Abstract
Ionic substitution forms an essential pathway to manipulate the structural phase, carrier density and crystalline symmetry of materials via ion-electron-lattice coupling, leading to a rich spectrum of electronic states in strongly correlated systems. Using the ferromagnetic metal SrRuO3 as a model system, we demonstrate an efficient and reversible control of both structural and electronic phase transformations through the electric-field controlled proton evolution with ionic liquid gating. The insertion of protons results in a large structural expansion and increased carrier density, leading to an exotic ferromagnetic to paramagnetic phase transition. Importantly, we reveal a novel protonated compound of HSrRuO3 with paramagnetic metallic as ground state. We observe a topological Hall effect at the boundary of the phase transition due to the proton concentration gradient across the film-depth. We envision that electric-field controlled protonation opens up a pathway to explore novel electronic states and material functionalities in protonated material systems.
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Introduction
Strong electron correlation and interplay between multiple degrees of freedom (charge, spin, orbital and lattice) give rise to a variety of fascinating electronic and magnetic phases in transition metal oxides, such as ferromagnetism, superconductivity, other charge (or spin) ordered states, etc1,2,3,4. Due to the extreme sensitivity of these systems to external stimuli, the ability to control versatile functionalities can achieve unique physical phenomena5,6. Among other oxides, SrRuO3 forms a fascinating material system for its interesting electronic and ferromagnetic properties4. Along the studies, the electric-field control of its magnetism, called magnetoelectric coupling7,8,9, has obtained particular research interests due to its associated intriguing physics and potential applications. Despite extensive explorations of dielectric10,11, ferroelectric12,13, and ionic liquid14,18,19,20. Furthermore, some recent results demonstrated nicely the electric-field controlled THE in ultrathin SrRuO3 films through the dielectric and ferroelectric modulations11,13, although the resultant effect remains subtle, reminiscent of its electric-field controlled magnetic state. Clearly, the distinct and rich magnetic transition correlated to the carrier density and inversion symmetry makes SrRuO3 a perfect model system to explore the electric-field controlled electronic and magnetic phase diagram, which might trigger a wide range of device applications.
Here, we demonstrate an efficient and reversible tunability of both the structural and electronic phase transformations within SrRuO3 thick film through electrically controlled protonation during the ionic liquid gating. With increasing protonation concentration in this compound, the ferromagnetism was gradually suppressed, and eventually we discover a novel protonated compound of HSrRuO3, which shows an exotic paramagnetic metallic ground state. In addition, a pronounced tunable THE is observed near the boundary of the phase transformation, which suggests an effective strategy to design THE in this compound.
Results
Gate tunable structural transformation via proton evolution
Our experiments were performed on high quality epitaxial SrRuO3 films grown on SrTiO3 (001) substrates by pulsed laser deposition (see Methods section). To explore the tunability of electrically controlled protonation, we first performed an in-situ X-ray diffraction (XRD) measurements during the ILG, in which a positive voltage would drive the positively charged protons into the film. Figure 1a shows the gate voltage (VG) dependent θ–2θ scans around the SrRuO3 (002) peak, in which the SrRuO3 (002) peak shows no obvious shift with VG up to ~1.8 V, while a further increase of VG leads to a clear shift of the peak position to a lower angle (from 45.90° to 44.35°). This result suggests a large out-of-plane lattice expansion up to 3.3% for the SrRuO3 film, which is comparable with our recent result of pronation induced phase transformation from SrCoO2.5 to HSrCoO2.521. It is interesting to note that when the gate voltage is removed, the diffraction peak returns nearly back to the original position with a slight offset, and afterwards the phase transformation can be reversibly and reproducibly controlled with the application of positive VG (3.5 V) and zero voltage (Fig. 1b). Importantly, the in-plane lattice constants and crystalline quality of the film remain unchanged throughout ILG as evinced by the reciprocal space map**, rocking curves, and reflectivity measurements (Supplementary Fig. 1). Notably, the structural transformation possesses a threshold gating voltage, as well as a clear voltage dependence with higher voltage corresponding to shorter transition time (Supplementary Fig. 2), which is consistent with the diffusion model suggested by previous studies21,22,23.
As our film thickness is much larger than the screening length associated with electrostatic gating, we can readily single out the ionic (H+ or O2−) evolution as the dominant mechanism for the observed structural phase transformations. To identify the type of ion responsible for the phase transformation, ex-situ secondary-ion mass spectrometry (SIMS) was performed, as shown in Fig. 1c and Supplementary Fig. 3a. The result shows considerable numbers of protons distributed in the SrRuO3 film associated with a structural transformation after being gated at VG = 3.5 V, but not in the pristine sample or in the film gated at 1.5 V. On the other hand, the 18O isotopic calibration measurements24 suggest that the oxygen ion evolution is negligible for the gated samples (Supplementary Fig. 3b). Therefore, we can conclude that the ILG induced structural transformation is strongly associated with the protonation evolution. Finally, the reason why protons leave the sample at zero voltage should be attributed to the phase instability of HxSrRuO3. Although some protonated phases (e.g., HxSrCoO2.5) are nonvolatile and can remain stable in air even with the gating voltage turned off21, other protonated materials (e.g., HxWO3) do possess the volatile nature with a reversible phase transformation back into almost pristine state when the gate voltage is removed22. In the latter case, the detected hydrogen signal is attributed to the residual portion of protons in the film.
