A Direct Observation of Room-Temperature Stable Magnetism in

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A Direct Observation of Room-Temperature Stable Magnetism in LaAlO/SrTiO Heterostructures 3

3

Ming Yang, * Ariando, JUN ZHOU, Teguh Citra Asmara, Peter Krueger, Xiao Jiang Yu, Xiao Wang, Cecilia Sanchez-Hanke, Yuan Ping Feng, T. Venkatesan, and Andrivo Rusydi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12945 • Publication Date (Web): 01 Dec 2017 Downloaded from http://pubs.acs.org on December 2, 2017

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ACS Applied Materials & Interfaces

A Direct Observation of Room-Temperature Stable Magnetism in LaAlO3/SrTiO3 Heterostructures Ming. Yang1,2,6,†, Ariando1,†, JUN ZHOU,2,†, Teguh Citra Asmara1,2, Peter Krüger4, Xiao Jiang Yu2, Xiao Wang1, Cecilia Sanchez-Hanke 5, Yuan Ping Feng1, and T. Venkatesan1,3,*, Andrivo Rusydi 1,2,* 1

NUSNNI-NanoCore,

Department

of

Physics,

National

University

of

Singapore,

Singapore117411, Singapore. 2

Singapore Synchrotron Light Source, National University of Singapore, Singapore 117603,

Singapore. 3

Department of Electrical and Computer Engineering, National University of Singapore,

Singapore 117576, Singapore. 4

Graduate School of Science and Engineering, Chiba University, Chiba 263-8522, Japan.

5

National Synchrotron Light Source, Brookhaven National Laboratory, Upton, New York

11973, USA 6

Institute of Materials Research and Engineering, A*-STAR, 2 Fusionopolis Way, Singapore

138634, Singapore.

†These authors contribute equally to this work. *Correspondence

should

be

addressed

to:

A.R.

([email protected])

or

T.V.

([email protected])

Keywords: LaAlO3/SrTiO3 Heterostructures, Magnetism, 2DEG, Oxygen Vacancies, and Interfacial Antisite Defect

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ABSTRACT Along with an unexpected conducting interface between non-magnetic insulating perovskites LaAlO3 and SrTiO3 (LaAlO3/SrTiO3), striking interfacial magnetisms have been observed in LaAlO3/SrTiO3 heterostructures. Interestingly, the strength of the interfacial magnetic moment is found to be dependent on oxygen partial pressures during the growth process. This rises an important, fundamental question on the origin of these remarkable interfacial magnetic orderings. Here, we report a direct evidence of room-temperature stable magnetism in a LaAlO3/SrTiO3 heterostructure prepared at high oxygen partial pressure by using element-specific soft- X-ray magnetic circular dichroism at both Ti L3,2 and O K edges. By combining X-ray absorption spectroscopy at both Ti L3,2 and O K edges and first-principles calculations, we qualitatively ascribe that this strong magnetic ordering with dominant interfacial Ti3+ character is due to the coexistence of LaAlO3 surface oxygen vacancies and interfacial (TiAl-AlTi) anti-site defects. Based on this new understanding, we revisit the origin of the weak magnetism in LaAlO3/SrTiO3 heterostructures prepared at low oxygen partial pressures. Our calculations show that LaAlO3 surface oxygen vacancies are responsible for the weak magnetism at the interface. Our result provides direct evidence on the host of roomtemperature stable magnetism and a novel perspective to understand magnetic and electronic reconstructions at such strategic oxide interfaces.


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INTRODUCTION The discoveries of two-dimensional electron gas (2DEG)

1-9

and magnetism

10-16

at the

interface of non-magnetic band insulators LaAlO3 and SrTiO3 have attracted tremendous research interest. The interfacial 2DEG is found to be LaAlO3-thickness-dependent,

8

in

which the interface becomes conducting only above a critical thickness of LaAlO3. Recently, it was reported that the interplay of oxygen vacancies (OV), lattice distortions, and electronic reconstructions determines this insulator-metal transition.

