Tuning the magnetism of epitaxial cobalt oxide thin films by electron beam irradiation

Tuning magnetic properties of perovskite thin films is a central topic of recent studies because of its fundamental significance. In this work, we demonstrated the modification of the magnetism of $\mathrm{L}{\mathrm{a}}_{0.9}\mathrm{C}{\mathrm{a}}_{0.1}\mathrm{Co}{\mathrm{O}}_3$ (LCCO) thin films by introducing a stripelike superstructure in a controllable manner using electron beam irradiation (EBI) in a transmission electron microscope. The microstructure, electronic structure, strain change, and origin of magnetism of the LCCO thin films were studied in detail using aberration-corrected scanning transmission electron microscopy, electron energy loss spectroscopy, and ab initio calculations based on density functional theory. The results indicate that the EBI-induced unit cell volume expansion accompanies the formation of oxygen vacancies and leads to the spin state transition of Co ions. The low spin state of $\mathrm{C}{\mathrm{o}}^{4+}$ ions depress the stripelike superstructure, while higher spin states of Co ions with lower valences are conductive to the formation of “dark stripes”. Our work clarifies the origin of magnetism of epitaxial LCCO thin films, benefiting a comprehensive understanding of correlated physics in cobalt oxide thin films.

Figure 1

The HAADF-STEM images of 35-nm-thick LCCO thin films in the initial state (a) and the irradiated state (b) by electron beam with 6 kGy carefully recorded in low-dose STEM (spot size of 8C). The hysteresis curves of the initial (red line) and irradiated with 6 kGy (black line) of the LCCO thin films with the thicknesses of 35 nm (c) and 70 nm (d), measured at 10 K.

Figure 2

HAADF-STEM images, demonstrating the structural evolution of the LCCO films during EBI. The images of 70-nm-thick LCCO thin film were recorded at the beginning of irradiation experiment (a) and after irradiation of 4 min (b) and 12 min (c), respectively. The images of 35-nm-thick film were recorded at the beginning of irradiation experiment (d) and after irradiation of 3 min (e) and 8 min (f), respectively. The insets show the corresponding FFT patterns. The red arrows indicate the superstructure reflections. “Dark stripes” gradually emerged and became denser in local areas (marked by the orange rectangles) with increasing exposure time.

Figure 3

The in-plane strain profiles as a function of distance $d$ from the substrate to the LCCO thin film of the initial and irradiated areas, determined by averaging data from the white rectangular areas in Fig. S4 [35] (the in-plane strain images of LCCO thin films using GPA method), respectively. The time evolution of lattice spacing at locations P1 (b) extracted from the 70-nm-thick LCCO thin film in Fig. 2 and P2 (c) extract from the 35-nm-thick LCCO thin film in Fig. 2 during EBI. The in-plane lattice spacings are marked by black squares, and the out-of-plane ones are marked by red dots.

Figure 4

The EELS fine structures across the stripe patterns of the 35-nm-thick LCCO thin films (a). The black line and red line were obtained from the dark and bright Co-O layers, respectively. (b) O K edges. (c) $\mathrm{Co}{\mathrm{L}}_{2,3}$ edges.

Figure 5

The optimized structures and magnetization of LCCO (a) and LCCO with O vacancies (c). Orange, cyan, and purple ions represent LS Co ions, IS $\mathrm{C}{\mathrm{o}}^{3+}$ ions, and HS $\mathrm{C}{\mathrm{o}}^{2+}$ ions, respectively. Red, blue, and magenta ions represent O, La, and Ca ions. The DOS of Co atoms of LCCO (b) and LCCO with O vacancies (d, e) at the corresponding positions given in (a) and (c).