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Magnetism Control by Doping in LaAlO/SrTiO Heterointerfaces Hong Yan, Zhaoting Zhang, Shuanhu Wang, Xiangyang Wei, Chang-Le Chen, and Kexin Jin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03275 • Publication Date (Web): 05 Apr 2018 Downloaded from http://pubs.acs.org on April 5, 2018

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ACS Applied Materials & Interfaces

Magnetism Control by Doping in LaAlO3/SrTiO3 Heterointerfaces Hong Yan, Zhaoting Zhang, Shuanhu Wang, Xiangyang Wei, Changle Chen and Kexin Jin∗ Shaanxi Key Laboratory of Condensed Matter Structures and Properties, School of Science, Northwestern Polytechnical University, Xi’an 710072, China

ABSTRACT The magnetic two dimensional electron gas at the oxide interfaces always is one of the key issues in spintronics, giving rise to intriguing magneto-transport properties. However, reports about the magnetic two dimensional electron gas remain elusive. Here we obtain the magnetic order at LaAlO3/SrTiO3 systems by introducing magnetic dopants at the La site. The transport properties with a characteristic of metallic behavior at the interfaces are investigated. More significantly, magnetic doped samples exhibit obvious magnetic hysteresis loops and the mobility is enhanced. Meanwhile, the photoresponsive experiments are realized by irradiating all samples with a 360 nm light. Compared with magnetisms, the effects of dopants on photoresponsive and relaxation properties are negligible because the behavior originates from SrTiO3 substrates. This work paves a way for revealing and better controlling the magnetic properties of oxide heterointerfaces. KEYWORDS: LaAlO3/SrTiO3 heterointerfaces, Transport properties, Magnetic two-dimensional electron gas, Magnetism, Persistent photoconductivity

∗

Author to whom correspondence should be addressed; electronic-mail: [email protected] 1

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1. INTRODUCTION The two-dimensional electron gas (2DEG) at the LaAlO3/SrTiO3 (LAO/STO) interface has received a great deal of attention in recent years owing to its potential for the exploration of novel physics and next generation of electronics.1-8 Emergent properties including superconductivity, magnetism, gating effect, and charge writing effect have been discovered.9-16 In particular, magnetic 2DEG at the interface is quite attractive due to its huge applications in magnetic functionality, such as spin generation and spin-to-charge conversion. There are various attempts to magnetic performances of the 2DEG of LAO/STO interfaces. Based on the analysis of X-ray magnetic circular dichroism, Lee et al. declared an in-plane ferromagnetic order for the 2DEG associated with Ti3+ in the dxy orbital of the anisotropic t2g band.17 Bi et al. observed magnetic phases at room temperature through the magnetic force microscopy.18 Furthermore, the results of scanning superconducting quantum interference device (SQUID) microscopy suggested that the ferromagnetism in isolated patches was unchanged with gate voltage.19 The observed magnetic hysteresis along with magneto-resistance oscillations was considered as a commensurability condition of edge states in a highly mobile 2DEG between substrate step edges.20 Meanwhile, strong anisotropy of the magnetoresistance indicated the magnetic ordering but no hysteresis was observed.21,22 Thereby, the 2DEG with spin polarization remains largely elusive although it helps to unravel new physics and possesses great values in practical applications.23-25 Additionally, it is necessary to find a way to create the magnetic order of 2DEG itself and thus the magnetic doping 2


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is provided to achieve the objectives. On the other hand, the transport of interfaces can be manipulated by various methods, such as the external perturbations, substitution or dopant, and electrostatic fields.26-34 Great efforts have been made to investigate the doped LAO/STO interfaces by introducing the ions into the conduction channel and drive the systems into a long range magnetically ordered state.35-38 Unexpectedly, no evidences for magnetic ordering at the LAO/STO interface doped with Ru at the Ti site and Tm, Lu at the La site have been exhibited, acting as sources of localized moments and spin-orbit scattering centers.35 In addition, as a powerful external perturbation, light can also modify the transport properties of 2DEG.39,40 Inspired by the idea of doping with magnetic ions above, we hereby dope 5% R (R= Fe, Co, Ni and Cu) at the Al site of LAO considering that magnetic ions are chosen for modulating the magnetic properties of LAO/STO interface and nonmagnetic Cu ion is used as a compared sample. We observe obvious evidence of a magnetic ordering at doped LAO/STO interfaces by SQUID magnetometry. In addition, the effect of magnetic ions on photoresponsive properties is also investigated. 2. EXPERIMENTAL DETAILS The polycrystalline targets of nominal LARO were prepared by a solid state reaction method (Figure S1). The films of LAO and LARO were deposited on the TiO2-terminated STO substrates using pulsed laser deposition by a KrF excimer laser (λ=248 nm) operating at 1 Hz and 2 J/cm2. Single crystal (001) STO substrates were etched in the buffered-HF and annealed at 970℃to achieve a smooth TiO2 termination. The substrates were held at 800℃ during deposition in a 4 × 10-3 Pa of oxygen 3



