Mishra K K, Shukla R, Krishna P S R, Babu P D, Achary S N, Katiyar R S, Scott J F
Department of Physics and Institute for Functional Nanomaterials, University of Puerto Rico, P.O. Box 70377, San Juan, PR 00936-8377, USA.
Phys Chem Chem Phys. 2020 Apr 7;22(13):6906-6918. doi: 10.1039/c9cp06124j. Epub 2020 Mar 17.
The intriguing coupling phenomena among spin, phonon, and charge degrees of freedom in materials having magnetic, ferroelectric and/or ferroelastic order have been of research interest for the fundamental understanding and technological relevance. We report a detailed study on structure and phonons of AlGaFeO (ALGF), a lead-free magnetoelectric material, carried out using variable temperature dependent powder neutron diffraction and Raman spectroscopy. Neutron diffraction studies suggest that Al ions are distributed in one tetrahedrally (BO) and three octahedrally (BO) coordinated sites of the orthorhombic (Pc2n) structure and there is no structural transition in the temperature range of 7-800 K. Temperature dependent field-cooled and zero-field-cooled magnetization studies indicate ferrimagnetic ordering below 225 K (T), and that is reflected in the low temperature powder neutron diffraction data. An antiferromagnetic type arrangement of Fe ions with net magnetic moment of 0.13 μ/Fe was observed from powder neutron diffraction analysis and it corroborates the findings from magnetization studies. At the magnetic transition temperature, no drastic change in lattice strain was observed, while significant changes in phonons were observed in the Raman spectra. The deviation of several mode frequencies from the standard anharmonicity model in the ferrimagnetic phase (below 240 K) is attributed to coupling effect between spin and phonon. Spin-phonon coupling effect is discernable from Raman bands located at 270, 425, 582, 695, 738, and 841 cm. Their coupling strengths (λ) have been estimated using our phonon spectra and magnetization results. BO (n = 4, 6) libration (restricted rotation) mode at 270 cm has the largest coupling constant (λ∼ 2.3), while the stretching vibrations located at 695 and 738 cm have the lowest coupling constant (λ∼ 0.5). In addition to the libration mode, several internal stretching and bending modes of polyhedral units are strongly affected by spin ordering.
具有磁、铁电和/或铁弹序的材料中，自旋、声子和电荷自由度之间有趣的耦合现象，因其在基础理解和技术相关性方面的重要性而备受研究关注。我们报告了一项关于无铅磁电材料AlGaFeO（ALGF）的结构和声子的详细研究，该研究采用了变温粉末中子衍射和拉曼光谱法。中子衍射研究表明，铝离子分布在正交（Pc2n）结构的一个四面体（BO）和三个八面体（BO）配位位点上，并且在7 - 800 K的温度范围内没有结构转变。变温场冷和零场冷磁化研究表明，在225 K（T）以下存在亚铁磁有序，这在低温粉末中子衍射数据中得到了体现。粉末中子衍射分析观察到铁离子呈反铁磁型排列，净磁矩为0.13 μ/Fe，这与磁化研究结果相符。在磁转变温度下，未观察到晶格应变的剧烈变化，而在拉曼光谱中观察到声子有显著变化。在亚铁磁相（低于240 K）中，几个模式频率偏离标准非谐模型，这归因于自旋和声子之间的耦合效应。自旋 - 声子耦合效应可从位于270、425、582、695、738和841 cm的拉曼谱带中辨别出来。利用我们的声子光谱和磁化结果估算了它们的耦合强度（λ）。270 cm处的BO（n = 4, 6）振动（受限旋转）模式具有最大的耦合常数（λ∼ 2.3），而位于695和738 cm处的伸缩振动具有最低的耦合常数（λ∼ 0.5）。除了振动模式外，多面体单元的几个内部伸缩和弯曲模式也受到自旋有序的强烈影响。
