Experimental and theoretical analysis of magnetic moment enhancement in oxygen-deficient EuO

We have carried out both experimental and theoretical studies of the magnetic properties of thin films of oxygen-deficient EuO. Accurate control of the oxygen vacancy concentration in these films was achieved by sputter codeposition of Eu and ${\text{Eu}}_2{\text{O}}_3$. The films were characterized by superconducting quantum interference device, x-ray reflectometry and polarized neutron reflectometry and the magnetic moment was found to increase monotonically with oxygen vacancy concentration. The electronic structure of ${\text{EuO}}_{1-x}$ was calculated using density-functional theory $\left(\text{DFT}+U\right)$. In agreement with previous studies, these calculations show that oxygen vacancies act as $n$-type dopants in EuO and that the excess electrons preferentially populate the majority spin branch of the spin-polarized conduction band. The observed increase in the magnetic moment originating from these excess electrons was accurately determined experimentally and found to be in good agreement with our quantitative predictions.

Figure 1

Magnetization curve (normalized to unity) and first derivative (inset) for a 106 nm ${\text{EuO}}_{1-x}$ film with $x_{\text{PNR}}=2.5\mathrm{\%}$. The continuous line is the Brillouin $J=7/2$ function with $T_c=70\text{K}$. Measured in a field of 1 T.

Figure 2

$T_c$ of 100-nm-thick ${\text{EuO}}_{1-x}$ samples (data points) fitted with Mauger’s model (line).

Figure 3

(Color online) PNR data (data points) and fit (lines) for varying oxygen vacancy concentrations, $x_{\text{PNR}}=2.5\mathrm{\%}$, 4.2%, and 9%.

Figure 4

(Color online) (a) Band structure and (b) density of states for bulk EuO calculated within $\text{LSDA}+U$ for $U_{\text{f}}=7.3\text{eV}$. Majority spin bands and DOS are in green (light), minority spin bands in blue (dark).

Figure 5

(Color online) Calculated band structure and density of states, for $2\times 2\times 2$ supercells of ${\text{EuO}}_{1-x}$, containing (a) one vacancy (b) two vacancies (c) three vacancies and (d) four vacancies. Majority spin bands and DOS are in green (light), minority spin bands in blue (dark). Note that the energy scale is shifted relative to that in Fig. 4 and between different calculations due to differing Fermi energies. Note also the different $k$-point path due to the choice of a simple-cubic supercell rather than fcc.

Figure 6

Magnetic moment of oxygen-deficient EuO as a function of the oxygen vacancy concentration as calculated by DFT and measured by PNR at 5 K.