Introduction

Due to the discontinuity in degrees of freedom such as lattice, charge, spin and orbital, the interface can show unique property that cannot be found in bulk materials1. One well-known example is the LaAlO3/SrTiO3 (LAO/STO) interface2, which can be tuned to exhibit conducting and magnetic properties, characterized by the two-dimensional electron gas (2DEG)2,3,4 accompanied with Kondo effect5 or ferromagnetic state6,7,8,9. This magnetic 2DEG has also been applied in electronic device such as magnetic tunnel junction10. Although this novel interface magnetism has been observed by different techniques5,6,7,8,9,10,11,12, some contradicting results were found in recent studies. For examples, the magnetic moment density can vary from 10−3 μB13 to 0.3 μB per unit cell7 and persist up to room temperature6,11. The data from magnetotransport5 and superconducting quantum interference device (SQUID)6 showed that the magnetic interaction is enhanced by increasing an oxygen pressure during the sample growth, or reducing oxygen vacancy. However, the study on X-ray absorption spectroscopy (XAS) clearly demonstrated that formation of the oxygen vacancy is crucial for ferromagnetism at the interface14. In order to resolve these inconsistencies, the interplay between itinerant electrons and local magnetic moments is emphasized15,16,17,18,19. The different types of magnetic interaction can be established at the LAO/STO interface by changing parameters such as carrier density17,18 and/or oxygen vacancy density19, leading to the variation in magnetic moment density and contradicting results as reported5,6,7,13,14. In this scenario15,16,17,18,19, the itinerant electrons can be provided by 2DEG. However, the origin of the local magnetic moment is still unclear and believed to be related to several factors such as localized electrons at Ti 3d orbitals9,14,15,18, cation antisite defect20 and oxygen vacancies21,22,23. Hence, more information is needed for better understanding of the magnetic origin, especially the nature of localized electrons exhibiting local magnetic moments at the interface between nonmagnetic oxides.

Several studies have shown that the lattice-mismatch-induced interface strain can tune the electronic structure in the SrTiO3-based systems, affecting the transport properties24,25,26 and interface magnetism27,28. In order to investigate the influence of lattice mismatch on the observed magnetism at the interface, we compare the LAO/STO interface with another oxide interface (La0.3Sr0.7)(Al0.65Ta0.35)O3/SrTiO3 (LSAT/STO), which has been proved to be able to be conducting by Huang et al.29. Both LAO and LSAT are band insulators which have bandgap larger than STO. The crystal structure of LAO and LSAT follows the perovskite ABO3-type lattice, with the polar AO/BO2 layer alternatively stacked along [100] axis. Moreover, both of them can induce 2DEG on STO substrate29. Considering the (pseudo)cubic lattice constant for LAO, LSAT and STO of 3.792 Å, 3.868 Å and 3.905 Å, respectively, the lattice mismatch is 3.0% at the LAO/STO interface and 1.0% at LSAT/STO.

In this paper, we present results from temperature-dependent and magnetic-field-dependent transport as well as X-ray absorption near edge structure (XANES) studies obtained for the LAO/STO and LSAT/STO interfaces. LAO and LSAT film are grown with various oxygen partial pressures PO2 (0.05–5 mTorr). Our data show that when PO2 is beyond 1 mTorr, the 26 unit cells (uc) LAO/STO interface begins to exhibit Kondo-like scattering, characterized by the resistance upturn (around 40 K) followed by the resistance saturation with negative isotropic magneto-resistance at low temperatures. In contrast, the LSAT/STO interfaces can always maintain the low-temperature metallicity when PO2 is increased up to 100 mTorr. The XANES studies performed at Ti L32-edge show that the Ti3+/Ti4+ ratio is larger at the LAO/STO interface, compared to the LSAT/STO interface. The Ti3+/Ti4+ ratio obtained from XANES should be regarded as a total number of electrons that occupy the Ti 3d orbitals, including the itinerant and localized electrons. Considering the similar itinerant carrier density for LAO/STO and LSAT/STO interfaces, the larger amount of localized Ti3+ ions the LAO/STO interface can be spin-polarized and scatter the mobile electrons, leading to the observed Kondo-like features.

