Electronic structure of ${\mathrm{La}}_{2/3}{\mathrm{Sr}}_{1/3}{\mathrm{MnO}}_3$: Interplay of oxygen octahedra rotations and epitaxial strain

Influence of epitaxial strain and oxygen octahedra rotations on electronic structure of ${\mathrm{La}}_{2/3}{\mathrm{Sr}}_{1/3}{\mathrm{MnO}}_3$ ultrathin films was systematically studied. The investigated films were grown by pulsed laser deposition on four different substrates: cubic (001)-oriented ${\mathrm{LaAlO}}_3$, (001) ${\left({\mathrm{LaAlO}}_3\right)}_{1/3}{\left({\mathrm{Sr}}_2{\mathrm{AlTaO}}_6\right)}_{2/3}$, (001) ${\mathrm{SrTiO}}_3$, and orthorhombic (110) ${\mathrm{DyScO}}_3$, providing a broad range of induced epitaxial strains. Magnetic properties were found to deteriorate with increasing value of the epitaxial strain, as expected due to unit cell distortion increasingly deviating from the bulk and effect of the magnetically inert layer. A combination of spectroscopic ellipsometry and magneto-optical Kerr effect spectroscopy was used to determine spectra of the diagonal and off-diagonal elements of permittivity tensor. The off-diagonal elements at room temperature confirmed the presence of two previously reported electronic transitions in the spectra of all films. Moreover they revealed another electronic transition around 4.3 eV only in the spectra of films grown under compressive strain. We proposed classification of this transition as a crystal field paramagnetic Mn $t_{2g}\to e_g$ transition. Ab initio calculations were employed to distinguish between the potential influence of oxygen octahedra rotations and distortions. The ab initio calculations indicated a negligible influence of oxygen octahedra rotations on magneto-optical properties of ${\mathrm{La}}_{2/3}{\mathrm{Sr}}_{1/3}{\mathrm{MnO}}_3$. They further supported the proposed classification of the additional electronic transition, showing a key role of strain in controlling the electronic structure of ultrathin perovskite films.

Figure 1

Structural parameters of ${\mathrm{La}}_{2/3}{\mathrm{Sr}}_{1/3}{\mathrm{MnO}}_3$ films as a function of in-plane strain along ${\left[1\text{−}10\right]}_m$ direction: $a_m$ and $b_m$ lattice parameters and orthorhombicity defined as $b_m/a_m$ (top panel), ${\gamma }_m$ angle and unit cell volume (bottom panel). Lines represent guides to the eye. The inset shows monoclinic unit cell, in which the lattice parameters are defined. The in-plane strain along ${\left[1\text{−}10\right]}_m$ direction was calculated from bulk value of LSMO $ab$ distance ${\left(ab\right)}_{\text{bulk}}=7.787$ Å [6] and measured $ab$ values in Tab. 1 as $(ab-{\left(ab\right)}_{\text{bulk}})/{\left(ab\right)}_{\text{bulk}}$.

Figure 2

Spectra of real and imaginary parts of the diagonal permittivity tensor elements of ${\mathrm{La}}_{2/3}{\mathrm{Sr}}_{1/3}{\mathrm{MnO}}_3$ films on four different substrate materials, extracted from spectroscopic ellipsometry measurements.

Figure 3

Spectra of Kerr rotation and Kerr ellipticity of ${\mathrm{La}}_{2/3}{\mathrm{Sr}}_{1/3}{\mathrm{MnO}}_3$ films on four different substrate materials. For clarity, spectra of samples deposited on LAO and DSO are magnified by ten.

Figure 4

Spectra of real and imaginary parts of the off-diagonal permittivity tensor elements of ${\mathrm{La}}_{2/3}{\mathrm{Sr}}_{1/3}{\mathrm{MnO}}_3$ films on four different substrate materials, calculated from diagonal permittivity tensor elements and magneto-optical Kerr effect spectra. For clarity, the spectra of samples deposited on LAO and DSO are magnified by ten.

Figure 5

Spectra of total density of states obtained by ab initio calculations. Top panel shows unstrained bulk LSMO and strained LSMO corresponding to growth on LAO and DSO substrates. Bottom panel shows bulk LSMO with no rotations of oxygen octahedra and bulk LSMO with quasireal tilt system (${\mathrm{a}}^{-}{\mathrm{a}}^{-}{\mathrm{c}}^{-}$).

Figure 6

Spectra of real and imaginary parts of the off-diagonal permittivity tensor elements of ${\mathrm{La}}_{2/3}{\mathrm{Sr}}_{1/3}{\mathrm{MnO}}_3$ films deposited on LAO (top panel) and DSO (bottom panel) substrates. Spectra obtained experimentally by means of spectroscopic ellipsometry and magneto-optical spectroscopy are compared to spectra obtained theoretically by ab initio calculations. The theoretical spectra were multiplied by factors of 0.065 (LAO) and 0.020 (DSO) to compensate for the temperature-induced decrease of magnetization. The decrease is different for both samples, for which the factors also differ.