Current-voltage characteristics of LiNbO3/La0.69Ca0.31MnO3 ...

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Oct 4, 2007 - Recently, Wu et al. deposited ferroelectric films of. Pb(Zr0.3 .... Wu, M. A. Zurbuchen, S. Saha, R. V. Wang, S. K. Streiffer, and J. F.. Mitchell, Phys ...
APPLIED PHYSICS LETTERS 91, 143509 共2007兲

Current-voltage characteristics of LiNbO3 / La0.69Ca0.31MnO3 heterojunction and its tunability S. M. Guo, Y. G. Zhao,a兲 and C. M. Xiong Department of Physics, Tsinghua University, Beijing 100084, China

W. G. Huang and Z. H. Cheng State Key Laboratory of Magnetism, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, China and Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, China

X. X. Xi Department of Physics, The Pennsylvania State University, University Park, Pennsylvania 16802, USA and Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, USA

共Received 9 June 2007; accepted 26 August 2007; published online 4 October 2007兲 The authors have fabricated LiNbO3 / La0.69Ca0.31MnO3 heterojunctions by growing LiNbO3 on La0.69Ca0.31MnO3 single crystals. Rectifying behavior was found in these junctions, which can be tuned by applied magnetic field. A band diagram is proposed to account for the junction behavior. A voltage pulse-induced resistive switching was also observed, which can be understood by considering the ferroelectric polarization at the junction interface. The ability to tune transport properties of ferroelectric-ferromagnetic heterojunctions by magnetic field and electric polarization is potentially significant for their electronic applications. © 2007 American Institute of Physics. 关DOI: 10.1063/1.2785113兴 Multiferroic materials are of great scientific and technological interest due to their magnetoelectric properties, originating from the coupling between ferroelectric and ferromagnetic order parameters.1 The interplay of the ferroelectricity and ferromagnetism allows for tuning of multiferroic devices by both magnetic and electric fields. Besides single-phase multiferroic materials, the combination of ferroelectric and ferromagnetic materials has also been intensively studied as an important approach to multiferroics. Material systems studied so far include various multilayers of ferroelectric and ferromagnetic materials and magnetic films on ferroelectric substrates. The structure with ferroelectric films grown on ferromagnetic substrates has rarely been studied because the suitable ferromagnetic substrates are not readily available. This structure is useful for studying the effect of magnetization on the ferroelectric property and their coupling at the interface on the characteristics of the heterostructure. Recently, Wu et al. deposited ferroelectric films of Pb共Zr0.3 , Ti0.7兲O3 on layered manganite La1.2Sr1.8Mn2O7 single crystals and found remarkable magnetoelectric effect.2 However, they did not investigate the effect of magnetization or ferroelectric polarization on the transport property of the heterostructure, a topic that has attracted much attention due to the potential for memory applications with multiple states.3 In this paper, we report a study on heterojunction between ferroelectric LiNbO3 共LNO兲 thin film and La0.69Ca0.31MnO3 共LCMO兲 single crystal, a colossal magnetoresistance 共CMR兲 material with a ferromagnetic Tc of about 230 K.4 A rectifying current-voltage 共I-V兲 characteristic was observed, which can be modified by applying a magnetic field. A resistive switching by voltage pulses was also observed indicating a clear influence of ferroelectric polarization on the junction property. These results demonstrate a兲

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the potential applications of ferroelectric-ferromagnetic heterojunctions for electronic devices. LNO films were grown on LCMO single crystal substrates by pulsed laser deposition. The description of the LCMO single crystal substrate and the growth condition of the LNO films can be found in our previous work.5,6 The thickness of LNO film is about 100 nm. Figure 1 shows a ␪-2␪ scan x-ray diffraction pattern for a LNO film on LCMO substrate. It indicates that the LNO film is oriented with 共012兲 axis parallel to the 共001兲 axis of the LCMO single crystal. The in-plane alignment of LNO film was investigated using phi scan and the result is shown in the inset of Fig. 1. It shows four 共104兲 peaks of LNO film separated by 90°, indicating the in-plane alignment of the film with the substrate. The I-V characteristics of the heterojunction in magnetic field were measured in a Quantum Design MPMS XL7 system with the applied field parallel to the film surface. The top

FIG. 1. X-ray diffraction pattern for LNO/LCMO heterojunction. The inset shows the phi scan for the 共104兲 peak of LNO film.

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FIG. 3. Band structure of 共a兲 LCMO and 共b兲 isotype p-LNO/ p-LCMO heterojunction.

