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Thickness Dependence of Magnetic Behavior of LaAlO3/ SrTiO3 Heterostructures Hai-Long Hu, Danyang Wang,* Allen Tseng, Zhigang Chen, Charlie Kong, Jiabao Yi, and Sean Li* in LAO/STO heterostructures. The first signature of magnetism at the LAO/STO interface was determined experimentally in magneto-transport measurements.[6] A large magnetization of 0.3–0.4 µB per interface Ti was also observed by torque measurement.[7] It was also discussed that the ferromagnetism was linked to dxy orbitals in the Ti t2g band through X-ray circular dichroism analysis.[8] Despite the evidence from various experimental studies, the origin and nature of magnetism have remained controversial. Fitzsimmons et al. showed that no magnetic moment in LAO/STO system was obtained from neutron reflectometry measurements,[9] whereas weak moments from LAO/STO heterostructures (≈2 × 10−3 µB per unit cell) were discovered by Li et al.[7] and Salmon et al.[10] through β-detected nuclear magnetic resonance. The tantalizing magnetic signals at LAO/STO interface were recently reported in a wide temperature range.[11] In addition, the recent theoretical reports suggested the magnetism to be induced by point defects[12] or oxygen vacancies[13] which produced ferromagnetism puddles. Because of these controversial reports, we studied the nature and mechanism of the induced magnetism in LAO/STO heterostructures with various thicknesses of LAO film. Kalisky et al. reported that the ferromagnetism was observed in LAO/STO heterostructures only when LAO reached a critical thickness.[14] However, this is opposed by the apparent magnetic behavior in LAO/STO heterostructures with a rather broad range of LAO thickness reported by a few other groups.[9,10,15,16] Therefore, there is a pressing need to clarify and understand the LAO thickness dependence of magnetic properties in LAO/STO. In this work, we experimentally and theoretically demonstrate the critical role of oxygen vacancy defects and the polar field on tuning the magnetic properties of LAO/STO heterostructures with different LAO layer thickness.

Thickness dependence of magnetic properties of LaAlO3 (LAO)/SrTiO3 (STO) heterostructures is systematically studied. It is found that the magnetic moment is rather robust at elevated temperatures in the samples grown under ultralow PO2 of 1 × 10−8 Torr, indicating room-temperature ferromagnetism in the LaAlO3/SrTiO3 heterostructures. Spin-glass-like behavior is observed in the LAO/STO heterostructures with the LAO thickness ranging from 5 to 90 unit cells (2–35.2 nm). As the LAO layer becomes thicker, the blocking temperature of spin-glass-like behavior decreases and magnetic moment weakens, which may be related to the reduced density of oxygen vacancies at the interfacial TiO2 layer. It is believed that the well-controlled magnetism by tuning the LAO thickness may open an avenue for the potential application of spintronics with LaAlO3/SrTiO3 heterostructures.

1. Introduction Interfaces between two correlated insulating oxides SrTiO3 (STO) and LaAlO3 (LAO) have been intensively studied because of the emergence of intriguing physical phenomena including 2D electron gas, superconductivity, and ferromagnetism.[1–5] This makes the LAO/STO system attractive for both fundamental research and potential applications in oxide-based electronics. Recent low-temperature transport and magnetization studies have demonstrated the presence of ferromagnetism

H.-L. Hu, Dr. D. Wang, A. Tseng, J. Yi, Prof. S. Li School of Materials Science and Engineering The University of New South Wales Sydney, New South Wales 2052, Australia E-mail: [email protected]; [email protected] Dr. Z. Chen School of Mechanical and Mining Engineering University of Queensland Brisbane, Queensland 4072, Australia Dr. Z. Chen Centre of Future Materials University of Southern Queensland Springfield Toowoomba, Queensland 4350, Australia Dr. C. Kong Mark Wainwright Analytical Centre University of New South Wales Sydney, New South Wales 2052, Australia The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/admi.201800352.

