Epitaxy, stoichiometry, and magnetic properties of Gd-doped EuO films on YSZ (001)

We have succeeded in preparing high-quality Gd-doped single-crystalline EuO films. Using Eu-distillation-assisted molecular beam epitaxy and a systematic variation in the Gd and oxygen deposition rates, we have been able to observe sustained layer-by-layer epitaxial growth on yttria-stabilized cubic zirconia (001). The presence of Gd helps to stabilize the layer-by-layer growth mode. We used soft x-ray absorption spectroscopy at the Eu and Gd $M_{4,5}$ edges to confirm the absence of ${\text{Eu}}^{3+}$ contaminants and to determine the actual Gd concentration. The distillation process ensures the absence of oxygen vacancies in the films. From magnetization measurements we found the Curie temperature to increase smoothly as a function of doping from 70 K up to a maximum of 125 K. A threshold behavior was not observed for concentrations as low as 0.2%.

Figure 1

(Color online) RHEED photographs of (a) clean and annealed YSZ (001), and Gd-doped EuO films with Gd concentrations of (b) 0.2%, (c) 2.0%, (d) 7.7%, and (e) 20%. The films were grown for 100 min using a $8.1–8.2\text{Å}/\text{min}$ Eu flux rate and a $8\times {10}^{-8}\text{mbar}$ oxygen pressure. The Gd flux rates were 0.006, 0.056, 0.51, and $0.7\text{Å}/\text{min}$ for (b)–(e), respectively. The RHEED electron energy was 20 keV with the beam incident along the [100] direction.

Figure 2

RHEED intensity oscillations of the specularly reflected electron beam, detected during deposition of a 1% Gd-doped EuO film on YSZ (001) grown using a $8.1–8.2\text{Å}/\text{min}$ Eu flux rate and a $8\times {10}^{-8}\text{mbar}$ oxygen pressure $P_{\text{Ox}}$.

Figure 3

(Color online) LEED photographs of epitaxial Gd-doped EuO films on YSZ (001), grown for 100 min using a $8.1–8.2\text{Å}/\text{min}$ Eu flux rate and a $8\times {10}^{-8}\text{mbar}$ oxygen pressure. The Gd concentration is (a) 1.0% and (b) 7.7%. The patterns were recorded at an electron beam energy of approximately 216 eV.

Figure 4

(Color online) Eu and Gd $M_{4,5}$ $\left(3d\to 4f\right)$ XAS spectra of (a) a 11.3% Gd-doped EuO film and (b) an undoped EuO film at 300 K. The net Gd $M_{4,5}$ contribution (c) is obtained after subtracting the EXAFS of the undoped EuO from the 11.3% Gd-doped EuO spectrum.

Figure 5

(Color online) The actual Gd/Eu ratio in Gd-doped EuO films determined from the Eu and Gd $M_{4,5}$ XAS spectra versus the nominal ratio given by the relative flux rates. Various oxygen pressures used are indicated and the Eu flux rate was $8.1–8.2\text{Å}/\text{min}$ in all cases. The dash line with $\text{slope}=1$ serves as guide to the eyes.

Figure 6

(Color online) Temperature dependence of the normalized magnetization of epitaxial Gd-doped EuO films on YSZ (001) for various Gd concentrations: panel (a) from undoped to 7.7% and panel (b) from 7.7% to 20%. The applied magnetic field was 10 G.

Figure 7

(Color online) The Curie temperature of Gd-doped EuO films on YSZ (001) versus the Gd concentration. The inset is a closeup for the low Gd concentration range. Dashed lines serve as guide to the eyes.

Figure 8

(Color online) Temperature derivative of the magnetization as a function of temperature for the undoped EuO, the 1.5% Gd-doped EuO, and the 7.7% Gd-doped EuO films on YSZ (001).

Figure 9

The magnetization as a function of the applied magnetic field of (a) the undoped EuO and (b) the 7.7% Gd-doped EuO films on YSZ (001) at 5 K. The insets are enlargement of the corresponding curves at low applied magnetic field.