9, 17

For conducting LaAlO3/SrTiO3

heterostructures, the formation of LaAlO3 surface OV leads to the transfer of 0.5 electrons from the LaAlO3 surface to the LaAlO3/SrTiO3 interface which compensates potential divergence caused by the polar discontinuity between polar LaAlO3 and non-polar SrTiO3. 9,17

Along with the 2DEG, different behaviors of magnetism have also been observed in the LaAlO3/SrTiO3 heterostructures.10-16 For LaAlO3/SrTiO3 heterostructures grown at oxygen partial pressure of 2.5×10-3 mbar, Kondo effect has been reported, which was attributed to the coupling between conducting Ti 3d electrons and interfacial magnetic ions.10 Transport, torque, and scanning superconducting quantum interference device (SQUID) measurements on LaAlO3/SrTiO3 samples grown at oxygen partial pressure of 1×10-5

11

and 8×10-5 mbar 12

suggested the coexistence of ferromagnetic order and superconductivity at the interface. Soft X-ray magnetic circular dichroism (XMCD) showed two-dimensional ferromagnetism with an interfacial Ti3+ character at low temperatures (≤ 10 K) in samples grown at low oxygen partial pressure of 1×10-5

13

or 1×10-4 mbar.15 Meanwhile, the magnetization strength

measured in the heterostructures prepared at high oxygen partial pressure of 1×10-2 mbar was found four times larger than those prepared at lower oxygen partial pressures.14 This indicates that the origin of magnetic ordering should be different for heterostructures prepared at Page 3 of 25 ACS Paragon Plus Environment


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different oxygen partial pressures. It is further found that the magnetism at LaAlO3/SrTiO3 interface can be effectively tuned by applying a gate voltage, 18 and the interfacial magnetism can also be affected by the thickness of LaAlO3. 19

Several scenarios such as lattice distortions,20 interfacial OV,21 surface Al vacancies,22 and intermixing of cationic atoms at the interface were proposed to explain the complicate magnetic ordering at LaAlO3/SrTiO3 interface. Given that the magnetic ordering in LaAlO3/SrTiO3 heterostructures depends on the growth environment, each of these models might be applicable to a specific case only. However, none of them addresses the origin of the strong magnetism in films prepared at high oxygen partial pressure.14

Here, we provide direct evidence of room-temperature, strong magnetism in LaAlO3/SrTiO3 heterostructures prepared at high oxygen partial pressure using XMCD and soft X-ray absorption spectroscopy (XAS). Supported by first-principles calculations, we reveal that this strong magnetic ordering is due to an interplay of LaAlO3 surface OV and interface (TiAl+AlTi) anti-site defects, which enhances Ti-O orbital hybridization, spin splitting of bands near the Fermi level and local crystal field splitting at the interface. Based on this result, we also propose that the magnetic ordering in LaAlO3/SrTiO3 heterostructures prepared at low oxygen partial pressure is instead induced by the formation of LaAlO3 surface oxygen vacancies alone.

RESULTS AND DISCUSSION The LaAlO3/SrTiO3 heterostructure is prepared by pulsed laser deposition of ten unit cells of LaAlO3 on TiO2-terminated SrTiO3 (001) substrates with an oxygen partial pressure of 1×10-2 mbar at 850 oC using a single-crystal LaAlO3 target. Results of SQUID and transport measurements of this sample have been reported elsewhere.14 The SQUID result shows that the magnetic property in these LaAlO3/SrTiO3 heterostructures is about four times stronger Page 4 of 25 ACS Paragon Plus Environment

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than that of LaAlO3/SrTiO3 heterostructures grown at lower oxygen partial pressures of ~10-3 − ~10-4 mbar. For convenience, the LaAlO3/SrTiO3 heterostructures grown at high oxygen partial pressure will be referred to as strong magnetic LaAlO3/SrTiO3, while samples grown at low oxygen partial pressure will be referred as weak magnetic LaAlO3/SrTiO3. Roomtemperature transport measurements on the strong magnetic LaAlO3/SrTiO3 show n-type conductivity with a reduced carrier density of ~8×1013 cm-2 and a higher sheet resistance of ~13 kΩ/ as compared to the weak magnetic LaAlO3/SrTiO3.14

In order to probe the magnetic and electronic properties of the strong magnetic LaAlO3/SrTiO3, synchrotron-radiation-based XMCD and XAS measurements, which are element-specific and magnetism-sensitive, are employed. In XMCD measurements, the polarized light is incident at a grazing angle of 20º from the surface with a propagation direction along the sample magnetization direction, so that the in-plane magnetic moments are measured (see the schematic picture in Fig. 1(a)). SQUID measurement shows that the sample is ferromagnetic at room temperature (see inset of Fig. 1(b)). We note that no magnetic impurities in the sample are probed by XAS and secondary ion mass spectroscopy down to the resolution limit.