Results

Temperature-dependent transport property

The data illustrating temperature-dependent sheet resistance RS(T) for 26 uc LAO/STO and LSAT/STO interfaces with different PO2 are shown in Fig. 1. For the LAO/STO interface in Fig. 1(a), the low-temperature metallic state (dRS /dT > 0) is preserved in the samples with PO2 below 1 mTorr, suggesting a normal 2DEG is established at the interface. When PO2 is above 1 mTorr, the LAO/STO interfaces show a clear upturn of sheet resistance (dRS /dT < 0) below 40 K and RS becomes gradually saturated (dRS /dT ≈ 0) under further cooling. These features are different from a normal 2DEG with the low-temperature metallic state. However, for the 26 uc LSAT/STO interface in Fig. 1(b), the metallic state of 2DEG can be always maintained down to 2 K when PO2 is changing from 0.05–50 mTorr. Only a slight resistance upturn at ~15 K can be observed when PO2 is increased to 100 mTorr. Hence, when increasing the PO2, the metallic state is less favored at the interface with the lager lattice mismatch.

Figure 1
figure 1

Temperature dependence of sheet resistance RS(T).

(a) Rs(T) curves for 26 uc LAO/STO with PO2 from 0.05–5 mTorr. (b) Rs(T) curves for 26 uc LAO/STO with PO2 from 0.05–100 mTorr. Inset: the schematic view of 2DEG exists at the LAO/STO (left) and LSAT/STO (right) interfaces with different lattice mismatch.

Full size image

Usually, the upturn of sheet resistance is caused by either carrier scattering with low carrier mobility μS, or carrier localization with low carrier density nS. In Fig. 2(a), all the LAO/STO interfaces show the decreasing nS from 100 to 2 K, probably due to the localization of the oxygen-vacancy-induced carriers as reported in the SrTiO3-δ30,31. However, carrier localization alone cannot explain resistance upturns in Fig. 1(a). All the samples studied here, exhibit a similar low-temperature nS (2–3 × 1013 cm−2 at 2 K) independent on PO2 and it is in contradiction with the resistance upturn, which has been observed only for the sample with high PO2. On the other hand, for carrier mobility μS in Fig. 2(b), the LAO/STO interfaces with high PO2 (1 and 5 mTorr) show the decreasing μS below 40 K on cooling, while the increasing μS during cooling is observed at the interfaces with low PO2 (0.05–0.5 mTorr). This data is consistent with the appearance of resistance upturn (metallic state) at the high-PO2 (low-PO2) interface. So, the upturns of RS(T) must be ascribed to the carrier scattering, instead of carrier localization.

Figure 2
figure 2

Carrier density nS and carrier mobility μS.

(a) nS(T) and (b) μS(T) curves for 26 uc LAO/STO interface prepared with PO2 from 0.05–5 mTorr. (c) nS(T) and (d) μS(T) curves for 26 uc LSAT/STO interface prepared with PO2 from 0.05–5 mTorr. (e) μS as a function of PO2 for both interfaces. The blue and orange lines are guides to the eye, representing LSAT/STO and LAO/STO interfaces, respectively.

Full size image

Figure 2(c) presents the nS as a functional of temperature at the LSAT/STO interfaces. As can be seen the trend is similar to that of the LAO/STO samples. In particular, if compared with the LAO/STO interface, the LSAT/STO interfaces prepared at PO2 = 0.05−0.5 mTorr exhibit a larger decrease of carrier density at low temperature, indicating the low PO2 could create more oxygen vacancies at the LSAT/STO interface than at the LAO/STO. The carrier mobility μS of all the LSAT/STO samples is increasing under cooling, as shown in Fig. 2(d). At the interface with a small lattice mismatch, e.g. LSAT/STO, μS is always higher as compared to the interface with a large lattice mismatch, e.g. LAO/STO. Moreover, even though the carrier density nS at 2 K is almost independent of PO2, the carrier mobility μS at 2 K is very sensitive to PO2 at both LAO/STO and LSAT/STO samples. As shown in Fig. 2(e), when PO2 is increased from 0.05–5 mTorr, μS at 2 K is reduced by factor of 300 for the LAO/STO (from 1,000 to 3 cm2V−1s−1) and 15 for the LSAT/STO (from 23,000 to 1,500 cm2V−1s−1) interface, respectively. For the low-PO2 interfaces, the high carrier mobility might be due to the oxygen vacancy formation in the STO bulk. Comparison of the LAO/STO and LSAT/STO interfaces properties at different PO2, reveals that: 1) the resistance upturn is caused by carrier scattering with low μS and 2) μS is more sensitive to PO2 at the interface with a larger lattice mismatch.

