庞磁电阻La_(0.8)Sr_(0.2)MnO_3薄膜的微结构研究

Microstructures of colossal magnetoresistant La_(0.8)Sr_(0.2)MnO_3 thin films

利用激光分子束外延方法在(001)SrTiO3衬底上制备庞磁电阻La0.8Sr0.2MnO3薄膜。发现高密度、尺度均匀的纳米聚集物弥散地分布在La0.8Sr0.2MnO3薄膜中。Z衬度像、EDS及电子衍射综合分析表明,这些纳米聚集物的内部成分出现起伏,主要特征是富Mn贫La,Mn La的比值在其核心处达到极大值,形成立方结构的第二相MnO。LSMO薄膜由两种正交结构的取向畴组成。MnO与畴1的取向关系为:[101]LSMO,1[100]MnO和(010)LSMO,1(010)MnO;与畴2的关系为:[101]LSMO,2[100]MnO和(010)LSMO,2(001)MnO。在LSMO STO外延体系中,还观察到大量的失配位错。失配位错的类型分为刃型失配位错,位错线沿〈100〉方向;螺型失配位错,位错线沿〈110〉方向。刃型位错是基体中原有位错在薄膜生长过程中沿生长表面的扩展造成的;螺型失配位错的形成与降温过程中LSMO正交结构的取向畴的形成有关。

Colossal magnetoresistant La_(0.8)Sr_(0.2)MnO_3 thin films were deposited on (001) SrTiO_3 substrate by laser molecular beam epitaxy. Highly dense and dimensional uniform nano-agglomerates were found embedded in the thin film of La_(0.8)Sr_(0.2)MnO_3 grown under the oxygen pressure of P_(O2)=2×10^(-2)Pa. EDS compositional analysis revealed that the nano-agglomerates were rich of manganese and poor of lanthanum. The ratio of Mn/La in the nano-agglomerates was fluctuated. A salient feature of this compositional fluctuation within the nanoscale was the formation of cubic MnO secondary phase, which appeared as the core of the nano-agglomerates. The present La_(0.8)Sr_(0.2)MnO_3 film was domain-oriented and two domains were identified on the basis of orthorhombic lattice. The orientation relationships between La_(0.8)Sr_(0.2)MnO_3 domains and MnO were determined as: [101]_(LSMO,1)//[100]_(MnO) and (010)_(LSMO,1)//(010)_(MnO);[10(1)]_(LSMO,2)//[100]_(MnO) and (010)_(LSMO,2)//(001)_(MnO) The domain structures and compositional inhomogeneities within nanoscale result in a textures microstructure, which is one of the most important parameters for tunning electronic properties in the colossal magnetoresistance oxides. Moreover, two types of misfit dislocations were found in the LSMO/STO epitaxial system. One was pure edge misfit dislocations with line directions of 〈100〉; The other was pure screw misfit dislocations with line directions of 〈110〉. The formation of pure edge misfit dislocations is believed to result from the propagation of pre-existing dislocations in the SrTiO_3 substrate during the deposition process, whereas that of pure screw misfit dislocations seems closely related to the configuration of orthorhombic oriented domains during the cooling stage.