Figures 1(c) and 1(d) show the XMCD signatures of the strong magnetic LaAlO3/SrTiO3 taken at the Ti L3,2 and O K edges, respectively. It is noted that these signatures are observed at room-temperature, while in a previous study,

13

the XMCD signals were observed at 10 K.

Interestingly, these signatures have similar features as those of Ta-doped TiO2, 23 especially at ~457.5 eV, ~459 eV, and ~463 eV at the Ti L3,2 edges and ~530 eV at the O K edge. In Tadoped TiO2, the magnetism arose from Ti vacancies introduced by the inclusion of Ta that enhanced p-d hybridizations,24

indicating that magnetism in the strong magnetic

LaAlO3/SrTiO3 may arise from a similar mechanism. By measuring XMCD as a function of Page 5 of 25 ACS Paragon Plus Environment


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applied field, a hysteresis loop is obtained (see Fig. 1(b)), which is similar to that obtained from SQUID measurement (see inset of Fig. 1(b)). Coercive force from both SQUID and XMCD measurements is similar, 0.23±0.02 kOe. This further supports that what we observe is truly intrinsic interfacial magnetic property of the LaAlO3/SrTiO3 heterostructures.

Figure 2(a) shows room-temperature, O K edge XAS of the strong magnetic LaAlO3/SrTiO3, compared to that of SrTiO3 bulk and the weak magnetic LaAlO3/SrTiO3. The pre-peak structure at the O K edge, which comes from the SrTiO3 side, is due to the transitions from O 1s to O 2p-Ti t2g hybridized states, particularly the low energy excitation of t2g, 24-25 which is our main interest. For the weak magnetic LaAlO3/SrTiO3, the intensity of t2g decreases and the energy position of t2g slightly shifts to lower energy as compared to SrTiO3. In contrast, for the strong magnetic LaAlO3/SrTiO3, the intensity of t2g increases and the energy position of t2g shifts to even lower energy than the other two cases.

Figure 2(b) shows the XAS at Ti L3,2 edges of the strong magnetic LaAlO3/SrTiO3 and bulk SrTiO3, which arises from the excitation of electrons from the occupied Ti 2p band into the unoccupied Ti 3d band (see Fig. S1 in Supplemental Material). Four main peaks, L3-t2g, L3-eg, L2-t2g, and L2-eg, are identified, from which we define the following two energy separations, ∆L3 = E(L3-t2g) – E(L3-eg)

(1)

∆L2 = E(L2-t2g) – E(L2-eg),

(2)

and

where E is the energy position of a given peak. The measured energy positions and separations of the peaks are summarized in Table 1. Both ∆L3 and ∆L2 of the strong magnetic LaAlO3/SrTiO3 are slightly larger than those of SrTiO3, indicating an increase of the crystal field splitting. And the overall intensity of the Ti L3,2-edge of the strong magnetic LaAlO3/SrTiO3 is slightly higher (~6.5% more) than that of bulk SrTiO3. Combined with the Page 6 of 25 ACS Paragon Plus Environment

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increase of the t2g pre-peak in the XAS at O K edge (cf. Fig. 2(a)), this indicates an enhancement of Ti 3d–O 2p hybridization in the strong magnetic LaAlO3/SrTiO3.

Given the experimental results above, we establish a comprehensive understanding of the origin of the electronic and magnetic properties of LaAlO3/SrTiO3 grown at different oxygen partial pressures. Previous studies suggested that the magnetism can be induced either by anion or cation defects in LaAlO3 or SrTiO3. 21-22, 26 For anion defects, OV in either SrTiO3 or LaAlO3 side of LaAlO3/SrTiO3 can explain the n-type interface conductivity.9, 17 Here, we calculate the layer-dependent formation energy of OV to determine which case is more likely to form. (Details of the first-principle calculations are given in the Supplemental Material). Figure 3 shows that the formation energy of OV is lowest at the LaAlO3 surface, consistent with previous calculations.

