Exploring the intrinsic limit of the charge-carrier-induced increase of the Curie temperature of Lu- and La-doped EuO thin films

Raising the Curie temperature $T_{\mathrm{C}}$ of the highly spin-polarized semiconductor EuO by doping it with rare-earth elements is a strategy to make EuO more technologically relevant to spintronics. The increase of $T_{\mathrm{C}}$ with free carrier density $n$ and the surprisingly low dopant activation $p$, found in Gd-doped EuO thin films [Mairoser et al., Phys. Rev. Lett. 105, 257206 (2010)], raised the important question of whether $T_{\mathrm{C}}$ could be considerably enhanced by increasing $p$. Using a low-temperature growth method for depositing high-quality Lu-doped EuO films we attain high dopant activation ($p$) values of up to 67%, effectively more than doubling $p$ as compared to adsorption-controlled growth of Lu- and Gd-doped EuO. Relating $n, p$, and lattice compression of La- and Lu-doped EuO films grown at different temperatures to the $T_{\mathrm{C}}$ of these samples allows us to identify several different mechanisms influencing $T_{\mathrm{C}}$ and causing an experimental maximum in $T_{\mathrm{C}}$. In addition, scanning transmission electron microscopy in combination with electron energy loss spectroscopy measurements on La-doped EuO indicate that extensive dopant clustering is one, but not the sole reason for dopant deactivation in rare-earth doped EuO films.

Figure 1

(a) Dependence of $T_{\mathrm{C}}$ and $n$ on $x$ in ${\mathrm{Eu}}_{1–x}{\mathrm{Lu}}_x\mathrm{O}$ films grown at different substrate temperatures $T$ on $\mathrm{LuAl}{\mathrm{O}}_3$ and measured at 5 K; (b) calculated dopant activation $p$ as a function of the doping concentration in atomic percent in ${\mathrm{Eu}}_{1–x}{\mathrm{Lu}}_x\mathrm{O}$ for the same samples. (c) Dependence of $T_{\mathrm{C}}$ on $n$ in ${\mathrm{Eu}}_{1–x}{\mathrm{Lu}}_x\mathrm{O}$ films grown at different substrate temperatures $T$ measured at 5 K. The numbers at the data points denote the doping concentrations in atomic percent. (d) ${\mathrm{Eu}}_{1–x}{\mathrm{Lu}}_x\mathrm{O}$ out-of-plane lattice spacing $c$ and calculated $c$ lattice strain as compared to bulk EuO as functions of $x$, derived from the 002 peak positions in θ-2θ scans. The full symbols represent data points of samples grown at $T=250{}^{\circ }\mathrm{C}$ and the empty symbols represent data points of samples grown at $T=400{}^{\circ }\mathrm{C}$. For comparison, the measured $c$ axis of a 300-nm-thick undoped EuO film grown on $\mathrm{LuAl}{\mathrm{O}}_3$ at $T=325{}^{\circ }\mathrm{C}$ is included. At this thickness, this film is expected to be relaxed [20]. (e)–(h) show the same measurements and calculation results as (a)–(d) but with ${\mathrm{Eu}}_{1–x}{\mathrm{La}}_x\mathrm{O}$ films. Please note that the lower growth temperature of these samples was $T=275{}^{\circ }\mathrm{C}$.

Figure 2

Spectroscopic imaging of a ${\mathrm{La}}_{0.08}{\mathrm{Eu}}_{0.92}\mathrm{O}$ film grown on an undoped EuO buffer layer on a $\mathrm{YAl}{\mathrm{O}}_3$ substrate in the adsorption-controlled growth regime. (a) HAADF-STEM image of the sample. Atomic-resolution spectroscopic images of the ${\mathrm{La}}_{0.08}{\mathrm{Eu}}_{0.92}\mathrm{O}$ region using the signal from the $\mathrm{La}\text{−}{M}_{4,5}$ and $\mathrm{Eu}\text{−}{M}_{4,5}$ edges are shown in (b) and (c), respectively. (d) The combined spectroscopic image with lanthanum in turquoise and europium in red shows that the lanthanum signal is inhomogeneous, indicating clustering of lanthanum atoms. (e) A low-magnification EELS spectroscopic image using the $\mathrm{La}\text{−}{N}_{4,5}$ and $\mathrm{Eu}\text{−}{N}_{4,5}$ edges demonstrates that the lanthanum preferentially clusters the near [010] and [100] zone axes. (f) A high-resolution image of a selected region from (e). Scale bars in (b)–(f) are 1 nm.

Figure 3

STEM EELS of an 8% La-doped EuO film grown close to the flux-matching regime at a relatively low substrate temperature of $T=275{}^{\circ }\mathrm{C}$. (a) Overview image of the sample. The top 12 nm of the EuO film has been oxidized (see Supplemental Material [27]). (b) Atomic-resolution HAADF-STEM image, demonstrating that the interface between the film and the substrate lacks impurity phases. Chemical maps of the lanthanum (c) and europium (d) show homogeneous distribution of these elements. (e) shows a combined color map of the film with lanthanum in turquoise and europium in red. The La content of the $\mathrm{LaAl}{\mathrm{O}}_3$ capping layer (bright turquoise) is 20%. Scale bars in (c)–(e) are 5 nm.