Correlations between oxygen octahedral distortions and magnetic and transport properties in strained ${\mathrm{La}}_{0.5}{\mathrm{Sr}}_{0.5}\mathrm{Co}{\mathrm{O}}_3$ thin films

We quantitatively evaluated oxygen octahedral distortions in epitaxial thin films of the itinerant ferromagnet, ${\mathrm{La}}_{0.5}{\mathrm{Sr}}_{0.5}\mathrm{Co}{\mathrm{O}}_3$ (LSCO), and investigated their impact on strucure-property relationships. The compressively strained film on a $\mathrm{LaAl}{\mathrm{O}}_3$ substrate has a lower electrical resistivity and higher ferromagnetic transition temperature than the tensilely strained film on a ${\mathrm{La}}_{0.30}{\mathrm{Sr}}_{0.70}{\mathrm{Al}}_{0.65}{\mathrm{Ta}}_{0.35}{\mathrm{O}}_3$ substrate. The magnetic anisotropy is also found to depend on the type of strain, with perpendicular magnetic anisotropy induced in the compressively strained film and in-plane magnetization seen in the tensilely strained film. Our synchrotron x-ray-diffraction measurements and quantitative analysis reveal distinct oxygen octahedral distortions accommodated in these films and show that the out-of-plane and in-plane Co–O bond lengths of the compressively strained film are comparable to the in-plane and out-of-plane bond lengths of the tensilely strained film. These results indicate that the bond-length changes in LSCO modify hybridization between Co $3d$ and O2p orbitals, leading to the strain-dependent properties. These results highlight the significant role of octahedral distortions for structure-property relationships in the strained LSCO films.

Figure 1

X-ray-diffraction characterizations of LSCO films on LAO and LSAT substrates. (a) $2\theta -\theta$ profiles of LSCO films on LSAT (blue) and LAO (red) substrates. (b) Reciprocal space mappings around the (103) LAO (left) and LSAT (right) Bragg reflection for the LSCO films.

Figure 2

Temperature dependence of the longitudinal resistivity ${\rho }_{xx}$ (top) and magnetization $M$ (bottom) of the LSCO films on (a), (c) LAO and (b) (d) LSAT substrates. For magnetization measurements, a magnetic field of 1000 Oe was applied in parallel and perpendicular directions to the film surface during the cooling process and measurement.

Figure 3

Magnetic-field dependence of the anomalous Hall resistivity ${\rho }_{\mathrm{AHE}}$ (top) and magnetization $M$ (bottom) of LSCO films on (a), (c) LAO and (b), (d) LSAT substrates. For magnetization measurements, a magnetic field of 5 T was applied in directions parallel and perpendicular to the sample during the cooling process and measurement. All data in (a)–(d) were taken at 20 K.

Figure 4

$\left(1/21/2L\right), \left(1/23/2L\right), \left(3/21/2L\right)$, and $\left(3/23/2L\right)$ profiles for the compressively and tensilely strained LSCO films (on the LAO and LSAT substrates). The black lines are profiles calculated by the optimized model of the LSCO/LAO. The peaks marked with the asterisk (*) in the profiles are originally forbidden but appear with weak intensity probably due to multiple scatterings in the substrate. The inset shows the profiles around the $\left(3/21/23/2\right), \left(-3/21/23/2\right), \left(3/2-1/23/2\right)$, and $\left(-3/2-1/23/2\right)$ reflections. In the profiles for the tensilely strained film, relatively high background signal from the LSAT substrate were observed, which hinder quantitative analysis on half-order reflection intensities.