Subtle Interplay between Localized Magnetic Moments and Itinerant

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Subtle Interplay between Localized Magnetic Moments and Itinerant Electrons in LaAlO3/SrTiO3 Heterostructures Hai-Long Hu,† Rong Zeng,† Anh Pham,† Thiam Teck Tan,† Zhigang Chen,‡ Charlie Kong,§ Danyang Wang,*,† and Sean Li*,† †

School of Materials Science & Engineering, The University of New South Wales, Sydney, New South Wales 2052, Australia School of Mechanical & Mining Engineering, The University of Queensland, Brisbane, Queensland 4072, Australia § Mark Wainwright Analytical Centre, The University of New South Wales, Sydney, New South Wales 2052, Australia ‡

ABSTRACT: Clarification of the role of magnetic ordering and scattering in two-dimensional electron gas has become increasingly important to understand the transport and magnetic behavior in the LaAlO3 (LAO)/SrTiO3 (STO) heterostructures. In this work, we report the sheet resistance of the LAO/STO heterostructures as functions of temperature, magnetic field, and field orientation. An unexpected resistance minimum was discovered at ∼10 K under a sufficiently high inplane magnetic field. An anisotropic magnetoresistance (MR) is clearly identified, indicating the presence of magnetic scattering which may be related to the interaction between itinerant electrons and localized magnetic moments in the LaAlO3/SrTiO3 heterostructures. It is believed that the high concentration of oxygen vacancies induced by the ultralow oxygen partial pressure during the deposition process plays a predominant role in the occurrence of the anisotropic MR. KEYWORDS: LaAlO3/SrTiO3 heterostructures, oxygen vacancy, anisotropic magnetoresistance, itinerant electrons, localized magnetic moments

1. INTRODUCTION The interface engineering between two insulating oxides of SrTiO3 (STO) and LaAlO3 (LAO) has attracted enormous interest due to the unique physical phenomena of the oxides such as tunable interfacial conductivity, superconductivity, and ferromagnetism.1−3 Although polar discontinuity,4 cation intermixing,5 oxygen vacancies,6 etc., have been considered to be responsible for these physical properties, the actual mechanisms are still under debate due to the complexity of this correlated system. An especially intriguing feature of this LAO/STO system is the existence of localized magnetic moments in the proximity of itinerant d orbital electrons, resulting in the coexistence of interesting phenomena. For instance, electronic phase separation was observed at the interface of LAO/STO due to the selective occupancy of interface sub-bands of the nearly degenerate Ti orbital in STO.7 In addition, there were also some reports on the coexistence of superconductivity and ferromagnetism at the interface of LAO/STO.3,8,9 Inhomogeneous magnetic and superconducting electron layers generated at different lateral puddles or electrons at different depths from the interface were proposed to explain these observed © XXXX American Chemical Society

phenomena. However, the key issue concerning whether and how the itinerant electrons interact with the localized magnetic moments remains unresolved. On the other hand, it was reported the itinerant electrons can be gate-tuned from light dxy bands to heavy dxz/dyz bands with preferred axes along crystalline directions. However, the preferred crystalline directionality may also arise from the localized magnetic moments.10 A feasible way to investigate the interaction of itinerant electrons and magnetic moments is to measure the resistance under the effect of a magnetic field. The previously reported possible magnetic ordering11 and prominent spin−orbit interaction12 have been evidenced by the anisotropic magnetoresistance (MR). In this work, we obtained a resistance minimum of the LAO/STO heterostructures at 10 K under the in-plane applied magnetic field. To understand the resistance minimum, the magnetic scattering was investigated through the analysis of anisotropic MR as a function of the strength and Received: February 4, 2016 Accepted: May 17, 2016

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DOI: 10.1021/acsami.6b01518 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. Eleven unit-cell LAO/STO grown under 5 × 10−8 Torr. (a) Schematic diagram of the LAO thin film deposited on the top of STO with small mismatch. Magnetic field orientation for the subsequent magnetoresistance measurement is also indicated (Ĥ refers to the magnetic field; φ refers to the orientation degree (φ = 0° is the out-of-plane direction, while φ = 90° is the in-plane direction), and J ̂ refers to the applied current). (b) RHEED patterns and oscillations of 11 uc LAO/STO. (c) XRD θ−2θ diffraction patterns for LAO/STO. (d) Atomic force microscopy (AFM) images showing the step-flow surface morphology of LAO/STO heterostructures.

