X-ray absorption and x-ray magnetic circular dichroism in bulk and thin films of ferrimagnetic $\mathrm{GdTi}{\mathrm{O}}_3$

Perovskite rare-earth titanates are prototypical Mott insulators in which ${\mathrm{Ti}}^{3+}$ ions with $3d^1$ electronic configuration exhibit ferromagnetic or antiferromagnetic spin order, depending on the rare-earth size. This peculiar magnetic behavior has, however, been barely studied with element-specific probes, either in bulk or in thin films. The recent finding of fingerprints of ferromagnetism in two-dimensional electron gases at oxide interfaces involving rare-earth titanates has produced a surge of the interest in these complex materials. Harnessing the interfacial magnetic states in these heterostructures calls for a better understanding of their insufficiently explored magnetic states in bulk and especially in thin film form. In this paper, we combine high-resolution transmission electron microscopy with x-ray absorption spectroscopy and x-ray magnetic circular dichroism (XMCD) to determine the structural, electronic, and magnetic structure of $\mathrm{GdTi}{\mathrm{O}}_3$ in bulk and thin film form. In both cases, we find that the sample surface is strongly overoxidized but a few nm below, Ti is mostly 3+ and shows a large XMCD. We provide evidence for the ferrimagnetic nature of $\mathrm{GdTi}{\mathrm{O}}_3$ with antialigned Gd and Ti sublattices and show that, just as in antiferromagnetic $\mathrm{LaTi}{\mathrm{O}}_3$ or ferromagnetic $\mathrm{YTi}{\mathrm{O}}_3$, Ti carries no orbital moment.

Figure 1

HAADF images of the GTO crystal oriented along the ⟨101⟩ direction (a) and the $\langle 001\rangle$ direction (b). Fast Fourier transform images are shown in the inset. Please note that some spatial drift is present. Bottom panels present the GTO crystal structure along the $\langle 101\rangle$ direction (c) and the $\langle 001\rangle$ directions (d).

Figure 2

(a) Low-magnification HAADF image of the GTO crystal edge. The blue rectangle represents the region where EEL spectra at the Ti $L_{2,3}$ edge (b) are extracted. The data are shown as symbols and in the fine lines are linear combination of simulations obtained from multiplet calculations. The proportion of ${\mathrm{Ti}}^{4+}$ is shown in (c). A XAS spectrum measured at 10 K is displayed in (d) and its simulation using experimental 3+ and 4+ spectra is presented in (e).

Figure 3

(a) Magnetic field dependence of the magnetization at 4 K. (b) Temperature dependence of the magnetization measured with a field of 0.01 T. XAS at the $\mathrm{Ti}{L}_{3,2}$ (c) and Gd $M_{5,4}$ (f) edges recorded in total electron yield (TEY) for right (dark colors) and left (light colors) circularly polarized light. The incoming light is 45°, with respect to the normal incidence of the film. An in-plane magnetic field of 1 T is applied. Corresponding XMCD signals acquired at the Ti L edge (d) and the Gd M edges (g) with the associated integrated signal (e), (h).

Figure 4

(a) X-ray diffraction 2θ-ω spectrum of a 60 nm GTO film grown on LAO(001). Inset: RHEED diagram at the end of the growth along a [100] pseudocubic direction. ϕ scans of the (202) reflections of LAO (b) and GTO (c) on the same sample. XAS at room temperature in TEY (d), TFY (e) and XEOL (f) in a 80-nm GTO film.

Figure 5

(a) Magnetic field dependence of the magnetization at 5 K. (b) Temperature dependence of the magnetization in a field of 0.1 T. XAS at 5 T (c), XMCD (d) and XMCD integral (e) at the $\mathrm{Ti}{L}_{3,2}$ edge measured in TFY. XAS at 5 T (f), XMCD (g) and XMCD integral (h) at the $\mathrm{Ti}{L}_{3,2}$ edge measured in XEOL. (i) XAS and (j) XMCD simulations for a ${\mathrm{Ti}}^{3+}$ ion in the GTO structure using the MultiX code. (k) Simulated XMCD integral.