17, 27

The presence of OV at the LaAlO3-side of LaAlO3/SrTiO3

was also observed using high-energy optical conductivity. 9

These LaAlO3 surface OV trigger electron transfer into the interface,

9, 17

creating Ti3+ states.

Since the extra electrons decrease the number of unoccupied Ti 3d states, the corresponding XAS peak intensity should be reduced, particularly the t2g peak at both resonant edges. Furthermore, a previous XAS study on LaxSr1-xTiO3 showed that Ti3+ decreases the splitting between t2g and eg states (i.e., ∆L3 and ∆L2).

25

However the present XAS data of the strong

magnetic LaAlO3/SrTiO3 show just the opposite effects, namely increased t2g intensity and increased t2g-eg splitting. This clearly indicates that the LaAlO3 surface OV alone cannot explain the strong magnetism in sample grown at high oxygen partial pressure.

The high oxygen pressure used to grow the strong magnetic LaAlO3/SrTiO3 samples opens the possibility of forming cation defects, 28-29 such as cationic (Sr, La, Al, and Ti) vacancies or anti-site defects. However, XMCD data (cf. Fig. 1) show that the magnetic signal comes from Page 7 of 25 ACS Paragon Plus Environment


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both Ti and O atoms, indicating that the magnetic ordering should involve both Ti and O atoms. The strong magnetic signal in Ti atoms rules out cationic vacancies alone, because they create magnetic moments on O atoms. On the other side, various interfacial cation intermixing has been proposed at LaAlO3/SrTiO3 interface, including A-site (La-Sr) B-site (Ti-Al)

30-32, 34

30-33

and

inter-diffusion, but without conclusive evidence yet. The (SrLa+LaSr)

anti-site defect enhanced the polar electric field and the LaSr at SrTiO3 side introduced more electrons at interface, in contrast with our XAS data. Furthermore, our calculations demonstrate that the interfacial (TiAl+AlTi) anti-site defects are energetically favorable at high oxygen partial pressure condition. Thus interfacial (TiAl+AlTi) anti-site defects are the most likely cation defects in the strong magnetic LaAlO3/SrTiO3. However, (TiAl+AlTi) anti-site defects alone lead to p-type conductivity, inconsistent with the transport measurements. 14

The above discussions strongly show that an interplay of LaAlO3 surface OV and cation defects interfacial (TiAl+AlTi) anti-site defects is responsible for the strong magnetic LaAlO3/SrTiO3. To confirm this, we calculate the formation energy and XAS (discussed later) for one LaAlO3 surface Ov in the LaAlO3/SrTiO3 supercell with and without interfacial (TiAl+AlTi) anti-site defects at oxygen-rich condition. We find that the coexistence of LaAlO3 surface OV and interfacial (TiAl+AlTi) anti-site defects is stable. Furthermore, their formation energies further reduce with an increase of LaAlO3 thickness and a decreased density of LaAlO3 surface OV (see Table S1 and Supplementary Information).

Figure 4 shows the optimized structure of LaAlO3/SrTiO3 supercell with a LaAlO3 surface Ov and an interfacial (TiAl+AlTi) anti-site defect, superimposed with the spin density. Figure 4(a) indicates the magnetic moments are mainly from interface Ti atoms with minor contributions from interface O atoms, consistent with XMCD results (cf. Fig. 1). The interplay of LaAlO3 surface OV and interfacial (TiAl+AlTi) anti-site defects leads to a magnetic moment of 0.19 µB Page 8 of 25 ACS Paragon Plus Environment

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per Ti atom in the interfacial TiO2 sub-layer. It is noted that the TiAl defects are non-magnetic, as excess electrons are transferred to the interface. Bader charge analysis shows the charge state of TiAl is almost the same as the Ti atoms in bulk SrTiO3. Figures 4(b) and 4(c) show the projected density of states (PDOS) of Ti atom and its nearest-neighbor O atom at the interfacial TiO2 sub-layer, respectively. They confirm that the spin density is mainly contributed by the Ti dxy orbitals. Interestingly, comparing Fig. 4(b) and Fig. 5(b) (the case with LaAlO3 surface OV only) shows that AlTi defect in SrTiO3 further reduces the interface symmetry, enhances the distortion of interfacial oxygen octahedra (see Fig. S6 in the supporting information), increases the spin-polarization of interfacial Ti, and enhances the hybridization of the Ti t2g and O 2p orbitals (see Figs. 4(b) and 4(c)). This hybridization is evident in the XAS and XMCD at both O K and Ti L3,2 edges. The magnetic ordering here prefers a ferromagnetic coupling with 156 meV lower than the antiferromagnetic coupling. While the LaAlO3 surface OV ensure the n-type conductivity despite the (TiAl+AlTi) anti-site defects. It is noted that LaAlO3/SrTiO3 with (TiAl+AlTi) anti-site alone (without surface OV) is non-magnetic. All these findings demonstrate the importance of the interplay of LaAlO3 surface Ov and interfacial (TiAl+AlTi) anti-site defects in determining the strong magnetism in LaAlO3/SrTiO3 heterostructure prepared at high oxygen partial pressure.