Figure 2. (a) HRTEM image of 11 uc LAO/STO. (b) Electron diffraction pattern for the interface of LAO/STO in a [001] projection (the scale bar is 10 nm). pressure of 5 × 10−8 Torr using a single-crystal LaAlO3 target. The laser energy density was 1.275 J/cm−2, and the repetition rate was 0.5 Hz. The films were grown with real-time monitoring by in situ reflective high-energy electron diffraction (RHEED) to verify the layer-by-layer growth mode of the LAO thin films. Subsequently, the samples were cooled to room temperature in the oxygen partial pressure of deposition at a rate of 20 °C/min without a postannealing process. We also deposited an 11 uc LAO thin film under an oxygen partial pressure of 1 × 10−5 Torr while other deposition parameters remained unchanged for purposes of comparison. The cross-sectional samples were prepared by using a high-resolution focused ion beam (FIB) and characterized using a field-emission transmission electron

orientation of the applied magnetic field and temperature. It revealed that the interaction of itinerant electrons with the localized magnetic moments in the LAO/STO heterostructures may be the predominant mechanism for this interesting anisotropic MR.

2. EXPERIMENTAL DETAILS The LAO ultrathin films were prepared by laser-molecular beam epitaxy (L-MBE) with a KrF excimer laser (λ = 248 nm) on TiO2terminated (100) STO single-crystal substrates. The 11 unit-cell (uc) thick LaAlO3 film was grown at 850 °C under an oxygen partial B


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Figure 3. M−H hysteresis loops for 11 uc LAO/STO grown under 5 × 10−8 Torr after the subtraction of the diamagnetic at different temperatures (in plane). M−H loops for the 1 × 10−5 Torr samples at 5, 10, and 20 K are also given for purposes of comparison.

Figure 4. Rs−T and MR−T under parallel magnetic fields. (a) R−T plots under different magnetic fields. The two solid curves (4, 6, 8, and 10 T) are the fittings to the Kondo model. (b) Magnetoresistance as a function of magnetic field at different temperatures (the applied magnetic field is parallel to the surface of the LAO/STO heterostructures). microscope (TEM). The electrical properties of the samples were measured using Van−Der−Pauw (VDP) geometry with an aluminum wire contact. The transport measurements were performed using the Physical Property Measurement System (PPMS). The magnetic field (up to 10 T) perpendicular to the sample surface was used to measure the magnetoresistance with a current of 50 μA. The nominal sheet carrier density n2D was determined by the Hall coefficient of n2D = −t/ RHe, where t is the film thickness, RH is the Hall coefficient, and e is the charge of an electron. The mobility μ was determined by the sheet resistance Rs and n2D, where μ = 1/en2DRs. The direct current (DC) magnetic properties were measured using a superconducting quantum interference device (SQUID) magnetometer with a reciprocating sample option and a resolution of up to 10−8 emu. Magnetization was measured in a temperature range of 1.9−50 K. A moderate magnetic field applied parallel to the sample surface with a maximum of 1000 Oe was used to complete the measurement.

epitaxial LAO/STO heterostructures in a persistent twodimensional layer-by-layer growth mode. Figure 1a shows a schematic diagram of thin film LAO epitaxially deposited on a TiO2-terminated (100) STO single-crystal substrate. The orientation of the applied magnetic fields for the MR measurement was indicated. Figure 1b shows the RHEED patterns of the as-grown 11 uc LAO thin film on a TiO2terminated STO substrate under an extremely low oxygen partial pressure of 5 × 10−8 Torr. A sharp, streaky RHEED pattern along with clear Kikuchi lines can be maintained up to 11 oscillations (equivalent to a thickness of ∼4.4 nm), demonstrating a layer-by-layer growth. The XRD θ−2θ scan patterns show only the strong (00l) reflections (Figure 1c), indicating the films are highly c-axis oriented. Figure 1d shows the topographic image of the LAO thin film deposited under 5 × 10−8 Torr taken by an atomic force microscope. Typical stepflow growth is clearly observed. Figure 2a shows the HRTEM image of the LAO/STO heterostructures taken from a [001] projection. A highly coherent lattice across the interface was revealed. The electron diffraction pattern of the LAO film overlaps with that of the substrate STO pattern as shown in

3. RESULTS AND DISCUSSION LAO possesses a pseudocubic perovskite structure with the space group R3̅c, similar to that of STO. It is also a band insulator with a wide bandgap energy of 5.6 eV. The lattice mismatch between LAO and STO is ∼3%. The small lattice mismatch between LAO and STO is essential for the growth of C


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Figure 5. Rs−T and MR−T plots under a perpendicular magnetic field. (a) Rs−T plots under different perpendicular magnetic fields. (b) MR as a function of magnetic field at different temperatures (the applied magnetic field was oriented perpendicular to the surface of the LAO/STO heterostructures).

