Goswami Sudipta, Bhattacharya Dipten, Ghosh Chandan K, Ghosh Barnali, Kaushik S D, Siruguri V, Krishna P S R
Department of Solid State Physics, Indian Association for the Cultivation of Science, Kolkata, 700032, India.
Nanostructured Materials Division, CSIR-Central Glass and Ceramic Research Institute, Kolkata, 700032, India.
Sci Rep. 2018 Feb 27;8(1):3728. doi: 10.1038/s41598-018-21803-1.
Using high resolution powder x-ray and neutron diffraction experiments, we determined the off-centered displacement of the ions within a unit cell and magnetoelectric coupling in nanoscale BiFeO (≈20-200 nm). We found that both the off-centered displacement of the ions and magnetoelectric coupling exhibit nonmonotonic variation with particle size. They increase as the particle size reduces from bulk and reach maximum around 30 nm. With further decrease in particle size, they decrease precipitously. The magnetoelectric coupling is determined by the anomaly in off-centering of ions around the magnetic transition temperature (T ). The ions, in fact, exhibit large anomalous displacement around the T which is analyzed using group theoretical approach. It underlies the nonmonotonic particle-size-dependence of off-centre displacement of ions and magnetoelectric coupling. The nonmonotonic variation of magnetoelectric coupling with particle size is further verified by direct electrical measurement of remanent ferroelectric hysteresis loops at room temperature under zero and ∼20 kOe magnetic field. Competition between enhanced lattice strain and compressive pressure appears to be causing the nonmonotonic particle-size-dependence of off-centre displacement while coupling between piezo and magnetostriction leads to nonmonotonicity in the variation of magnetoelectric coupling.
通过高分辨率粉末X射线和中子衍射实验，我们确定了纳米级BiFeO（≈20 - 200nm）晶胞内离子的偏心位移和磁电耦合。我们发现，离子的偏心位移和磁电耦合都随粒径呈现非单调变化。随着粒径从体相减小，它们会增大，并在约30nm处达到最大值。随着粒径进一步减小，它们会急剧下降。磁电耦合由磁转变温度（T ）附近离子偏心的异常决定。实际上，离子在T 附近表现出较大的异常位移，这是用群论方法分析的。这是离子偏心位移和磁电耦合粒径非单调依赖性的基础。通过在零磁场和~20kOe磁场下室温下对剩余铁电磁滞回线的直接电学测量，进一步验证了磁电耦合随粒径的非单调变化。增强的晶格应变和压缩压力之间的竞争似乎导致了偏心位移的粒径非单调依赖性，而压电和磁致伸缩之间的耦合导致了磁电耦合变化的非单调性。
