Oxygen vacancies and induced changes in the electronic and magnetic structures of ${\mathrm{La}}_{0.66}{\mathrm{Sr}}_{0.33}\mathrm{Mn}{\mathrm{O}}_3$: A combined ab initio and photoemission study

The effect of oxygen vacancies on the electronic and magnetic properties of ${\mathrm{La}}_{0.66}{\mathrm{Sr}}_{0.33}\mathrm{Mn}{\mathrm{O}}_3$ (LSMO) has been investigated by means of ab initio calculations within the density-functional formalism combined with photoemission. The simulations show that the introduction of oxygen vacancies causes a shift of the valence-band features toward higher binding energies and an increase of the degree of covalency of Mn bondings. The Mn magnetic moments undergo some changes, keeping, however, the situation relatively close to the ideal nondefective system: in none of the different vacancy configurations, a drastic charge or spin rearrangement occurs. There is, though, an important vacancy-induced drawback: half-metallicity, typical of the perfectly stoichiometric system, is generally lost due to defective bands that cross the Fermi level. Photoemission experiments performed on epitaxial thin films of LSMO with different contents of oxygen vacancies grown by pulsed laser deposition essentially confirm theoretical predictions. Our findings clearly indicate that the control over oxygen deficiency should therefore be experimentally achieved to avoid unwanted consequences in terms of spin-injection efficiency.

Figure 1

(Color online) Scheme of the LSMO tetragonal unit cell: purple, red, green, and blue spheres represent Sr, La, O, and Mn atoms, respectively. The vertical $z$ axis represents the [001] direction. The Bravais lattice is shown as thin blue rods, whereas the yellow rods highlight the octahedral oxygen cage around Mn atoms. Also shown are the $z$ coordinates, atomic plane, and system notation used for the oxygen vacancies.

Figure 2

(Color online) Density of states for the oxygen vacancy in ${\left(\mathrm{Mn}{\mathrm{O}}_2\right)}_1$ layer (red bold line), compared to the corresponding values in ideal LSMO (black thin line). Panel (a) shows the total DOS. The PDOS is shown in (b), (d), (e), (g), (h), (k), and (l) for oxygen atoms at $z=0$, $z=c∕6$, $z=c∕3$, $z=c∕2$, $z=c∕2$, $z=-c∕3$, $z=-c∕6$, and $z=-c∕6$, respectively. Mn PDOS at $z=c∕6$, $c∕2$, and $-c∕6$ are shown in panels (c), (f), and (j), respectively. Spin-up and spin-down bands are reported on the positive and negative $y$ axes, respectively. The zero of the energy scale refers to the Fermi level and the Gaussian smearing is set to $0.1\mathrm{eV}$.

Figure 3

Band structures along the main symmetry lines in the tetragonal Brillouin zone for the ideal structure [panels (a) and (b) show the spin-up and spin-down bands, respectively] and for the B structure [panels (c) and (d) show the spin-up and spin-down contributions, respectively]. The zero of the energy scale is set to $E_F$. The results are obtained within $\mathrm{GGA}+U$, using $U=2\mathrm{eV}$ and $J=0.7\mathrm{eV}$.

Figure 4

(Color online) Total density of states for (a) structure A (vacancy in SrO layer), (b) structure C (vacancy in LaO plane), and (c) structure D [vacancy in ${\left(\mathrm{Mn}{\mathrm{O}}_2\right)}_2$ layer]. The bold red lines show the DOS relative to the defective system, whereas the black thin line refers to ideal LSMO without vacancies. The zero of the energy scale is set to $E_F$; spin-up and spin-down bands are reported on the positive and negative $y$ axes, respectively. The inset in panel (a) shows the PDOS for Mn at $z=0.5$ and $z=c∕6$ (solid and dashed lines, respectively). The inset in panel (b) shows the PDOS for the Mn at $z=0.5$ and $z=c∕6$ (dashed and solid lines, respectively). The insets in panel (c) shows the Mn PDOS (bold red line) in the defective plane compared to the corresponding atom in the ideal structure (thin solid line).

Figure 5

(Color online) (a) Comparison between TDOS calculations for defective LSMO with vacancies in the $\mathrm{Mn}{\mathrm{O}}_2$ planes and XPS data from the oxygen deficient sample ${\mathrm{S}}_2$. Inset: $30\mathrm{keV}$ RHEED from sample ${\mathrm{S}}_2$. (b) Calculated TDOS for ideal and defective LSMO, with vacancies in planes (A) SrO, (B) ${\left(\mathrm{Mn}{\mathrm{O}}_2\right)}_1$, (C) LaO, (D) ${\left(\mathrm{Mn}{\mathrm{O}}_2\right)}_2$. The calculated DOSs have been convoluted with a Gaussian function with $0.84\mathrm{eV}$ FWHM in order to take into account the experimental broadening.

Figure 6

(Color online) Ultraviolet photoemission spectra from LSMO(001) samples. The photon energy is $38\mathrm{eV}$ for ${\mathrm{S}}_1$ and $21.2\mathrm{eV}$ for ${\mathrm{S}}_2$ and ${\mathrm{S}}_3$.

Figure 7

(Color online) $\mathrm{Mn}3s$ spectra from the three samples of LSMO, ${\mathrm{S}}_1–{\mathrm{S}}_3$.