splitting of the spin bands, resulting in the stabilization of magnetic order.17,18 Therefore, the interaction between the itinerant electrons and localized magnetic moments may be the most reasonable mechanism to cause the observed phenomenon. The magnetic signal is robust at elevated temperatures as shown in Figure 3. The magnetic moment was found to be non-zero up to 50 K. Such temperature dependence is consistent with previously reported results, indicating the existence of an ordered state at elevated temperatures.7 The oxygen vacancies induced by the extremely low partial oxygen pressure during the deposition process act as donors to increase the electron density in our samples. With the high applied magnetic field, the magnetic scattering originating from the interaction between the itinerant electrons and localized magnetic moments in the heterostructures may occur, resulting in an unexpected resistance minimum. To further investigate the nature of the observed resistance minimum in the LAO/STO heterostructures, the in-plane MR of LAO/STO as a function of temperature is plotted in Figure 4b. The measurement of MR (defined as the change in fielddependent resistance against the zero-field resistance, ΔR/R(0) = [R(B) − R(0)]/R(0)) was used to differentiate the weak localization quantum correction to electron−electron scattering from the Kondo effect. The Kondo contribution to MR is usually trivial and isotropic but negative in sign. It is interesting to note that the sign of MR is negative at 10 K but positive at the other temperatures. This temperature corresponds to the temperature range of the resistance minimum occurrence in the applied magnetic field beyond 4 T. It should be noted that the MR ratio can reach ∼950% in 10 T at 2 K, decreases to ∼350% at 5 K, and is ∼ −200% at 10 K. Such a large negative MR cannot be explained by the classical orbital effect. The observed behavior can be ascribed to the spin scattering of conduction electrons by the localized magnetic moments in the LAO/STO heterostructures. In fact, the spin scattering has been explored theoretically with the s-d model, the Anderson model, and the Kondo model based on the interaction between itinerant carriers and localized magnetic moments.19 In this case, the resistance upturn may be attributed to the magnetic scattering that is related to the misalignment between the electron spinning and the applied magnetic field, which acts in the same manner as temperature to excite the electrons from the ground state to the excitation state. To reveal the mechanism of electron scattering in the LAO/ STO heterostructures, the onset temperature (TK) and

Figure 2b, indicating the high quality of epitaxial growth of LAO thin films. Figure 3 plots the well-defined M−H hysteresis loops measured at different temperatures, demonstrating the temperature dependence of localized magnetic moments that appeared in the as-deposited LAO/STO samples. The theoretical calculation suggests that the robust ferromagnetism is not an intrinsic property of the LAO/STO heterostructures.13 Such a localized ferromagnetism in the nonmagnetic LAO/STO system is possibly related to Ti3+ that is induced by the high concentration oxygen vacancies in the sample.14 Because the oxygen partial pressure used to deposit our samples is extremely low compared to that used in the published reports, this may be the case in our work. The magnetic moments of the LAO/STO samples deposited under a higher oxygen partial pressure of 1 × 10−5 Torr are significantly lower than those of the low oxygen partial pressure samples at 5, 10, and 20 K. This implies a strong dependence of magnetic properties on the level of oxygen vacancies. Figure 4a shows the resistance of the LAO/STO heterostructures as a function of temperature. It can be seen that the in-plane magnetic field has a significant impact on the measured resistance. When the temperature is lower than 10 K, the resistance measured in the applied field of 10 T is almost 1 order of magnitude higher than that without the applied magnetic field. In addition, the sign of the temperature dependence dR/dT changes from positive to negative when a sufficiently strong magnetic field (≥4 T) is applied, and a resistance minimum is seen at ∼10 K. In general, the resistance minimum arising from the intrinsic Kondo effect or weak localization cannot be maintained under the effect of a magnetic field. However, an unexpected Kondo effect can be induced by a magnetic field due to the electron tunneling effect.15 This magnetic field-induced Kondo effect also gives rise to the resistance minimum. One possible origin of this behavior could be the influenced electronic structure of the Ti 3d states affected by oxygen vacancies. The excess charge originating from the eliminated oxygen atoms in the interfacial TiO2 plane leads to a redistribution in the occupancy of the five 3d orbitals. The dominant contribution to the magnetic moment was ascribed to the 3dxy spin-up occupancy.16 On the other hand, an oxygen vacancy adds two extra electrons at the interface to preserve the neutrality. These two electrons are most likely to reside in the vicinity of the oxygen vacancy. This enhances the charge density and increases the exchange D


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Figure 6. Sheet resistance under magnetic fields as a function of the angular position on the LAO/STO heterostructures at different temperatures: (a) 1 T and (b) 10 T.