Magnetotransport property

In Fig. 3, the low-temperature (T = 2 K) magneto-resistance, defined by MR = [R(H)-R(0)]/R(0), is shown for the LAO/STO and LSAT/STO interfaces with different PO2. The positive MR is observed in all the samples except the LAO/STO sample with PO2 = 5 mTorr. The LSAT/STO interface always exhibits the larger positive MR than LAO/STO interface with the same PO2. For both interfaces, the magnitude of positive MR is consistently reduced with increasing PO2 value. The positive MR is induced by the Lorentz-force-driven helical path for mobile carriers32 and it can be enhanced by increasing μS33,34. This is consistent with our observation that the larger positive MR appears at the higher mobility interface, of which the lattice mismatch is smaller and PO2 is lower. However, the negative MR at the LAO/STO sample with PO2 = 5 mTorr is out of this picture, since the Lorentz force alone cannot induce the negative MR. The strong spin-orbit coupling may induce the large negative MR35, but it does not correlate with the RS(T) data for the LAO/STO interface with PO2 = 5 mTorr. Two reasonable mechanisms can be proposed to explain the upturn in RS(T) and negative MR–one is the spin-related Kondo scattering5,15,36 and the other is orbital-related weak anti-localization37,38,39.

Figure 3
figure 3

PO2-dependent MR with field perpendicular to the 2DEG plane at 2 K.

The MR curves for (a) LAO/STO and (b) LSAT/STO interfaces with different PO2. The inset shows the applied magnetic field H perpendicular to the 2DEG plane.

Full size image

In order to distinguish these two different mechanisms, the MR curves with different field orientations are shown in Fig. 4(a) for the LAO/STO interface with PO2 = 5 mTorr. The sample exhibits no observable difference in MR curves with changing the field orientation and MR (H = 9 T) is always negative. This isotropic and negative MR confirms the spin-related Kondo-like scattering for the resistance upturn5,15,36. On the other hand, the metallic LSAT/STO interface exhibits the clear anisotropic MR, as shown in Fig. 4(b). The positive MR is gradually suppressed by increasing the angle θ between the sample normal and field direction. Moreover, the negative MR appears when the in-plane field (θ = 90°) is applied. The angular-dependent MR, which is defined by AMR = [R(θ)-R(90°)]/R(90°) in Fig. 4(c), clearly presents the isotropic MR at the 5 mTorr LAO/STO interface, medium anisotropic MR at the 0.05 mTorr LAO/STO and 5 mTorr LSAT/STO interface and strong anisotropic MR at the 0.05 mTorr LSAT/STO interface. This suggests the AMR can be enhanced by lowering PO2 and reducing lattice mismatch.

Figure 4
figure 4

The AMR behavior for both interfaces with different PO2.

The MR curves with different θ at 2 K for (a) LAO/STO and (b) LSAT/STO interfaces with PO2 = 5 mTorr. The angle θ lies between the magnetic field and the normal of the interface, as shown in the inset of Fig. 4(a). (c) The AMR curves for both interfaces with PO2 = 0.05 and 5 mTorr.