FIG. 2. 共Color online兲 共a兲 I-V characteristic of LNO/LCMO heterojunction at 230 K under magnetic fields of 0 and 5 T, respectively. The upper inset is the layout of the heterojunction for measurement. The bottom inset is the replot for the I-V data of 230 K with and without magnetic field by the equation J ⬃ exp共−qVD / kBT兲关exp共qV / nkBT兲-1兴. 共b兲 The bias voltage dependence of MR at 230 K. The inset exhibits the temperature dependence of MR at the bias voltages of +0.1 and −0.1 V, respectively.

In electrode was pressed on the LNO film and the bottom Au electrode was deposited on the back of LCMO. The bias voltage 共+1 to − 1 V兲 with a step of 0.05 V and an interval of 2 s was applied between the top and bottom electrodes, and the current was measured in a two-probe arrangement using a Keithley 2400. The voltage polarity and the layout of the device are illustrated in the inset of Fig. 2共a兲. Figure 2共a兲 shows the I-V characteristics of a LNO/ LCMO heterojunction at 230 K under 0 and 5 T magnetic fields. They show a diodelike characteristic, which is dependent on the magnetic field. For a certain bias voltage, the junction current increases with magnetic field and returns to the initial value when the magnetic field is withdrawn, indicating a negative magnetoresistance 共MR兲. The MR is defined as 共RH-R0兲 / R0 ⫻ 100%, where RH 共R0兲 are the junction resistance 共defined as V / I兲 with 共without兲 magnetic field. MR versus bias voltage at 230 K is illustrated in Fig. 2共b兲, where a MR of nearly −50% is observed near the zero bias. The temperature dependence of MR for the bias voltages of 0.1 and −0.1 V is exhibited in the inset of Fig. 2共b兲. The maximum MR value appears around 230 K, the metal-insulator transition temperature 共T P兲 of the LCMO single crystal, indicating its correlation with LCMO. The contact between the Au electrode and LCMO is Ohmic and the contact resistance 共⬃2 ⍀兲 is much less than the junction resistance. The resistance for the bulk LCMO along the direction perpendicular to the surface is also less than 0.5 ⍀, which can be ignored compared to the total junction resistance. According to the band diagram, the barrier of In/LNO junction is small,6 and the I-V behavior should be dominated by LNO/LCMO junction. This is consistent with the remarkable MR because LCMO/LNO junction could be tuned by magnetic fields rather than In/LNO junction.

The diodelike I-V characteristics have been observed previously in heterojunctions composed of manganites or ferroelectric materials and explained by considering the energy band bending at the interfaces due to the work function differences between different materials.6,7 However, there has been no report of rectifying property in CMR/ ferroelectric heterostructures. LCMO can be regarded as a degenerate p+-type conductor at low temperatures and nearly degenerate p-type semiconductor around T P.5 Below T P, its Fermi level 共EF兲 is located in the valence band 共e1g ↑ 兲 with work function of about 5.3 eV and the energy gap of 1 eV.4 The electron affinity of LNO is about 1.1 eV with an energy gap of 3.9 eV.8 It has been reported that LNO films can be treated as a p-type semiconductor with its EF close to the top of the valance band.8,9 The band diagram of the LNO/LCMO isotype heterojunction below T P of LCMO is plotted schematically in Fig. 3. ⌬EC and ⌬EV are the discontinuities in the valence and conduction band edges, respectively, between LCMO and LNO. As illustrated in the band diagram, the diffusion of carriers results in the depletion of LCMO and accumulation of LNO at the p-p heterojunction interface.10 The band bending in the depletion region obstructs the carrier transport through the heterojunction, while the band bending in the accumulation layer aids the strength of the current.10 It has been reported that11 both the barrier height and the effective carrier density of LCMO at the interface could be regulated by applying a magnetic field. In order to estimate the magnetic field effect on the energy barrier of the junction, we fit the I-V data in Fig. 2共a兲 with thermoinic emission equation J ⬃ Js关exp共qV / nkBT兲-1兴,12 where Js ⬃ exp共−qVD / kBT兲, kB is the Boltzmann constant, V is the bias voltage, VD is the built-in potential, and n is the ideal factor of the junction. Good fitting for the lower bias voltages 共V ⬍ 0.4 V兲 is illustrated in the bottom inset of Fig. 2共a兲, which gives VD0 = 0.377 eV, n0 = 6.8, VDH = 0.365 eV, and nH = 7.4 for the junction, where VD0共VDH兲 and n0共nH兲 are the built-in potential and ideal factor without 共with兲 magnetic field, respectively. With the low bias voltage approximation, MR= RH / R0 − 1 = J0 / JH − 1 = 共nH / n0兲exp共−q␦VDH / kBT兲 − 1, where ␦VDH 共=VD0-VDH兲 is the variation of the built-in potential influenced by the magnetic field, J0共JH兲 is the current density, and n0共nH兲 is the ideal factor of the junction without 共with兲 magnetic field, respectively. At 230 K with zero bias, the MR of the junction can reach −50%, which corresponds to ␦VDH ⬃ 0.016 eV. This value is consistent with the difference of VD obtained from the fitting of the bottom inset of Fig. 2共a兲.