Our calculations also explain the weak magnetism observed in LaAlO3/SrTiO3 heterostructures grown at low oxygen partial pressures. Previous results

16, 23

and our

calculation suggest that LaAlO3 surface OV is the energetically most favorable defect in LaAlO3/SrTiO3 at low oxygen partial pressures. Figure 5 shows that LaAlO3 surface OV induces magnetic moments with dominant contribution from the interfacial Ti dxy orbitals, consistent with XMCD result, 13 but with a weak magnetic moment of 0.08 µB per Ti atom at the interfacial TiO2 sublayer. It is also noted the p-d orbital hybridization between interfacial Ti and O atoms in the weak magnetic LaAlO3/SrTiO3 is not as profound and has less Page 9 of 25 ACS Paragon Plus Environment


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deformed TiO6 octahedral crystal field as compared with those of the strong magnetic LaAlO3/SrTiO3. All these calculated ground state and XAS simulations agree with experimental XAS data (cf. Figs. 2(a) and 2(b)).

To further corroborate the consistency of the interplay between LaAlO3 surface OV and (TiAl+AlTi) anti-site defects with the XAS and XMCD spectra, we calculate the theoretical XAS of three cases: SrTiO3 bulk, and the strong and weak magnetic LaAlO3/SrTiO3 heterostructures. The calculated spectra (Fig. 2(c)) are qualitatively consistent with experimental results (cf. Fig. 2(a)). The lowest excitation is indeed t2g for all cases, but its relative energy position is different for each case. For the weak magnetic LaAlO3/SrTiO3, the calculated intensity of t2g decreases and its energy position moves toward low energy. For the strong magnetic LaAlO3/SrTiO3, the intensity of t2g increases and its energy position further shifts toward lower energy. It is noted that the eg is broad and occurs at 4-5 eV higher than t2g, consistent with previous results.9 Using the same method above, we calculate the Ti d DOS projected into t2g and eg states [Fig. 2(d))]. For the strong magnetic LaAlO3/SrTiO3, t2g shifts toward lower energy as compared to bulk SrTiO3 and the weak magnetic LaAlO3/SrTiO3. For the weak magnetic LaAlO3/SrTiO3, t2g slightly shifts toward lower energy as compared to SrTiO3. It is noted that although core-hole effects are neglected here, it does not affect our conclusions about the relative changes of t2g intensity and energy position.

We note that our XMCD results on the interfacial magnetism in LaAlO3/SrTiO3 heterostructures are in line with previous study, 18 in which Bi et al. found that the magnetism can be effectively tuned by applying a gate voltage. When the LaAlO3/SrTiO3 heterostructure was gated with less conducting, room-temperature stable ferromagnetism was observed using magnetic force microscopy. In contrast, the magnetism was much destabilized if the sample was gated more conducting. All these findings corroborate our XMCD results demonstrating Page 10 of 25 ACS Paragon Plus Environment

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a stable ferromagnetic signal at room temperature for the less conducting samples prepared at high oxygen partial pressure than those at low oxygen partial pressure14.

Based on our experimental data and theoretical calculations, we explain the origin of various magnetic signals on LaAlO3/SrTiO3 heterostructures grown under different oxygen partial pressures as follows. For samples grown under low oxygen partial pressure (or weak magnetic samples), surface OVs introduce extra electrons into the LaAlO3/SrTiO3 interface

9, 17

, and

lead to a (Ti3+-O2- -Ti4+)-like electronic configuration, which is then responsible for the weak magnetism. In this case, the exchange energy JDE is proportional to the p-d hybridization strength, i.e. JDE ~ tpd.