DOI: 10.1002/admi.201800352

Adv. Mater. Interfaces 2018, 1800352

2. Results and Discussion Figure 1a shows the XRD θ–2θ diffraction patterns of LAO films with various thicknesses. Only the strong (00l) reflections are detected, indicating all the films are highly c-axis oriented. As the increase of LAO thickness, a continuous shift of the (002) peak toward lower angles is found. For the 90 uc LAO

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Figure 1. a) XRD θ–2θ diffraction patterns for LAO/STO with different LAO thickness, showing the subtle changes in LAO (002) peak positions demonstrated by a dashed line. b) Out-of-plane lattice constant c of LAO thin films plotted as a function of out-of-plane strain levels.

thin film, its out-of-plane lattice constant c was 3.785 Å, which is almost the same as bulk LAO lattice constant 3.791 Å with pseudo-cubic structure at room temperature. Thus, the strain in this 35.2 nm thick LAO film is trivial. The out-of-plane lattice constant c contracts as the LAO thickness decreases, as shown in Figure 1b, suggesting the increasing level of out-of-plane compressive strain, e.g., the 5 uc sample suffers from a strain up to 1.19% along c-axis direction. Figure 2a shows the cross-sectional high-resolution transmission electron microscopy (HRTEM) image of the 5 uc LAO/ STO heterostructure. Highly coherent lattice across the interface was revealed. The electron diffraction pattern of LAO film cannot be resolved from that of the substrate STO, as shown in Figure 2b, implying the high quality of epitaxial growth of LAO thin films. Figure 2c shows the AFM topographic image of 5 uc LAO with the typical step-flow terrace. The average surface roughness value (Ra) is found to be 0.165 nm. X-ray diffraction reciprocal space mapping (RSM) was performed on 5 and 90 uc samples. Figure 2d shows the RSM around (002) reflection for 5 uc. RSMs around (002) and (103) reflections for 90 uc sample are shown in Figure 2e,f, respectively. The diffraction spots of the LAO films locate right above that of STO, i.e., the in-plane lattice parameter of the film is almost the same as that of the substrate, suggesting the LAO is fully strained along the inplane direction. Based on the RSM results, the out-of-plane and in-plane lattice parameters of 90 uc LAO were determined to be c = 3.785 Å and a = b = 3.901 Å, respectively. The out-of-plane lattice constant c of LAO was in good agreement with that calculated from θ–2θ scan, as indicated in Figure 1b. Figure 3a shows the zero field cooled (ZFC) and FC M–T curves measured at an applied magnetic field of 100 Oe for LAO films with various thicknesses. A typical spin-glass-like behavior with a bifurcation of ZFC and FC curves was demonstrated, indicating the thermal irreversibility affected by the heating or cooling mode during magnetic measurement. The spin glass generally occurred when the magnetic spin of constituting atoms appeared in disordered and frustrated manner. Moreover, visible cusps were present in ZFC curves of 5, 10, and 20 uc samples, which can be explained by the interaction of spins occurring in the thin films. The blocking temperatures of 5, 10, and 20 uc samples were ≈70, ≈25, and ≈15 K, respectively. Well-defined M–H hysteresis loops were obtained for samples with different LAO thickness at both 2 and 300 K as shown in


Figure 3b,c, respectively. Strong thickness dependence of magnetic moments was observed at both temperatures, although the magnetization substantially decreased as the measuring temperature increased from 2 to 300 K. The change of the saturated magnetic moment for the samples with LAO thickness is summarized in Figure 3d. It is obvious that the thinner samples exhibit a higher saturated magnetic moment at both 2 and 300 K. The spin-glass behavior in our samples illustrates the magnetic frustration owing to the competition between multiple ground states with different spin configurations.[17] The oxygen vacancies produced during deposition process turn out to be the main factor that contributes to the spin-glass behavior. To understand the effect of oxygen partial pressure on the magnetic properties of the 5 uc samples grown under 1 × 10−3 and 1 × 10−5 Torr, their M–T curves and M–H loops were measured as shown in Figure 4. It is clear that spin-glass behavior was only observed in sample grown under ultralow oxygen partial pressure, i.e., 1 × 10−8 Torr. The Curie temperature of both 1 × 10−3 Torr and 1 × 10−5 Torr samples was found to be ≈19 K (Figure 4a). Figure 4b–d shows that the magnetization decreases as the measuring temperature increases from 2 to 10 K. The saturated magnetic moments at various temperatures are summarized in Table 1. Greatly enhanced magnetic moment was achieved in sample deposited under 1 × 10−8 Torr at all the measuring temperatures (2–300 K). In marked contrast to the nondetectable magnetization in samples grown under relatively high oxygen pressures at room temperature, robust magnetic behavior with a saturated room-temperature magnetic moment f of 2.62 × 10−6 emu was obtained in 1 × 10−8 Torr sample, as shown in Table 1. The oxygen vacancies can generate electrons and act as donors to increase the electron density (in Figure S3, Supporting Information).For the emerging magnetism in LAO/STO heterostructures, it is believed that the magnetism was associated with the oxygen vacancies at the interfacial TiO2 layer and the generated Ti3+ ions, which was due to the low oxygen partial pressure during growth.[8,13] XPS was employed to examine the fraction of Ti3+, which is estimated by the Ti3+/(Ti3+ + Ti4+) peak area ratio in the XPS spectra as shown in Figure S4 in the Supporting Information. The fraction of Ti3+ was found to be 4.93% and 6.74% in samples grown under 1 × 10−5 and 1 × 10−8 Torr, respectively. This implies that a nonnegligible fraction of Ti3+ introduced by oxygen vacancies leads