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sample were measured from +1 to − 1 V, so the polarization of the LNO had been biased with negative voltage. The electroresistance 共ER兲, defined as 共RHRS / RLRS − 1兲, increases linearly with the pulse voltage value, as illustrated in Fig. 4共b兲. The ferroelectric hysteresis measured at 1000 Hz was shown in Fig. 4共c兲, indicating ferroelectricity of the LNO film. The pulse voltage-induced resistance modulation in ferroelectric heterostructures has generally been explained by the barrier regulation at the interface by ferroelectric polarization.16 Since the nucleation of the ferroelectric domain reversal starts at the interface of electrodes,17 small change of the polarization could dramatically change the energy barriers at the interface via the depolarization field screening effect.16 The pulse voltage used to induce the resistive switching in the LNO/LCMO junction is very small 共0.02 V兲 compared to those previously reported in the nonferroelectric transition metal oxide junctions,18 where several voltages are needed to induce the resistive switching.

FIG. 4. 共a兲 Pulse voltages 共upper panel兲 and their induced resistive switching of the heterojunction at room temperature 共lower panel兲. 共b兲 Variation of ER with pulse voltage value. 共c兲 Ferroelectric hysteresis loop of LNO/ LCMO heterojunction.

The effect of magnetic field and bias voltage on the thickness of the depletion layer could be described by the equation d1 = 兵2NA2␧1␧2␧0共VD − V兲 / 关qNA1共␧1NA1 + ␧2NA2兲兴其1/2,12 where d1 is the thickness of the depletion layer of LCMO, NA1 共NA2兲 is the acceptor concentration in LCMO 共LNO兲, ␧1 共␧2 , ␧0兲 is the dielectric constant of LCMO 共LNO, vacuum兲, and q is the charge of the carrier. Adopting the parameters8,13 ␧1 = 20, ␧2 = 31, VD ⬃ VD1 = 0.35 V, NA2 Ⰶ NA1 ⬃ 10−21 cm−3, and V = 0, we can get d1 ⬃ 12 Å. According to the above equation, d1 decreases with the increase of forward bias voltage, leading to the decrease of the MR due to the weight decrease of the depletion layer in the total junction resistance. While with the reverse bias voltage increasing, it has been proposed5,14 that the band bending increases at the interface leading to the increase of scattering of carriers transporting from e1g spin up band in LCMO to the t2g spin down band at the interface, which counteracts the negative MR. On the other hand, the magnetic field can change NA1,11 which can tune the thickness of the depletion layer according to the formula of d1. It has been predicted that magnetic field can induce abrupt carrier density increase around T P of LCMO,15 which should be contributed to the maximum negative MR appearing at around T P of LCMO, as shown in the inset of Fig. 2共b兲. The LNO/LCMO junction property can also be tuned by the ferroelectric polarization in the LNO layer. Figure 4共a兲 shows a resistive switching behavior of a LNO/LCMO junction induced by electric pulses of different voltages 共V P兲 at room temperature. The resistance of the junction was measured with a constant low measurement voltage, while pulse voltages of different values with 2 ms width were applied. Under the measurement voltage of −0.01 V, the junction was switched between a low resistive state 共LRS兲 after a positive pulse voltage and a high resistive state 共HRS兲 after a negative pulse voltage. It should be pointed out that before the resistive switching measurement, the I-V curves of the