Full size image

X-Ray absorption near edge structure (XANES)

In order to clarify the origin of the Kondo-like scattering at the LAO/STO interface, Ti L32-edge XANES spectra are compared in Fig. 5(a) for TiO2-terminated STO substrate (t-STO, reference for substrate), Ti2O3 (reference for Ti3+), 10 uc LAO/STO and LSAT/STO interfaces with PO2 = 5 mTorr. The XANES is a powerful tool to examine the low-density Ti3+ ions under a strong Ti4+ background40. As can be seen, the LAO/STO interface exhibits a higher intensity around Ti3+ states (see reference Ti2O3 spectrum) peaks denoted by red dash line as compared with LSAT/STO. Moreover, a linear combination fit analysis based on t-STO and Ti2O3 reference spectra revealed a Ti3+/Ti4+ ratio in a range of ~10% for the LAO/STO interface. In contrast, linear combination fit analysis for the LSAT/STO sample results in a negligible Ti3+/Ti4+ ratio of about 1% which is below the uncertainty range of XANES. Here we want to stress that Ti3+/Ti4+ ratio obtained from XANES should be proportional to the total number of electrons that occupy the Ti 3d orbitals, including the mobile 2DEG and the localized Ti3+ ions. Given that both interfaces show similar nS (3−4 ×1013  cm−2 from Hall measurement) of mobile 2DEG at room temperature, the larger amount of localized Ti3+ ions is expected at the LAO/STO interface. One localized Ti3+ ion can provide one unpaired electron, which can be spin-polarized and provide the local magnetic moment to scatter the mobile 2DEG at low temperatures, leading to the Kondo-like scattering at the LAO/STO interface.

Figure 5
figure 5

The X-ray absorption near edge structure (XANES) and possible 2DEG location with respect to localized Ti3+ ions.

(a) XANES for t-STO and Ti2O3 for reference (top), 10 uc LSAT/STO and LAO/STO interface with PO2 = 5 mTorr (middle) and residual values of both interfaces after being fitted by linear combination between t-STO and Ti2O3 (bottom). The red dashed lines indicate the peak positions for Ti3+. (b) The spatially-separated localized Ti3+ ions and mobile 2DEG at the metallic interface, where the spin scattering from the localized Ti3+ is weak. (c) The localized Ti3+ ions overlap the mobile 2DEG, resulting in the strong spin scattering and Kondo-like interface.

Full size image

Discussions

Our transport data demonstrate that the Kondo-like scattering is induced at the LAO/STO interface with high PO2, but not at the LSAT/STO interface. The XANES analysis and Hall measurement identify a large amount of localized Ti3+ ions at the LAO/STO interface, where the itinerant 2DEG can be scattered by the localized Ti3+ ions with local magnetic moments to show the Kondo-like effect. However, there are still two questions needed to be addressed in our discussion. The first is why there are more localized Ti3+ ions at the LAO/STO interface; the second is why PO2 can influence the Kondo-like scattering.

For the first question, the different lattice mismatch at the LAO/STO and LSAT/STO interfaces is emphasized. As well documented, most of the localized electrons are located near the interface, where the interface disorders can lift the mobility edge for Anderson localization41,42,43,44. However, the interface disorders such as cation antisite defect20 and oxygen vacancies21,22,23 that may induce local magnetic moment should be at the same level for both interfaces, because the LAO/STO and LSAT/STO interfaces were fabricated under the same condition including laser energy, growth temperature and oxygen pressure. By contrast, the interface lattice distortion, especially for the STO layer that is close to the interface, must be much larger at the LAO/STO interface than the LSAT/STO interface due to the larger lattice mismatch and symmetry breaking at the LAO/STO interface. Such lattice distortions including the tetragonal-like TiO6 deformation25 and octahedral tilting27,45 would narrow the Ti 3d band, resulting in electron localization and magnetic interface27,28. Hence, when increasing the lattice mismatch from LSAT/STO to LAO/STO interface, the larger structural distortion is expected to produce more localized Ti3+ ions and stronger Kondo-like scattering.