Crucially, the presence of positively charged protons are known as electron donor within the materials21,22. To trace the associated valence state evolution in Ru, we performed in-situ hard X-ray absorption experiments near Ru K-edges for both pristine and protonated (with a gate voltage of 3.5 V) samples, as shown in Fig. 1d, along with two referenced compounds (RuO2 and Ru metal)25. Clearly, a significant energy shift towards lower energy region was observed with respect to that of the pristine state, suggesting the reduction of the Ru valence state from +4 to +3 due to the electron do** associated with protonation.
Reversible control of magnetism through proton evolution
Since the phase transformation can be gradually controlled during ILG, the current study provides a unique opportunity to investigate the evolution of electronic state in SrRuO3 through protonation. Figure 2a shows the temperature dependent resistivity ρXX(T) for SrRuO3 with different VG during ILG, in which the thin film remains metallic throughout the gating. However, a careful analysis reveals that the kink feature, which can be observed at ~160 K (Curie temperature TC) for the pristine sample, gradually smooths out and eventually disappears (inset of Fig. 2a). These results suggest a possible suppression of ferromagnetism during ILG, as the kink feature is a typical characteristic for ferromagnetism in SrRuO3. This magnetic transition can also be observed in the magnetoresistance (MR = ρXX(H)/ρXX(0)−1) measurements, as shown in Fig. 2b. As VG increases, the typically negative butterfly-like MR gradually decreases, and more interestingly, with the gating voltage of 2.5 V, we observed a positive parabolic MR, representing a conventional paramagnetic metallic state.
To clearly investigate the evolution of the ferromagnetic state in SrRuO3 under ILG, we measured the magnetic-field dependent Hall resistivity at different VG. The pristine SrRuO3 film exhibits a well-defined hysteresis loop attributed to the anomalous Hall effect (AHE) associated with the ferromagnetic state. As VG increases, the hysteresis loop (at 2 K) is gradually suppressed and eventually turns into a linear response with VG of 2.5 V, as shown in Fig. 2c. Figure 2d summarizes the AHE resistivity (extracted at μ0H = 0 T) at different temperatures under various VG, in which the anomalous Hall resistivity gradually decreases and eventually disappears with the increase of VG. Furthermore, the electron carrier density increases by about 2.61 × 1022 cm−3 from pristine to the 2.5 V gated sample (insert in Fig. 2c), which is consistent with the change of Ru valence state from +4 to +3 (corresponding to 1.65 × 1022 cm−3), and such a significant increase of carrier density further supports the scenario that the intercalated hydrogen serves as an effective electron donor into SrRuO3 system. It is interesting to note that the diffusion process would also be strongly correlated with the gating temperature, leading to a dramatically different magnetic state for the sample gated at different temperature (Supplementary Fig. 4).
The magnetic evolution during ILG was also studied with the in-situ magneto-optic Kerr effect (MOKE) measurements, which measures the ac inter-band Hall conductivity and has the same origin as the intrinsic AHE (i.e., the anomalous velocity due to the Berry curvature in momentum space26). Similar to the Hall measurements, as VG increases the square-like MOKE hysteresis loop is gradually suppressed and eventually disappears (Fig. 2e, f), indicating that the ferromagnetism is indeed weakened by the ILG induced protonation. Furthermore, the element-specific X-ray magnetic circular dichroism (XMCD) measurements at the Ru L3,2 edges clearly show the suppression of ferromagnetism in Ru ions in the protonated sample (Supplementary Fig. 5). Undoubtedly, all these experimental observations provide strong evidences that the protonated HxSrRuO3 sample undergoes an exotic ferromagnetic to paramagnetic phase transition with the protonation induced electron modulation. More importantly, we revealed a novel protonated compound of HSrRuO3 with paramagnetic metallic as ground state. Similar to the structural transformation, the modulation of the ferromagnetic state is also reversible when cycling VG (Supplementary Fig. 6). The slight suppression of AHE signal (as well as the magnetization) after removing the gating voltage as compared to the pristine samples (Supplementary Fig. 6 and Supplementary Fig. 7) should be attributed to the residual protons previously observed in the structural modulation and SIMS measurements (Supplementary Fig. 1b, c).
