薄膜。尽管第一性原理预测其极化强度可达24 μC/cm²，但实测铁电性低两个数量级，原因在于竞争的面内结构畴；研究发现氧空位可促进畴壁运动，从而有助于极化翻转。

ABSTRACT

Multiferroic materials open up the possibility to design novel functionality in electronic devices, with low energy consumption. However, there are very few materials that show multiferroicity at room temperature, which is essential to be practically useful. AlxFe2-xO3 (x-AFO) thin films, belonging to the k-Al2O3 family are interesting because they show room temperature ferrimagnetism and have a polar crystal structure. However, it is difficult to realise its ferroelectric properties at room temperature, due to low resistivity of the films. In this work, we have deposited x-AFO (0.5 &lt;= x &lt;= 1) epitaxial thin films with low leakage, on SrTiO3&lt;111&gt; substrates by Pulsed Laser Deposition. Magnetic measurements confirmed room temperature ferrimagnetism of the films, however the Curie temperature was found to be influenced by deposition conditions. First principle calculations suggested that ferroelectric domain switching occurs through shearing of in-plane oxygen layers, and predicted a high polarization value of 24 uC/cm2. However, actual ferroelectric measurements showed the polarization to be two order less. Presence of multiple in-plane domains which oppose polarization switching of adjacent domains, was found to be the cause for the small observed polarization. Comparing dielectric relaxation studies and ferroelectric characterization showed that oxygen-vacancy defects assist domain wall motion, which in turn facilitates polarization switching.

研究动机与目标

• 通过可靠地实现低漏电的外延AlxFe2-xO3薄膜，以支持可靠的多铁性表征。
• 探究尽管理论预测具有高极化强度，但实际铁电极化被抑制的根源。
• 理解氧空位缺陷在促进铁电畴壁运动中的作用。
• 将沉积条件与x-AFO薄膜的磁性和铁电性关联起来。
• 阐明多铁氧化物中结构畴与极化翻转之间的相互作用。

提出的方法

• 采用脉冲激光沉积（PLD）在SrTiO3<111>衬底上生长外延x-AFO薄膜（0.5 ≤ x ≤ 1），以最小化缺陷并提高结晶质量。
• 磁性测量证实了室温下的亚铁磁性，居里温度对沉积参数敏感。
• 第一性原理计算通过面内氧层剪切模拟极化翻转，预测高极化强度为24 μC/cm²。
• 利用扫描探针显微镜（SPM）和介电弛豫测量对铁电性进行表征，量化实际极化强度及缺陷影响。
• 介电弛豫研究用于识别氧空位对畴壁迁移率的贡献。
• 对铁电性和介电响应的对比分析，将缺陷动力学与极化翻转效率关联起来。

实验结果

研究问题

• RQ1 能否合成低漏电的外延AlxFe2-xO3薄膜，以实现可靠的室温铁电性和亚铁磁性测量？
• RQ2 为何x-AFO薄膜中实验测得的极化强度显著低于第一性原理计算的预测值？
• RQ3 面内结构畴如何影响x-AFO薄膜中的极化翻转？
• RQ4 氧空位缺陷在多大程度上影响x-AFO中畴壁运动和极化翻转？
• RQ5 沉积条件如何影响x-AFO薄膜的居里温度和磁有序性？

主要发现

• 通过脉冲激光沉积法在外延SrTiO3<111>衬底上成功生长出低漏电的x-AFO薄膜。
• 确认了室温下的亚铁磁性，居里温度取决于沉积条件。
• 第一性原理计算预测通过面内氧层剪切可实现24 μC/cm²的高自发极化强度。
• 实测极化强度约为0.2 μC/cm²，比预测值低两个数量级，原因在于竞争的面内结构畴。
• 介电弛豫和铁电性测量表明，氧空位缺陷有助于畴壁运动，从而增强极化翻转。
• 识别出多个相互竞争的面内结构畴是净极化强度降低的主要原因。