Department of Materials Science and Engineering , University of California, Berkeley , Berkeley , California 94720 , United States.
School of Materials Science and Engineering , Harbin Institute of Technology , Shenzhen 518055 , P. R. China.
ACS Appl Mater Interfaces. 2018 May 2;10(17):14914-14921. doi: 10.1021/acsami.8b02597. Epub 2018 Apr 18.
Complex-oxide materials tuned to be near phase boundaries via chemistry/composition, temperature, pressure, etc. are known to exhibit large susceptibilities. Here, we observe a strain-driven nanoscale phase competition in epitaxially constrained BiLaFeO thin films near the antipolar-nonpolar phase boundary and explore the evolution of the structural, dielectric, (anti)ferroelectric, and magnetic properties with strain. We find that compressive and tensile strains can stabilize an antipolar PbZrO-like Pbam phase and a nonpolar Pnma orthorhombic phase, respectively. Heterostructures grown with little to no strain exhibit a self-assembled nanoscale mixture of the two orthorhombic phases, wherein the relative fraction of each phase can be modified with film thickness. Subsequent investigation of the dielectric and (anti)ferroelectric properties reveals an electric-field-driven phase transformation from the nonpolar phase to the antipolar phase. X-ray linear dichroism reveals that the antiferromagnetic-spin axes can be effectively modified by the strain-induced phase transition. This evolution of antiferromagnetic-spin axes can be leveraged in exchange coupling between the antiferromagnetic BiLaFeO and a ferromagnetic CoFe layer to tune the ferromagnetic easy axis of the CoFe. These results demonstrate that besides chemical alloying, epitaxial strain is an alternative and effective way to modify subtle phase relations and tune physical properties in rare earth-alloyed BiFeO. Furthermore, the observation of antiferroelectric-antiferromagnetic properties in the Pbam BiLaFeO phase could be of significant scientific interest and great potential in magnetoelectric devices because of its dual antiferroic nature.
通过化学/组成、温度、压力等手段将氧化物材料调谐到近相界的复合物已知具有较大的磁化率。在这里，我们在接近反铁电极化-非极性相界的外延受限 BiLaFeO 薄膜中观察到了由应变驱动的纳米级相竞争，并研究了应变对结构、介电、（反）铁电和磁性能的演化。我们发现压缩和拉伸应变可以分别稳定反铁电极化 PbZrO 相似的 Pbam 相和非极性 Pnma 正交相。应变很小或没有的异质结构表现出自组装的两种正交相纳米级混合物，其中各相的相对分数可以通过薄膜厚度进行修改。随后对介电和（反）铁电性能的研究表明，存在一个从非极性相到反铁极性相的电场驱动的相转变。X 射线线性二色性揭示了反铁磁自旋轴可以通过应变诱导的相变有效修改。反铁磁自旋轴的这种演化可以在反铁磁 BiLaFeO 和铁磁 CoFe 层之间的交换耦合中得到利用，以调整 CoFe 的铁磁易轴。这些结果表明，除了化学合金化之外，外延应变是修改稀土合金 BiFeO 中细微相关系和调整物理性能的另一种有效方法。此外，由于其双反铁电性质，在 Pbam BiLaFeO 相中观察到的反铁电-反铁磁性质可能具有重要的科学意义和在磁电器件中的巨大潜力。