35-36

For samples prepared at high oxygen partial pressure (or strong

magnetic samples), besides surface OV, interfacial (TiAl+AlTi) anti-site defects significantly enhance the orbital hybridization between interfacial Ti and O atoms, as well as deformed TiO6 octahedral crystal field, which increases interfacial spin-splitting of the Ti t2g bands and thus strong magnetism as shown in XMCD measurements (Figs. 1(c) and 1(d)). Furthermore, using XMCD sum rule

37-38

, the ratio between orbital and spin magnetic moment is found to

be ~0.25. This means that the orbital magnetic moment contributes significantly to the strong magnetism. 39

CONCLUSION In conclusion, our result reveals the importance of coupling of charge and orbital to magnetic properties in LaAlO3/SrTiO3 heterostructures which can be tuned by the oxygen partial pressure during sample preparation. The remarkable magnetism observed in LaAlO3/SrTiO3 at room temperature opens up further potential applications of this exciting oxide interface in future spintronic devices.



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METHODS Sample Growth. Ten unit cells of LaAlO3 are deposited by using pulsed laser deposition and a single crystal LaAlO3 target on a TiO2-terminated (001) SrTiO3 substrate in an oxygen partial pressure of 1.0×10-2 mbar. Before deposition, the SrTiO3 substrate is treated by buffered hydrofluoric acid and then annealed at 950 oC in oxygen, which results in an atomically-flat SrTiO3 surface.14 The laser pulse (248 nm) energy density is 1.8 J cm-2 and the repetition rate is 1 Hz. The in situ REED monitored growth process shows layer-by-layer growth for samples. After growth, all samples are cooled to room temperature in the same oxygen partial pressure used during the deposition.

XMCD, XAS, and SQUID Measurements. The soft X-ray absorption (XAS) and soft X-ray magnetic circular dichroism (XMCD) measurements are done using a combination of left and right circularly-polarized lights in total electron yield mode and applied external magnetic field (H) from 0 to +0.5 kOe on heterostructure (Figure 1a). The photon energy is chosen to be resonant at Ti L3,2-edge, which comes from Ti 2p3d electron transitions, and XMCD hysteresis is further performed at this resonant edge (Figure 1b). This experimental geometry and method allow us to probe intrinsic ferromagnetic property. All measurements are done at room temperature. The inset of Figure 1b shows that the ferromagnetic signal as measured by a SQUID magnetometer has a maximum saturation magnetization of 8 emu/cm2.

DFT Calculations. Spin-polarized calculations are carried out by using density-functionaltheory-based Vienna ab initio simulation package (VASP) with the Perdew–Burke–Ernzerhof (PBE) approximation for the exchange-correlation functional.38,39 The projector-augmented wave (PAW)40 pseuodopotentials, as implemented in VASP, are used for the interaction between ionic cores and valence electrons. The cut-off energy for the expansion of planePage 12 of 25 ACS Paragon Plus Environment

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wave basis is set to 500 eV. The effective on-site Coulomb repulsion of U=1.2 and U=11 has been applied for d and f states in Ti and La, respectively. The LaAlO3/SrTiO3 interface is modeled by placing four unit cells of LaAlO3 on TiO2-terminated four unit cells of SrTiO3 (001) substrate with 15 Å vacuum to minimize Coulomb interaction between neighboring surfaces. The (TiAl+AlTi) anti-site defect is introduced by exchanging an interfacial Ti atom with an Al atom at 2×2×1 LaAlO3/SrTiO3 interface slab. A 8×8×8 and 4×4×1 Gamma-pointcentered k-point mesh is used for SrTiO3 bulk and LaAlO3/SrTiO3 interface slab, respectively. For the FM and AFM calculations, two (TiAl+AlTi) anti-site defects with a distance of 3.905 Å are included in a 3×3×1 LaAlO3/SrTiO3 slab, and 2×2×1 Gamma-point-centered k-point meshes are used. The electronic convergence is set to 1.0×10-6 eV. The force on each atom is optimized to be smaller than 0.01 eV/Å, while the bottom layer of SrTiO3 substrate is fixed. The dipole correction is applied to remove artificial dipole interactions.41

Theoretical Calculations of XAS. The theoretical XAS spectra are calculated within density functional theory using VASP. The calculation of XAS at O K edge is based on O-p density of state (DOS) broadened with energy-dependent Lorentzian (Γ0=0.156 eV for core-hole) and arctan-like step function for bulk SrTiO3, the strong magnetic LaAlO3/SrTiO3 heterostructure (i.e., the case with the interplay of surface oxygen vacancies and (TiAl+AlTi) anti-site defects) and the weak magnetic LaAlO3/SrTiO3 heterostructure (i.e., the case with surface oxygen vacancies). The Ti-d-DOS is decomposed into t2g and eg contributions and Gaussian broadened with 0.2 eV full-width at half maximum.

