Microstructure and Electrical Conductivity of (Y, Sr)CoO_3-δ Thin Films Tuned by the Film-Growth Temperature

Abstract

Perovskite-based rare earth cobaltates have been the topic of extensive research since they possess fascinating electrical and magnetic properties [1,2]. Y x Sr 1−x CoO 3−δ (YSCO) compounds with roomtemperature ferromagnetic properties were reported. In particular, the YSCO compound with x≈0.25 exhibits the highest Curie temperature (T c ≈335 K) among the perovskite-based cobaltates and the properties of YSCO could be modulated by its chemical composition [3]. In contrast to the considerable studies on the YSCO bulk materials, investigations on the microstructure and the properties of the YSCO thin films are limited. In this work, we report a detailed study on the microstructural and electrical properties of the Y x Sr 1−x CoO 3−δ (x=0.25) films, which were prepared at different growth temperatures on the (La 0.289 Sr 0.712 )(Al 0.633 Ta 0.356 )O 3 (LSAT) substrates.The YSCO thin films are grown on single crystalline LSAT (001) substrates at 800 °C and 900 °C, respectively, using a magnetron sputtering system. The flowing pressure is 1 mbar with the mixed ambient of Ar and O 2 at the ratio of 1:1. After the film growth, the films are annealed for 15 minutes in a mixture gas of Ar and O 2 (400 mbar) in a ratio of 1:1, and then cooled down to room temperature in 30 minutes. Cross-sectional specimens were prepared by focused ion beam (FIB) technique (FEI Dualbeam Helios NanoLab 600i). Transmission electron microscopy (TEM) observation and selected area electron diffraction (SAED) are carried out on a JEOL 2100 microscope, operated at 200 kV. Energy-dispersive X-ray spectroscopy (EDS) data and high-angle annular dark-field (HAADF) images are obtained in a JEOL ARM 200F. The electrical properties of the films are measured using a Physics Property Measurement System (PPMS, Quantum Design). (001) substrates at 800 °C and 900 °C, respectively, viewed along the [100] zone axis of the substrates. From the image of the cross-sectional samples the thickness of the films is measured as about 100 nm. The films prepared at 800 °C show a homogeneous contrast and a sharp interface to the substrate, indicating a perfect epitaxy of the films. In contrast, In Figure 1 (b) nano-scale columnar contrast is evident in the YSCO films prepared at 900 °C, which comes from second phase. The EDS spectrum of the second-phase particles is inserted in (b), which indicates that the secondary phase is Y 2 O 3 . Importantly, it can be seen that Y 2 O 3 columns either start at the film-substrate interface or form within the film in a self-assembled manner. The size of the Y 2 O 3 columns is typically in a range of 4−10 nm in the in-plane direction and 60−80 nm in the out-of-plane direction. Electronic address: