Structural and magnetic analysis of epitaxial films of Gd-doped ZnO

Zn${}_{(1-x)}$Gd${}_x$O epitaxial films prepared by reactive magnetron sputtering were studied using a combination of synchrotron based x-ray absorption spectroscopies, x-ray diffraction, and classical magnetometry. With nominal Gd concentrations of up to 16$\%$ only paramagnetism is found. Reducing the oxygen content in the sputter gas provokes phase separation, however, the samples remain paramagnetic. The experimental findings are corroborated by first-principles density-functional theory calculations. In all cases a paramagnetic behavior is found with Gd atomic moments comparable to the experiment. The structural degradation with increasing Gd concentration is consistently found by experiment as well as theory.

Figure 1

XRD data for the 1.3$\%$, 4$\%$, 7$\%$, and 16$\%$ Gd:ZnO films. (a) displays the ZnO(002) reflection. The FWHM shown in the inset depends strongly on the amount of Gd in the films. The substrate (006) peak of Al${}_2$O${}_3$ is displayed in (b) for comparison. The spectra are shifted vertically for clarity.

Figure 2

XANES and XLD spectra at the Zn $K$ edge for the ion-implanted and the sputtered Gd:ZnO films. With increasing Gd concentration, the size of the XLD signal is reduced.

Figure 3

Isotropic XANES (a), XLD (b), and XMCD (c) of 1.3$\%$, 4$\%$, 7$\%$, and 16$\%$ Gd:ZnO at the Gd $L_3$ edge. The implanted sample is shown for comparison as a dotted line.

Figure 4

Element-specific XMCD(H) curves at the Gd $L_3$ edge for the ion-implanted $3\times {10}^{15}$ Gd/cm${}^2$ ZnO film and the sputtered 1.3$\%$, 4$\%$, and 16$\%$ Gd:ZnO samples. The XMCD(H) was measured at 6.5 K. All curves are normalized for comparison.

Figure 5

XMCD(H) curves taken at 6.4 and 41.5 K and SQUID $M\left(H\right)$ at 5 K. The Brillouin function for $J=S=7/2$ is shown as well.

Figure 6

SQUID $M\left(T\right)$ behavior recorded after field-cooled and zero-field-cooled conditions. The diamagnetic background has been subtracted from all data sets.

Figure 7

SQUID $M\left(H\right)$ curves taken at 5 K for Gd${}_2$O${}_3$ nanoparticles and 16$\%$ Gd:ZnO, the latter concentration once grown with an (Ar:O${}_2$) ratio of (10:0.8) and once with (10:0.5). The inset shows the $M\left(T\right)$ curves under FC and ZFC conditions for the 16$\%$ Gd:ZnO (10:0.5) sample.

Figure 8

XRD 2$\theta$ scan of 16$\%$ Gd:ZnO grown with an (Ar:O${}_2$) ratio of (10:0.5) in comparison to the 16$\%$ Gd:ZnO sample grown under optimized conditions.

Figure 9

Relaxed structure of 3.7$\%$ Gd at substitutional sites (a) and at interstitial sites (b) for a $3\times 3\times 3$ cell of Gd:ZnO. The red (dark gray) atoms are O, gray are Zn, and green (large gray) are Gd.

Figure 10

Density of states for Gd-doped ZnO with a Gd concentration of 3.7$\%$ within GGA + U. The Fermi level is set at zero.