Regarding the influence of PO2, calculations have shown that the Kondo effect is observable with low density of oxygen vacancy (high PO2), if the oxygen vacancy interacting with Ti 3d orbitals it can induce local magnetic moments19,21,22. Here, we argue that PO2 can also tune the location of itinerant electrons, resulting in a stronger Kondo-like scattering for the higher PO2. When PO2 is low, not only the interface but also the bulk region of the STO substrate become conducting due to the oxygen vacancy. In this case, the conductive bulk region of STO could weaken the confinement potential of the interface electrons, so the itinerant electrons can travel away from the interface where the localized Ti3+ ions are located41,42,43,44, leading to a weaker magnetic scattering. Therefore, the mobile electrons are spatially separated from the localized Ti3+ as shown in Fig. 5(b) and the spin-relate Kondo-like scattering from localized Ti3+ is very weak. When PO2 is increasing, the propagation depth of mobile carriers in the STO substrate is greatly reduced46. In other words, by increasing PO2 the mobile electrons are pushed to the interface with a better confinement47. So, as schematically shown in Fig. 5(c), the itinerant electrons are much closer to the localized Ti3+ ions and the stronger interaction between itinerant carriers and localized Ti3+ are expected. It leads to the Kondo-like features, including resistance upturn and saturation, low carrier mobility and isotropic negative MR, which are observed at the sample with increasing PO2. This model can also explain the AMR behavior at the metallic interface as shown in Fig. 4(b). By applying an in-plane magnetic field, the Lorentz force will drive the mobile carriers along the 2DEG normal to interact with the localized Ti3+ close to the interface. So, when the magnetic field is changed from out-of-plane to in-plane (θ from 0°–90°), the spin-relate scattering arising from the localized Ti3+ begins to take effect to suppress the positive MR and eventually show the negative in-plane MR.

Conclusions

In summary, the crucial roles of lattice mismatch and growth oxygen pressure in Kondo-like effect has been demonstrated by comparing LAO/STO and LSAT/STO interfaces. For the LAO/STO interface with 3.0% lattice mismatch, the Kondo-like effect appears in the 26 uc sample when PO2 is above 1 mTorr. For the LSAT/STO interface with 1.0% lattice mismatch, the metallic state is always preserved up to PO2 of 100 mTorr. From the XANES and Hall measurement, a larger amount of the localized Ti3+ is identified at the LAO/STO interface compared to the LSAT/STO interface. Those localized Ti3+ ions can be spin-polarized and scatter the mobile electrons, leading to the observed Kondo-like features. Our results demonstrate that the Kondo-like effect at the SrTiO3-based interface can be dually-controlled by lattice mismatch and PO2, paving the path for engineering the interface magnetism at the functional oxide heterostructures.

Methods

Sample fabrication

26 unit cells (uc) of LAO and LSAT layers were deposited onto a TiO2-terminated STO (001) substrates by pulsed laser deposition using a KrF laser (λ = 248 nm). The LAO and LSAT single crystal targets are used for deposition. During the deposition, the laser repetition is kept at 1 Hz, laser fluence at 1.8 J/cm2, growth temperature at 760 °C and PO2 varies from 0.05–100 mTorr. The deposition is monitored by in-situ reflection high energy electron diffraction (RHEED), from which the growth rate of 22–24 seconds per unit cell can be seen.

Magnetotransport measurements

The Hall bar is patterned on samples for measuring the transport property. The length of bridge is 160 μm and the width is 50 μm. The transport property measurements were conducted in Physical Property Measurement System (Quantum Design, PPMS).

X-Ray absorption near edge structure (XANES) measurements

The XANES data have been recorded for the 10 uc LAO/STO and LSAT/STO interface with PO2 = 5 mTorr. The thickness is chosen at 10 uc to guarantee the access to the interface during XANES measurements at the Ti L32-edge. The XANES spectra were collected at the SINS beam-line at the Singapore Synchrotron Light Source (SSLS). To avoid possible contamination and surface modification, experiments were performed in UHV chamber with a background pressure of about 2×10−10 mbar. All XANES spectra presented here were recorded ex-situ and at X-ray incident angle of 90° using total electron yield (TEY) mode.

Additional Information

How to cite this article: Han, K. et al. Controlling Kondo-like Scattering at the SrTiO3-based Interfaces. Sci. Rep. 6, 25455; doi: 10.1038/srep25455 (2016).