Direct observation of multivalent states and $4f\to 3d$ charge transfer in Ce-doped yttrium iron garnet thin films

Due to their large magneto-optic responses, rare-earth-doped yttrium iron garnets, ${\mathrm{Y}}_3\mathrm{F}{\mathrm{e}}_5{\mathrm{O}}_{12}$ (YIG), are highly regarded for their potential in photonics and magnonics. Here, we consider the case of Ce-doped YIG (Ce-YIG) thin films, in which substitutional $\mathrm{C}{\mathrm{e}}^{3+}$ ions are magnetic because of their $4f^1$ ground state. In order to elucidate the impact of Ce substitution on the magnetization of YIG, we have carried out soft x-ray spectroscopy measurements on Ce-YIG films. In particular, we have used the element specificity of x-ray magnetic circular dichroism to extract the individual magnetization curves linked to Ce and Fe ions. Our results show that Ce doping triggers a selective charge transfer from Ce to the Fe tetrahedral sites in the YIG structure. This, in turn, causes a disruption of the electronic and magnetic properties of the parent compound, reducing the exchange coupling between the Ce and Fe magnetic moments and causing atypical magnetic behavior. Our work is relevant for understanding magnetism in rare-earth-doped YIG and, eventually, may enable a quantitative evaluation of the magneto-optical properties of rare-earth incorporation into YIG.

Figure 1

(a) Schematic diagram of the YIG crystal structure. (b) Fe cations at $a$ and $d$ sites are connected to each other by a common corner of the oxygen anions. (c) FMR spectra of the (001)- and (111)- oriented Ce-YIG films for the frequencies $f=5$, 11, and 17.5 GHz. (d) The FMR peak linewidths as a function of the resonance frequencies. Straight lines are the best fits based on Eq. (2) given in the text.

Figure 2

Ce-${\mathrm{M}}_{4,5}$ XAS and XMCD of (a) Ce-YIG(001) and (b) Ce-YIG(111) thin films at 300 K. The spectral features of mixed ${\mathrm{Ce}}^{3+}\left(f^1\right)$ and ${\mathrm{Ce}}^{4+}(f^0,f^1\mathrm{L})$ ground state contributions are well understood from the experimental ${\mathrm{Ce}\text{−}\mathrm{M}}_{4,5}$ XAS data extracted from Ref. [30], given in (c). The XMCD $({\sigma }^{-}-{\sigma }^{+})$ in panels (a) and (b) arises mainly from the ${\mathrm{Ce}}^{3+}$ contribution. (d) The ${\mathrm{Ce}}^{3+}$ XAS was separated out by fitting the experimental data to the extracted data. Panel (e) shows integration of the ${\mathrm{Ce}}^{3+}$ XAS and XMCD spectra. A detailed sum-rule analysis is given in the text.

Figure 3

Fe-${\mathrm{L}}_{2,3}$ XAS and XMCD of (a) YIG(111), (b) Ce-YIG(001), and (c) Ce-YIG(111) thin films at 300 K. Total XAS $({\sigma }^{-}+{\sigma }^{+})$ in the inset (a) reveals that the YIG film has only ${\mathrm{Fe}}^{3+}$ while the Ce-YIG films exhibit an additional ${\mathrm{Fe}}^{2+}$ contribution. The A/B peak ratio along with the growing $L_2$ pre-edge feature (shown by arrows) hints at the presence of ${\mathrm{Fe}}^{2+}$ in the films. Panel (d) shows the calculated XMCD spectra for $a\text{−}{\mathrm{Fe}}^{3+},d\text{−}{\mathrm{Fe}}^{3+}$, and $d\text{−}{\mathrm{Fe}}^{2+}$ contributions. From this, it appears that the $\mathrm{Fe}\text{−}{\mathrm{L}}_3$ XMCD of YIG film has two positive peaks for $a$ sites and a large negative peak for $d$ sites. Panel (e) shows a growing extra XMCD peak (shown by arrow) that confirms the ${\mathrm{Fe}}^{2+}$ in the Ce-YIG films. Panel (f) shows the integration of Fe-XAS and XMCD spectra of YIG film. Please refer to the text for the sum-rule analysis.

Figure 4

(a) Out-of-plane XMCD (symbols) and model (solid lines) magnetization curves of $d$ and $a$ sites of YIG(111) film given in the upper and lower side of the panel, respectively. For the sake of clarity, only the positive field range is plotted. The evolution of magnetizations in the $d$ and $a$ sites $({\mathbf{M}}_{\mathit{d}}$ and ${\mathbf{M}}_{\mathit{a}}$, respectively) is given for the ${\mathbf{M}}_{\mathit{d}}/{\mathbf{M}}_{\mathit{a}}$ antiparallel coupling $\left(J_{d/a}\right)$ in two cases: $\left(\mathrm{i}\right)$ $J$ (top sketches) for YIG film and $\left(\mathrm{ii}\right)$ $J/3$ (bottom sketches) for the Ce-YIG films. The solid lines in the XMCD hysteresis data of (b) Ce-YIG(001) and (c) Ce-YIG(111) films are given for a guided view. Panels (d) and (e) represent the simulated magnetization loops for (001)- and (111)-oriented Ce-doped YIG films, respectively.

Figure 5

Schematic diagram of the energy levels at $\mathrm{Fe}\left(3d^6\right)\text{−}\mathrm{O}\text{−}\mathrm{Fe}\left(3d^5\right)$ superexchange coupling. $\theta$ is the angle between the $d\text{−}\mathrm{Fe}$ and $a$-Fe sites. The $d^5\text{−}{d}^5$ coupling is a strong antiferromagnetic while the spin-down electron of $d^6$ weakens the coupling by freely moving to empty spin-up states of $d^5$ orbitals.