Figure 7. Electrical properties of LAO/STO heterostructures. (a) Sheet resistance, Rs, as a function of temperature for samples grown under different oxygen partial pressures. (b) SdH oscillations as a function of magnetic field under different tilting angles at 2 K for the 5 × 10−8 Torr sample. (c) Hall resistance for the 5 × 10−8 Torr sample. (d) Carrier density and mobility as a function of temperature for samples grown under different oxygen partial pressures.

magnetic field are determined with the following equation:20 SgμBHK = kBTK, where S, g, μB, and kB are the impurity quantum number, Lande factor, Bohr magnetron, and Boltzmann constant, respectively. From Figure 4b, we can see that the temperature dependence of logarithmic resistance over the 4 T applied field can be fitted by Rfit(T) = R0 + R1T2 + R2T5 + RK(T/TK), where R0 is the residual resistance, R1T2 is the contribution from the Fermi liquid properties, R2T5 is from the lattice vibrations, and R0, R1, and R2 are constants. For the numerical fitting of this model, an empirical form for the universal resistivity function was employed:

where TK′ =

TK 1/ S

(2

− 1)1/2

Therefore, the onset temperatures TK = 21.35 K for 4 T, TK = 22.05 K for 6 T, TK = 37.90 K for 8 T, and TK = 40.7 K for 10 T were experimentally determined. TK increased with an increase in the magnetic field. It is clearly indicated that the applied magnetic field can effectively tune the interaction of itinerant electrons with localized magnetic moments in the LAO/STO heterostructures. The negative MR was observed at 10 K and vanished at 20 K, turning positive due to the disruption of the exchange interaction between the magnetic centers by thermal excitations. Therefore, it can be summarized that the observed

⎛ T ′ 2 ⎞S ⎟ RK(T /TK ) = RK(T = 0)⎜ 2 K 2 ⎝ T + TK′ ⎠ E


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resistance decreases with decreasing temperature. The sample grown under a lower oxygen partial pressure exhibited lower resistance over the entire temperature range, suggesting the critical role of oxygen vacancies in determining the transport properties of the LAO/STO heterostructures. Figure 7b shows the Shubnikov−de Haas (SdH) oscillations, ΔRxx, as a function of magnetic field with different tilting angles at 2 K for the 5 × 10−8 Torr sample. The two-dimensional nature of the conduction in our LAO/STO heterostructures is indicated by angle-dependent SdH quantum oscillations. The amplitude of the SdH oscillations decreases as the tilting angle increases, demonstrating that the conduction electrons are confined to the interface, and the perpendicular component of the applied magnetic field contributes to the visible SdH oscillations. The Hall resistance Rxy exhibits a linear dependence on the applied magnetic field at temperatures below 100 K, as shown in Figure 7c. The linear relationship between Rxy and the perpendicular applied field suggests a single band transport. However, the Hall resistance at 10 K shows an anomalous behavior with a change in slope at high magnetic fields, probably due to the coupling of itinerant carriers and localized magnetic impurities.23,24 On the other hand, no anomalous Hall resistance was observed at any of the other temperatures. Although oxygen vacancies may induce localized moments as indicated in Figure 3, they either did not couple or just weakly coupled with the carriers. The transport property is controlled by the applied magnetic field through modulation of the carrier density at 10 K. This is in good agreement with the emergence of a resistance minimum under high magnetic fields (≥4 T) at 10 K, as shown in Figure 4a. In addition, for the sample grown under 10−5 Torr, a carrier density of between approximately 1013 and 1014 cm−2 is indicative of a two-dimensional-confined conducting channel. The high carrier density of ∼1016 cm−2 for the sample grown under ultralow oxygen partial pressure (5 × 10−8 Torr) is due to the synergistic effect of the oxygen vacancy with the two-dimensional electron gas (2DEG) in the LAO/STO heterostructures. The magnitude of the carrier density in our ultralow growth pressure sample implies a three-dimensional bulk-like character, but the existence of 2DEG at the interface cannot be excluded because the formation of 2DEG was reported at the interface and oxygen deficient samples were deposited at low oxygen pressures.13,25,26 The film deposition process is critical to the reduction of single-crystal STO. The oxygen vacancy-reduced STO plays a role in the observed anomalous electrical properties of the LAO/STO heterostructures.27,28 Therefore, the interaction of itinerant electrons contributed by oxygen vacancies with localized magnetic moments in the LaAlO3/SrTiO3 heterostructures was responsible for the demonstrated anisotropic MR.