Supporting Information: Details of calculation method on defect formation energy, defect configurations, density of states, XAS and XMCD measurements.



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ACKNOWLEDGEMENTS: This work is supported by Singapore National Research Foundation

under its Competitive Research

Funding

(NRF-CRP 8-2011-06

and

NRF2008NRF-CRP002024), MOE-AcRF Tier-2 (MOE2015-T2-1-099, MOE2015-T2-2-065, and MOE2015-T2-2-147), NUS-YIA, and FRCs. We thank Centre for Advanced 2D Materials and Graphene Research Centre at National University of Singapore to provide the computing resource.

FIG. 1. Room-temperature X-ray magnetic circular dichroism (XMCD) measurements of LaAlO3/SrTiO3 interface. (a) Schematic diagram of the experimental configurations. (b) XMCD hysteresis of LaAlO3/SrTiO3 heterostructure at Ti L3-t2g. Inset is SQUID hysteresis. Page 14 of 25 ACS Paragon Plus Environment

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XMCD of LaAlO3/SrTiO3 heterostructure at Ti L3,2 (c) and O K edges (d). The error bar of the XMCD is comparable to the size of the (solid, blue) circles.

FIG. 2. Experimental and simulated X-ray absorption spectroscopy (XAS) of bulk SrTiO3, and the strong and weak magnetic LaAlO3/SrTiO3. (a) and (b) Comparison of experimental XAS at O K and Ti L3,2 edges, respectively. (c) and (d) Calculated O K edge XAS and Ti-d DOS, respectively. To emphasize the changes, only the contribution of the interface atoms are shown in the weak magnetic and strong LaAlO3/SrTiO3. The XAS of the weak magnetic LaAlO3/SrTiO3 is reproduced from Ref. 13.



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FIG. 3. (a) Atomic structure of LaAlO3/SrTiO3 with OV, where the dashed circle line denotes the location of the OV. “Hard” (“Easy”) denotes that OV are difficult (easy) to form. For clarity, only one SrTiO3 layer near the interface is shown while we use 4 uc SrTiO3 as substrate in calculations. (b) The layer-dependent formation energy of OV at LaAlO3/SrTiO3 under oxygen-rich and oxygen-poor experimental conditions.


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FIG. 4. (a) Structural guide for LaAlO3/SrTiO3 with LaAlO3 surface OV and interfacial (TiAl+AlTi) anti-site defects (side view). Spin density is visualized by blue color with an isosurface value of 1.0×1.0-3 e/Å3. The partial density of states (PDOSs) of Ti atom (b) and its neighbour O atom (c) at the interfacial TiO2 sublayer. The scale of the O PDOS is 1/20 of Ti PDOS. The Fermi energy is shifted to 0 eV.



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FIG. 5. (a) Structural guide for LaAlO3/SrTiO3 with LaAlO3 surface OV (side view), where spin density is visualized by blue color with an iso-surface value of 1.0×1.0-3 e/Å3. The partial density of states (PDOSs) of a Ti atom (b) and its neighbour O atom(c) at the interfacial TiO2 sublayer. The scale of the O PDOS is 1/20 of Ti PDOS.


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Table 1: Energy positions and separations of the X-ray absorption at Ti L3,2 edges of LaAlO3/SrTiO3 and bulk SrTiO3. SrTiO3

LaAlO3/SrTiO3

E(L3-t2g)

457.65 eV

457.55 eV

E(L3-eg)

459.95 eV

459.95 eV

2.30 eV

2.40 eV

E(L2-t2g)

463.00 eV

462.90 eV

E(L2-eg)

465.40 eV

465.40 eV

2.40 eV

2.50 eV

∆L3

∆L2



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