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Figure 2. a) High-resolution transmission electron microscopy (HRTEM) images of 5 uc LAO/STO. b) Electron diffraction pattern for both STO and LAO along the [001] zone axis (the scale bar is 5 nm). c) X-ray RSM around (002) of 5 uc LAO/STO. d) Atomic force microscopy (AFM) images of 5 uc LAO/STO. e) X-ray RSM around (002) of 90 uc LAO/STO. f) X-ray RSM around (103) of 90 uc LAO/STO.

to ferromagnetic puddles, which was corresponding to the enhanced magnetic moment as shown in Figure 4 and Table 1. As the oxygen partial pressure is lowered, the oxygen vacancies would induce magnetic moments deep inside the bulk STO substrate.[13] Therefore, the magnetic behavior related to oxygen vacancies and the formed Ti3+ in the LAO/STO heterostructures was demonstrated. The decreased blocking temperature of spin-glass-like behavior and the decreased magnetic moment with increased LAO thickness might be related with the decreased strain level affected by the different LAO thickness in LAO/STO samples. This out-of-plane compressive strain will induce the AlO6 octahedral distortion and thus affect the orbital structure and bond length.[18,19] Tetragonal-like distortion was observed in coherently grown heterostructures.[20] Once the symmetry in LAO/STO was broken, the magnetic


properties might be affected. Moreover, the 2DEG behaviors were largely different in tensile-strained and compressively strained LAO/STO.[21] Carrier density as a function of temperature for LAO/STO heterostructures grown under 1 × 10−8 Torr with various LAO thicknesses and the electric polar field were investigated, as show in Figure 5. As the perovskite LAO and STO are both wide bandgap insulators, the deposition of an epitaxial LAO film on a TiO2-terminated, (001)-oriented STO substrate contributes to the emergence of interfacial conductivity when the LAO thickness is thicker than the critical thickness of 4 unit cells (uc). As our films (5–90 uc) are thicker than the critical thickness, the increasing of electrostatic potential might be formed as the LAO thickness is increased,[22] leading to the proposed electronic reconstruction with charge transferred toward the topmost TiO2 plane in the STO substrate. In addition, as the LAO

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Figure 3. Magnetic properties of LAO/STO heterostructures grown under 1 × 10−8 Torr with various LAO thicknesses. a) Zero field cooled (ZFC) and field cooled (FC) magnetization as a function of temperatures (M–T curves). The measurements were performed during heating; b) M–H loops measured at 2 K; c) M–H loops measured at 300 K; d) Saturated magnetic moment as a function of LAO thickness.

Figure 4. Magnetic properties of 5 uc LAO/STO grown under different oxygen partial pressures. a) δM/δT versus T curve under 100 Oe zero field cooling (ZFC) for 5 uc LAO/STO grown under 1 × 10−3 Torr and 1 × 10−5 Torr. Temperature-dependent M–H loops for 5 uc LAO/STO grown under b) 1 × 10−3 Torr, c) 1 × 10−5 Torr, and d) 1 × 10−8 Torr.