This work was supported by the National Science Foundation of China 共Grant Nos. 50425205, 10674079, 10474132, and 50272031兲 and National 973 projects 共Grant Nos. 2006CB921502, 2002CB613505, and 2005CB724402兲. N. A. Spaldin and M. Fiebig, Science 309, 391 共2005兲; W. Eerenstein, N. D. Mathur, and J. F. Scott, Nature 共London兲 442, 759 共2006兲; R. Ramesh and N. A. Spaldin, Nat. Mater. 6, 21 共2007兲. 2 T. Wu, M. A. Zurbuchen, S. Saha, R. V. Wang, S. K. Streiffer, and J. F. Mitchell, Phys. Rev. B 73, 134416 共2006兲. 3 M. Gajek, M. Bibes, S. Fusil, K. Bouzehouane, J. Fontcuberta, A. Barthélémy, and A. Fert, Nat. Mater. 6, 296 共2007兲; J. F. Scott, ibid. 6, 256 共2007兲. 4 J. M. D. Coey, M. Viret, and S. von Molnar, Adv. Phys. 48, 167 共1999兲. 5 C. M. Xiong, Y. G. Zhao, Z. H. Zhao, Z. Q. Kou, Z. H. Cheng, H. F. Tian, H. X. Yang, and J. Q. Li, Appl. Phys. Lett. 89, 143510 共2006兲. 6 S. M. Guo, Y. G. Zhao, C. M. Xiong, and P. L. Lang, Appl. Phys. Lett. 89, 23506 共2006兲. 7 Y. Watanabe, Phys. Rev. B 57, R5563 共1998兲; H. Tanaka, J. Zhang, and T. Kawai, Phys. Rev. Lett. 88, 027204 共2002兲; J. R. Sun, C. M. Xiong, T. Y. Zhao, S. Y. Zhang, Y. F. Chen, and B. G. Shen, Appl. Phys. Lett. 84, 1528 共2004兲. 8 W. C. Yang, B. J. Rodriguez, A. Gruverman, and R. J. Nemanich, Appl. Phys. Lett. 85, 2316 共2004兲. 9 J. A. Chaos, J. Gonzalo, C. N. Afonso, J. Perrière, and M. T. GarcíaGonzález, Appl. Phys. A: Mater. Sci. Process. 72, 705 共2001兲. 10 R. C. Kumer, Int. J. Electron. 25, 239 共1968兲; B. W. Hakki, J. Appl. Phys. 52, 6054 共1981兲. 11 N. Nakagawa, M. Asai, Y. Mukunoki, T. Susaki, and H. Y. Hwang, Appl. Phys. Lett. 86, 082504 共2005兲; Y. W. Xie, J. R. Sun, D. J. Wang, S. Liang, W. M. Lü, and B. G. Shen, J. Phys.: Condens. Matter 19, 196223 共2007兲. 12 S. M. Sze, Physics of Semiconductor Devices, 2nd ed. 共Wiley, New York, 1981兲. 13 J. L. Cohn, M. Peterca, and J. J. Neumeier, Phys. Rev. B 70, 214433 共2004兲. 14 K. J. Jin, H. B. Lu, Q. L. Zhou, K. Zhao, B. L. Cheng, Z. H. Chen, Y. L. Zhou, and G. Z. Yang, Phys. Rev. B 71, 184428 共2005兲. 15 A. S. Alexandrov and A. M. Bratkovsky, Phys. Rev. Lett. 82, 141 共1999兲. 16 P. W. M. Blom, R. M. Wolf, J. F. M. Cillessen, and M. P. C. M. Krijn, Phys. Rev. Lett. 73, 2107 共1994兲; R. Meyera and R. Waser, J. Appl. Phys. 100, 051611 共2006兲; Y. Watanabe, Phys. Rev. B 59, 11257 共1999兲; M. Y. Zhuravlev, R. F. Sabirianov, S. S. Jaswal, and E. Y. Tsymbal1, Phys. Rev. Lett. 94, 246802 共2006兲. 17 A. Erbil, Y. Kim, and R. A. Gerhardt, Phys. Rev. Lett. 77, 1628 共1996兲; M. Dawber, K. M. Rabe, and J. F. Scott, Rev. Mod. Phys. 77, 1083 共2005兲. 18 Y. B. Nian, J. Strozier, N. J. Wu, X. Chen, and A. Ignatiev, Phys. Rev. Lett. 98, 146403 共2007兲; M. Quintero, P. Levy, A. G. Leyva, and M. J. Rozenberg, ibid. 98, 116601 共2007兲; D. S. Shang, Q. Wang, L. D. Chen, R. Dong, X. M. Li, and W. Q. Zhang, Phys. Rev. B 73, 245427 共2006兲. 1

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