To shed more light on the protonation induced magnetic transition, we carried out first-principles calculations (see Methods section). The optimized crystalline structure for HSrRuO3 is shown in the inset of Fig. 1a, in which the proton is bonded with the equatorial oxygen of Ru octahedral as the ground state, while its bonding with apical (or mixed equatorial and apical) oxygen would lead to higher system energy (Supplementary Fig. 8). Figures 3a, b present the calculated non-spin-polarized band structures for pristine and protonated HSrRuO3 samples, respectively. Clearly, the proton intercalation leads to a dramatically modified density of states (DOS) due to the significant splitting of the degenerated Ru t2g bands and shift of spectra weight toward lower energy. As shown in Fig. 3c, although the spin-resolved DOS shows significant splitting of majority (down) and minority (up) bands in pristine SrRuO3, the corresponding DOS in protonated HSrRuO3 shows a nearly equivalent spectral weight, indicating the absence of ferromagnetism in the latter. It has been established that the metallic ferromagnetism of SrRuO3 can be described within the framework of Stoner model4,27, in which the ferromagnetic ground state is favored when IN0 > 1, where I and N0 are the so-called Stoner factor and nonmagnetic DOS per spin at the EF, respectively. Accordingly, we calculated crystalline structures, as well as Stoner factors (and then IN0 value) for a series of protonated phases with different proton concentrations, as summarized in Fig. 3d. The results show that with increasing proton concentration, the lattice results in a dramatic expansion, being consistent with the XRD results, and the value of IN0 gradually decreases. According to the Stoner criterion, a non-magnetic (or paramagnetic) ground state would be favored for the case with IN0 < 1, therefore this theoretical calculation nicely explains our experimental observations of protonation induced ferromagnetic to paramagnetic transition in the SrRuO3 film.
As the protonation process can result in both electron do** and lattice expansion, we further calculated the Stoner parameters for cases with charge modulation and structural expansion independently involved, and the corresponding DOS results are summarized in Supplementary Fig. 9. In the former case, the Stoner parameter is 1.06 (1.09) for adding 0.5 (1.0) electron per Ru, indicating a rather stable ferromagnetic state. This calculation can also explain the reason why the ferromagnetism is so robust even for ultrathin SrRuO3 during the electrostatic gating10,11,12,14,\(\left( {\rho _{YX}^{\mathrm{T}}} \right)\) and characteristic fields (HC and HP) obtained at HxSrRuO3 gated with VG = 1.8 V. HC (black filled symbol) represents the coercive field and the HP (white open diamond) denotes the field where the topological Hall resistivity reaches its maximum.
It has been established that the SrRuO3 itself has a rather large spin-orbit coupling4, and recent studies demonstrate the breaking of inversion symmetry by inequivalent interfaces in ultrathin SrRuO3 system can result in large Dzyaloshinskii-Moriya (DM) interaction with the emergent THE11,13,16,17,18,20,30. Then one would immediately realize that our current system provides a suitable condition to manipulate the THE effect.
Interestingly, a close view of Fig. 2c reveals indeed that a distinct hump feature superposing on the AHE loop at VG 1.8 V (also see Fig. 4c). The fact that the in-situ Kerr signal exhibits only the conventional magnetization loops (Fig. 2e) as compared with the Hall measurements showing additional hump feature at the same temperature (Supplementary Fig. 11) suggests that this hump feature has a strong frequency dependence as observed by our recent studies31. We attribute the hump feature to the THE, as similar feature was widely observed in bulk MnSi32,33, EuO thin films34 and ultrathin SrRuO3 film11,13,16,17,18,20,30 as a hallmark of THE.
To quantitatively evaluate the THE, we estimated the topological Hall resistivity \(\rho _{YX}^T\) (as shown in Fig. 4c) by subtracting the AHE signal through linear fitting of the high field data. With this, we can construct a phase diagram for the topological Hall term \(\rho _{YX}^T\) in the T-H plane (Fig. 4d), showing that the THE clearly exists in a wide range of the T-H plane. Moreover, although the sign of the AHE component remains unchanged up to 100 K, the corresponding THE component changes sign with the increase of temperature. In particular, both positive and negative THE components can be identified at certain temperatures (e.g., 10 K) In addition, we confirm that the THE is driven by the magnetization reversal process due to the fact that the peak position of THE (HP) scales nicely with the coercive field (HC). We note that in previous studies of ultra-thin SrRuO3 film systems, the THE emerges as the consequence of the enhanced DM interaction, as well as reduced ferromagnetism due to the interface effect11,16,17. Similarly, in our system, the ILG induced large proton compositional gradient at the boundary of the ferromagnetic to paramagnetic phase transition would lead to an enhanced DM interaction as well as reduced ferromagnetism, and both favor the emergence of THE.
As it is demonstrated that the THE emerges during the ILG induced magnetic transition, one could expect that similar effect would also occur during the magnetic transition from paramagnetic to ferromagnetic phase. To confirm that, we performed in-situ AHE measurements during the proton out-diffusion process from a fully gated HSrRuO3 sample with the gating voltage turned off (Supplementary Fig. 12). The result clearly demonstrates that the AHE signal recovers gradually toward its pristine state as a function of dwell time, and more importantly a distinct THE emerges at the phase boundary.