atmosphere. The films without the annealing treatment were in situ cooled down to room temperature at a rate of 5 ℃/min under the same oxygen pressure. The MFP-3D atomic force microscopy (AFM) was used to characterize surface morphologies of the samples. X-ray reflectivity (XRR) was carried out by a PANAlytical X’Pert Pro X-ray diffractometer with Cu Kα X-ray source to estimate the thickness of films. Ultrasonic Al wire bonding was used for the electrode contact. All the low-temperature measurements were carried out in a closed-cycle He refrigerator with quartz glass windows. These measurements were performed in the Van de Pauw geometry with an applied magnetic field of 1 T. Magnetic characterization was performed using a Quantum Design SQUID magnetometer over a temperature range of 5 - 300 K. The magnetization vs. temperature curves were measured with an applied external magnetic field of 4 T. The magnetic hysteresis loops were measured from −5 to 5 T at 10 K. Films were irradiated using a light with a wavelength of 360 nm (~3.44 eV) and the power density was about 0.5 W/cm2. 3. RESULTS AND DISCUSSION LAO and LARO epitaxial films with a thickness of 5.3 nm show atomically smooth terrace surfaces (Figure S2 and S3). The temperature dependence of sheet resistance (R□) of the heterointerfaces is shown in Figure 1a. All the doped samples favor the metallic behavior of LAO/STO interface. At low temperature, there is a resistivity upturn at LAO/STO and LARO/STO interfaces except for the R =Cu sample. It is noted that the sheet resistance for the R = Cu sample is the smallest at the whole temperature range. Furthermore, the Hall measurements of all samples are performed 4


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(Figure S4). The obtained sheet carrier density (ns) and mobility (µ) are shown in Figure 1b and 1c, respectively. It is interesting to find that the low temperature mobility dominated by the disorder and localization is increased instead of the suppression by the doping. The electron mobility at T = 12 K obtained for LAO/STO and LARO/STO heterointerfaces are 15.4, 35.1, 32.8, 24.7, and 56.0 cm2/Vs, respectively. The magnetic properties of LAO/STO and LARO/STO samples are investigated using SQUID magnetometry. Through doping with local magnetic moments at the LAO side of interface in the form of 3d electrons of R3+ ions, we explore the possible generation of magnetic response at the interface. As expected, results of SQUID magnetometry show a distinct evidence for the magnetic transition in the temperature dependence of magnetization (Figure S5). It indicates that the magnetic moments can be generated through magnetic dopants. The Curie temperature (TC) is determined by the inflection point of the magnetization curve vs. temperature. And the TC values are ~ 40 K and ~ 51 K for the samples R =Co and Ni, respectively,41 which is roughly consistent with the resistance upturn temperature of 37 K and 55 K. The hysteresis loops of films measured at 10 K are shown in Figure 2. The magnetic hysteresis in the doped films LARO (R = Fe, Co and Ni) is remarkable under an applied field of 4 T at 10 K except for the Cu dopant. The coercivities HC of R =Fe, Co and Ni samples are 359 Oe, 308 Oe and 278 Oe, respectively. No magnetic signals are observed for LAO and R=Cu samples (Figure S6). Previous reports have demonstrated that the magnetic moment is not inherent to the interfaces and our dopants can generate the effect, 5



which is different from that of the Ru dopant.32 The observed hysteresis loops are associated with a long-range magnetic order. Thus, the magnetic 2DEG is obtained by the introduction of interface magnetism. More importantly, the carrier mobility is not deteriorated. In general, the origin of magnetism at LAO/STO interfaces is attributed to several factors, such as localized electrons at Ti 3d orbitals, the band structure itself and oxygen vacancies.20,