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Table 1. Magnetic pr operties for 5 uc LAO/STO samples. Ms (10−6 emu) 2 K

Ms (10−6 emu) 5 K

Ms (10−6 emu) 10 K

Ms (10−6 emu) 300 K

LAO/STO (1 × 10−3 Torr)

5.06

3.18

2.81

N/A


5.08

3.21

2.84

N/A


6.01

4.09

3.45

2.62

Samples/magnetic moment

N/A (Not available): Sample will not present the magnetic behavior at this temperature.

film growth was performed under low oxygen partial pressure, the oxygen vacancies were produced, which can be evidenced by the distinct increase of carrier density when LAO thickness increases as shown in Figure 5a. The formation energy of oxygen vacancies would decrease as the LAO thickness increased as shown in Table 2, leading to the generation of more oxygen vacancies, which can partially balance out the polar field in LAO. It was noted that the built-in polar field is always present during the LAO growth. Therefore, the built-in polar field in LAO/STO heterostructures was reduced by the increasing amount of transferred electrons from oxygen vacancies when LAO layer becomes thicker (Figure 5b). In the absence of polar field, all the defects will be electrically neutral, limited magnetic moment was demonstrated.[16] Thus, different magnetic behaviors were observed after tuning the thickness of LAO. We used the density functional theory to study the effect of LAO layers grown under low oxygen partial pressure on the magnetic properties of LAO/STO heterostructures. The first-principles calculations were done using the Vienna Ab initio simulation package (VASP) software with the projected augemented wave method[23,24] and the Perdew–Burke–Ernzerhof pseudopotential.[25] To simulate the magnetic proper2 × 2 asymmetric structies of the heterostructures, a ture consisting of 5 uc STO and 4 uc LAO with a minimum vacuum layer of 20 Å was used for the electronic calculation. And this asymmetric structure with dipole correction was performed for the electronic calculation with in-plane lattice of the optimized lattice of STO. Effective values of U = 4.0 eV

applied on Ti’s 3d, and U = 7.0 eV on La’s 4f orbitals within the Dudarev scheme[26] were used to account for the correlation effect in LAO/STO. A kinetic energy cut off 500 eV with 6 × 6 × 1 k-point grids was used for the geometrical optimization and k-point mesh of 9 × 9 × 1 was used to calculate the density of states. These values were consistent with previous studies in LAO/STO system.[27,28] Oxygen vacancies at different locations in LAO/STO were considered in this calculation. A single oxygen atom (Vo) at the interfacial TiO2 layer and the surface of LAO in the AlO2 layer was removed to investigate the oxygen vacancy effect with various LAO layers as shown in Figure 6. The formation energy of the oxygen vacancies at the TiO2 layer is 5.26 eV, which is larger than those in LAO layer. This is in good agreement with the reported result, i.e., preferred position of the oxygen vacancies in LAO/ STO heterostructures is on the LAO surface.[22] In addition, the formation energy of the oxygen vacancies on the LAO surface is significantly lower when the LAO film becomes thicker, as shown in Table 2. The magnetic moment of 0.53 µB/Ti was induced by the oxygen vacancy related defects at the interfacial TiO2 layer (Table 3). This high magnetic moment was attributed to the large concentration of oxygen vacancy formed during growth. This result also suggests the significant role of TiO2 interfacial layer acting as electron trapping from the oxygen vacancies. As the oxygen vacancy continuously steps away from the interfacial Ti, the decreased magnetic moment of 0.34 µB/Ti to 0.09 µB/Ti was obtained, which is in good agreement with the experimental results, i.e., magnetic moment was decreased with the increased thickness of LAO layer, as shown in Table 1 and Figure 4.

Figure 5. a) Carrier density for LAO/STO heterostructures grown under 1 × 10−8 Torr with various LAO thicknesses. b) Schematic diagram of band alignment for 5 uc LAO/STO (black line) and 90 uc LAO/STO (blue line).


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Table 2. Formation energy (eV) of one O vacancy on the TiO2 and LAO surface (S-LAO). TiO2 layer/S-LAO

TiO2 layer

1 layer LAO

2 layers LAO

3 layers LAO

4 layers LAO

5.26

4.78

4.81

4.31

3.95

3. Conclusions In conclusion, LaAlO3/SrTiO3 heterostructures with different LAO film thickness were prepared by L-MBE. Our experimental results demonstrated robust spin-glass-like behavior in 5 unit-cells LAO/STO heterostructures, which was induced by the interaction between the oxygen vacancy defects at the interfacial TiO2 layer and the polar field in LAO/STO samples. The decreased blocking temperature of spin-glass-like behavior and the decreased magnetic moment at each specific measured temperature was also observed with increasing LAO thickness. Tuning of magnetic behavior through changing the LAO layer thickness is desirable for the potential spintronic application with LaAlO3/SrTiO3 heterostructures.

