Kaspar Tiffany C, Spurgeon Steven R, Matthews Bethany E, Bowden Mark E, Heald Steve M, Wang Le, Kelley Ron, Paudel Rajendra, Isaacs-Smith Tamara, Comes Ryan B, Yin Xinmao, Tang Chi Sin, Wee Andrew T S, Chambers Scott A
Physical and Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland, WA, United States of America.
Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland, WA, United States of America.
J Phys Condens Matter. 2021 Jun 17;33(31). doi: 10.1088/1361-648X/ac0571.
The titanomagnetites (FeTiO,⩽ 1) are a family of reducible spinel-structure oxides of interest for their favorable magnetic, catalytic, and electrical transport properties. To understand the stability of the system during low temperature deposition, epitaxial thin films of FeTiOwere deposited by molecular beam epitaxy (MBE) on MgO(001) at 250-375 °C. The homogeneous incorporation of Ti, Fe valence state, and film morphology were all found to be strongly dependent on the oxidation conditions at the low substrate temperatures employed. More oxidizing conditions led to phase separation into epitaxial, faceted FeOand rutile TiO. Less oxidizing conditions resulted in polycrystalline films that exhibited Ti segregation to the film surface, as well as mixed Fe valence (Fe, Fe, Fe). A narrow window of intermediate oxygen partial pressure during deposition yielded nearly homogeneous Ti incorporation and a large fraction of Fe. However, these films were poorly crystallized, and no occupation of tetrahedral sites in the spinel lattice by Fewas detected by x-ray magnetic circular dichroism at the Fe L-edge. After vacuum annealing, a small fraction of Fewas found to occupy tetrahedral sites. Comparison of these results with previous work suggests that the low temperature deposition conditions imposed by use of MgO substrates limits the incorporation of Ti into the spinel lattice. This work suggests a path towards obtaining stoichiometric, well-crystallized FeTiOby MBE by utilizing high substrate temperature and low oxygen partial pressure during deposition on thermally stable substrates.
钛磁铁矿（FeTiO，⩽ 1）是一类具有可还原尖晶石结构的氧化物，因其良好的磁性、催化和电输运性能而备受关注。为了解该体系在低温沉积过程中的稳定性，通过分子束外延（MBE）在250 - 375 °C的MgO(001)上沉积了FeTiO外延薄膜。发现在所采用的低衬底温度下，Ti的均匀掺入、Fe的价态以及薄膜形态都强烈依赖于氧化条件。氧化性更强的条件导致相分离为外延的、有刻面的FeO和金红石TiO。氧化性较弱的条件导致多晶薄膜，其表现出Ti偏析到薄膜表面，以及混合的Fe价态（Fe²⁺、Fe³⁺、Fe⁴⁺）。沉积过程中中等氧分压的狭窄窗口产生了几乎均匀的Ti掺入和大量的Fe³⁺。然而，这些薄膜结晶性较差，并且在Fe L边通过X射线磁圆二色性未检测到Fe占据尖晶石晶格中的四面体位置。真空退火后，发现一小部分Fe占据了四面体位置。将这些结果与先前的工作进行比较表明，使用MgO衬底所施加的低温沉积条件限制了Ti掺入尖晶石晶格。这项工作提出了一条通过在热稳定衬底上沉积期间利用高衬底温度和低氧分压，通过MBE获得化学计量比、结晶良好的FeTiO的途径。