42-48

Here, the LAO/STO and R =Cu samples show no

magnetic signals, whereas the R =Fe, Co and Ni samples have the magnetic hysteresis, suggesting that oxygen vacancies make little contribution to the magnetism. Therefore, the magnetism is mainly created by our magnetic dopants. In addition, we also investigate the effect of magnetic ions on photoresponsive properties. Figure 3a shows the temporal evolution of resistance of all samples under the 360 nm light irradiation and the subsequent recovery in dark at 30 K. The other curves of temporal evolution of resistance under the irradiation are available in Figures S7-S11. For the purpose of better understanding the recovery process, we define a normalized resistance by ∆R/RD, where ∆R = RD − R(t), RD is the resistance in dark and R(t) is the resistance at time t. The details of measurement have been presented in our earlier report.49 Figure 3b shows the relative change in resistance of all samples at 30 K after the illumination. The relative changes in resistance of all heterointerfaces under the illumination are 77.2%, 59.8%, 35.5%, 55.3% and 25.4% at 30 K for LAO/STO and R = Fe, Co, Ni and Cu, respectively. Furthermore, we describe the decay process using the bi-exponential function by the following expression as shown by the solid lines in Figure 3b: 6


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R = A exp(t / τ 1 ) + B exp(t / τ 2 ) + C

(1)

where A and B are the magnitude, t is the time, τ1 and τ2 is the time constant for fast and slow relaxation process, respectively. The time impendent term C is presumably due to the heating effect lasting for longer time than time range concerned here. In Figure 3c, we also present the ∆R/RD data as a function of time for all samples at 300 K. A semilogarithmic plot of the time constant τ2 as a function of (1/T) for all the films is shown in Figure 4. The results of τ1 are shown in Figure S12. It can be seen that the turning point appears at ∼ 100 K, which is approximately close to the temperature of antiferrodistortive structural phase transition to a tetragonal symmetry. Meanwhile, we note that the time constant is dependent on temperature and the system recovers much faster at the higher temperatures. The effect of dopant on the time constant is negligible. To better understand the recovery process, we fit the data by the Arrhenius equation, τ = τ 0 exp(− ∆U ) , where τ0 is the high temperature limit of k BT

time constant, ∆U and kB are the activation energy for detrapping the photogenreated carriers and Boltzmann constant, respectively.50 The time constants are the characteristics of relaxation process in the photoinduced resistance change, and the similar phenomena has been reported in LaTiO3/SrTiO3 interface and manganites.51,52 Based on two temperature regions, two activation energies are obtained. At the low-temperature regime, the energy is located in the range of 1.0 to 1.5 meV. Nevertheless, the energy changes from 26.3 to 39.0 meV for all samples at high-temperature regime, which is much higher than that at low temperatures. The energy scale depends on different depths of localization at the subbands of STO. It 7



can infer from the smaller activation energy that the depth of localizations is shallower for our samples. As we all know, STO experiences a transition of dielectric function near ~ 105 K.53 At this temperature, the electrons are easily to be trapped and detrapped due to the loosely bound with electrons. Since the light energy is larger than the band gap energy of STO (3.25 eV) and less than that of films (LAO, 5.6 eV), the major influence on photoconductive behavior results from STO substrates. Therefore, the effects of our dopants on photoresponsive properties are puny compared with the magnetism. 4. CONCLUSIONS In summary, we have investigated the effect of magnetic dopants on the transport properties of LAO/STO interfaces. It presents a metallic behavior in all systems and the doping with ions at Al site significantly modifies the sheet resistance, carrier density and mobility. More significantly, obvious hysteresis loops with the enhanced mobility due to the dopants are obtained by an important influence from Al site. Furthermore, the similar photoresponsive properties at LAO/STO and LARO/STO interfaces are observed. Our work in this study is helpful for deep understandings of the magnetism in oxide 2DEG and suggests huge potential applications in magnetic functionality. Supporting Information The Supporting Information is available from the ACS Publications or from the author. X-ray diffraction pattern of targets; RHEED intensity oscillations; AFM images of 8