negative in-plane MR was mainly dominated by the occurrence of magnetic scattering based on the spin orientation. Figure 5 plots the sheet resistance and magnetoresistance as a function of temperature under the different magnetic fields, which were applied perpendicularly to the LAO/STO heterostructure surfaces. It shows that a double-resistance transition with two resistance minima occurred at the applied field beyond 6 T in the temperature range from 5 to 20 K (Figure 5a), showing a strong magnetic field dependence of resistance behavior in the LAO/STO heterostructures. Extrinsic factors such as terrace steps, local nonstoichiometry, and impurities may give rise to the imhomogeneities in the conducting channel,21 leading to the observed doubleresistance transition. To understand the effect of the magnetic field on resistance, the MR effect was measured with the applied magnetic field up to 10 T. The MR measured under the perpendicular magnetic field was positive over the full range of measured temperatures. From Figure 5b, it can be seen that MR exhibits a linear relationship with the applied field at 2 and 5 K, while the quadratic-like relationship is observed at 10 K. The positive quadratic-like MR, which is similar to that in the normal metallic systems, may be attributed to the classical orbital scattering of the Lorentz contribution, as the charge carriers tend to go in a cyclotron orbit in the out-of-plane direction. The MR of ≥2000% measured under 10 T at 2 K is reasonably large, and it dropped to ∼250% at 20 K. This observed large MR at a low temperature can be explained by the Kohler-rule orbital scattering theory of R(B,T) = R(0,T) + α2μ2B2 R(0,T); therefore, ΔR/R = α2μ2B2, where α is a material-dependent constant and μ is the mobility.22 To further investigate the temperature dependence of magnetic anisotropic properties in the LAO/STO heterostructures, the angular dependence of electrical resistance was studied under different applied magnetic fields. Figure 6 shows that the MR anisotropy phenomenon is absent at higher temperatures, for example, ≥30 K (for 1 T) and ≥50 K (for 10 T). Under these applied fields, the angular dependence of resistance at the lower temperatures roughly obeys a cosine relationship with the measured angles. With the applied magnetic field of 1 T, the value of the out-of-plane resistance (at 0°) was about 50% higher than that of the in-plane resistance (90°) at 2 and 5 K, while the resistance increased with increasing temperature but had a similar behavior in the angular dependence of resistance at 10 K. This is consistent with the lowest resistance measured in Figure 4a. In addition, the pronounced anisotropic MR behavior up to 20−30 K suggests that the magnetic properties of the LAO/STO heterostructures are strongly dependent on temperature. The anisotropic behavior of the out-of-plane and in-plane MR indicates that the conductivity is quasi-two-dimensional rather than three-dimensional because the orbital effect is suppressed by the in-plane magnetic field in this two-dimensional system. Although we do not have exact evidence of the contribution of Ti3+ to the localized magnetic moment induced by the oxygen vacancies, we believe that the unique deposition process with the ultralow oxygen partial pressure may be the predominant factor for the appearance of anisotropic magnetotransport in the LAO/STO system. This is similar to the notion that areas with an increased density of oxygen vacancies could consequently develop ferromagnetic puddles.14 Figure 7a shows the sheet resistance of the LAO/STO heterostructures deposited under different oxygen partial pressures. Both samples exhibit typical metallic behavior, e.g., the sheet

4. CONCLUSIONS In conclusion, the LAO/STO heterostructures, which were prepared by laser MBE under ultralow oxygen partial pressures, show a strong temperature dependence of anisotropic magnetoresistance. The resistance minimum was observed at 10 K in the heterostructures when the in-plane applied field was higher than 4 T. Contrary to the applied magnetic field that is parallel to the surface of the LAO/STO heterostructures, a weak magnetic ordering state could be field-tuned by manipulation of the perpendicularly applied magnetic field. The results show that a double-resistance transition occurred at the applied field beyond 6 T in the temperature range from 5 to 20 K, showing a strong magnetic dependence of resistance F


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behavior at the interface. It is believed that the observed anisotropic magnetoresistance phenomenon originated from the interaction of itinerant electrons with localized magnetic moments in the LAO/STO heterostructures.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research was supported by the Australian Research Council through Projects DP150103006 and DP140104373. This work was performed in part at the NSW Node of the Australian National Fabrication Facility, a company established under the National Collaborative Research Infrastructure Strategy to provide nano- and microfabrication facilities for Australia’s researchers.

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REFERENCES

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Subtle Interplay between Localized Magnetic Moments and Itinerant

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