4. Experimental Section The LAO thin films were prepared by laser-molecular beam epitaxy (L-MBE) with a KrF excimer laser (λ = 248 nm) on TiO2-terminated (100) STO single crystal substrates. LaAlO3 films with various thicknesses of 5 unit-cells (uc) (≈2 nm), 10 uc (≈4 nm), 20 uc (≈8 nm), and 90 uc (36 nm) were grown at 850 °C under oxygen partial pressures of 1 × 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 film growth was real-time monitored by in situ reflective high-energy electron diffraction (RHEED) to ensure the layer-by-layer growing mode of LAO thin film (Figure S1, Supporting Information). X-ray reflectometry was used to check the thickness of the 90 uc sample as shown in Figure S2 in the Supporting Information. The nominal thickness obtained through fitting was determined to be 35.2 nm, indicating the good thickness control with the aid of RHEED. After deposition, the samples were cooled to room temperature in the oxygen partial pressure of deposition at a rate of 20 °C/min without postannealing process. 5 uc LAO/STO samples were also deposited under different oxygen partial pressures (1 × 10−3 Torr and 1 × 10−5 Torr) for comparison (All the other deposition parameters remained the same). Surface morphology of the samples was imaged by an atomic force microscope operating in tapping mode. X-ray diffraction with Cu Kα radiation and four-bounce Ge (220) monochromator was used to characterize the crystallographic structure of LAO/STO. The interfacial structure of LAO/STO was characterized by the field-emission transmission electron microscope (TEM) at an accelerating voltage of 200 kV. Superconducting quantum interference devices magnetometer was used to measure the magnetic property, with a reciprocating sample option parallel to the sample’s surface and with a resolution up to 10−8 emu. The temperature dependence of magnetization was measured in a temperature range from 2 to 300 K at magnetic field up to 2000 Oe. The magnetic field was applied parallel to the sample surface. The sheet resistance, carrier density, and Hall mobility were measured using a physical property measurement system with a Van-Der-Pauw geometry. Direct contact with the LAO/STO interface was made through aluminum wire by wire bonding. The magnetic field (up to 10 T) perpendicular to the interface of the samples was used to measure the magnetoresistance with the current of 50 µA. The nominal sheet carrier density ΔRSdH was determined by the n2D = −B/eRxy, where B is applied magnetic field, Rxy is the Hall resistance, and e is the charge of an electron. The mobility μ was determined from the sheet resistance RS and by µ = 1/n2DRS. The valence states of Ti near the interface was determined by ESCALAB250Xi X-ray photoelectron spectrometer (XPS) using a monochromated Al Kα (hν = 1486.68 eV) source.

Figure 6. a) Schematic view of the 4 uc LaAlO3/TiO2 terminated-5 uc SrTiO3. The position of O vacancy is identified by a blue dashed circle. Four different locations of O vacancy were designed for this calculation. b) Total density of states of 4 uc LaAlO3/5 uc SrTiO3 sub (When the O vacancy is located at 4.). The inset is the partial density of states of Ti.


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Table 3. Magnetic moment of the interface Ti with the change of the position of O vacancy at the surface of LAO in AlO2 layer. O-vacancy position

TiO2

Interface Ti magnetic moment 0.53 µB

1 (AlO2) 2 (AlO2) 3 (AlO2)

4 (AlO2)

0.34 µB

0.09 µB

0.18 µB

0.10 µB

Supporting Information Supporting Information is available online from the Wiley Online Library or from the author.

Acknowledgements The research was supported by the Australian Research Council through projectsDP150103006 and DP170104831. This work was performed in part at the New South Wales (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.

Conflict of Interest The authors declare no conflict of interest.

Keywords LaAlO3/SrTiO3 heterostructures, magnetic behavior, spin-glass-like behavior, oxygen vacancy, spintronics application Received: March 5, 2018 Revised: March 29, 2018 Published online:

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