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LAO and LARO heterointerfaces; Hall resistance at different temperatures; results of magnetization; temporal evolution of the resistance and normalized resistance with light for all type of samples; time constant τ1 of all samples AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]* ORCID Kexin Jin: 0000-0001-5838-0315 Present Addresses Shaanxi Key Laboratory of Condensed Matter Structures and Properties, School of Science, Northwestern Polytechnical University, Xi’an 710072, China Notes The authors declare no competing financial interest. Acknowledgments This work is supported by the National Natural Science Foundation of China (Nos. 51572222, 11604265, and 61471301) and sponsored by Innovation Foundation for Doctor Dissertation of Northwestern Polytechnical University (No. CX201836). REFERENCES: (1) Ohtomo, A.; Hwang, H. Y. A High-Mobility Electron Gas at the LaAlO3/SrTiO3 Heterointerface. Nature 2004, 427, 423−426. (2) Lee, J.; Demkov, A. A. Charge Origin and Localization at n-type SrTiO3/LaAlO3 Interface. Phys. Rev. B 2008, 78, 193104. (3) Savoia, A.; Paparo, D.; Perna, P.; Ristic, Z.; Salluzzo, M.; Granozio, F. M.; di Uccio, U. S.; 9



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(43) Ruhman, J.; Joshua, A.; Ilani, S.; Altman, E. Competition between Kondo Screening and Magnetism at the LaAlO3/SrTiO3 Interface. Phys. Rev. B 2014, 90, No. 125123. (44) Pentcheva, R.; Pickett, W. E. Charge Localization or Itineracy at LaAlO3/SrTiO3 Interfaces: Hole Polarons, Oxygen Vacancies, and Mobile Electrons. Phys. Rev. B 2006, 74, No. 035112. (45) Yu, L. P.; Zunger, A. A Polarity-Induced Defect Mechanism for Conductivity and Magnetism at Polar–Nonpolar Oxide Interfaces. Nat. Commun. 2014, 5, No. 5118. (46) Rice, W. D.; Ambwani, P.; Thompson, J. D.; Leighton, C.; Crooker, S. A. Revealing Optically Induced Magnetization in SrTiO3 Using Optically Coupled SQUID Magnetometry and Magnetic Circular Dichroism. J. Vac. Sci. Technol. B 2014, 32, 04E102. (47) Rice, W. D.; Ambwani, P.; Bombeck, M.; Thompson, J. D.; Haugstad, G.; Leighton, C.; Crooker, S. A. Persistent Optically Induced Magnetism in Oxygen-Deficient Strontium Titanate. Nat. Mater. 2014, 13, 481-487. (48) Salluzzo, M.; Gariglio, S.; Stornaiuolo, D.; Sessi, V.; Rusponi, S.; Piamonteze, C.; De Luca, G. M.; Minola, M.; Marré, D.; Gadaleta, A.; Brune, H.; Nolting, F.; Brookes, N. B.; Ghiringhelli, G. Origin of Interface Magnetism in BiMnO3/SrTiO3 and LaAlO3/SrTiO3 Heterostructures. Phys. Rev. Lett. 2013, 111, 087204. (49) Yan, H.; Zhang, Z. T.; Wang, S. H.; Zhang, H. R.; Chen, C. L.; Jin, K. X. Modulated Transport Behavior

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and Its Perturbation by Electrostatic Fields in Oxide-Based Two-Dimensional Electron Gas. Phys. Rev. B 2012, 86, No. 075127. (52) Jin, K. X.; Chen, C. L.; Zhao, S. G.; Wang, Y. C.; Song, Z. M.; Yuan, X. Laser-induced Voltage in La0.85Sr0.015MnO3/Fe Heterostructure. J. Mater. Sci. Technol. 2006, 22, 4. (53) Kozuka. Y.; Hikita. Y.; Susaki, T.; Hwang, H. Y. Optically Tuned Dimensionality Crossover in Photocarrier-Doped SrTiO3: Onset of Weak Localization. Phys. Rev. B 2007, 76, No. 085129.

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Figures and Figure captions Figure 1. The temperature dependence of (a) sheet resistance, (b) carrier density, and (c) mobility for the LAO/STO and LARO/STO heterointerfaces. Figure 2. Magnetic hysteresis loops measured at 10 K for R= Fe, Co and Ni samples. Figure 3. (a) Time evolution of the resistance at LAO/STO and LARO/STO heterointerfaces under irradiation of light at 30 K. (b) The relative changes in resistance for LAO/STO and LARO/STO heterointerfaces at 30 K after switching off the irradiation. The recovery process follows a bi-exponential behavior as shown by solid lines. (c) Time evolution of the relative changes in resistance at LAO/STO and LARO/STO heterointerfaces at 300 K. Figure 4. Logarithm of the time constant τ2 as a function of reciprocal temperature. The solid lines are the fitting curves.

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Figure 1

Figure 2

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Figure 3

Figure 4

Table of Contents Graphic

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SrTiO3 Heterointerfaces

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