Highly epitaxial thin films of Pr0.7Sr0.3MnO 3 were grown on (100) SrTiO3 single crystal substrates by laser ablation. Similar to other manganite compounds, these Pr 0.7Sr0.3MnO3 films exhibited remarkable magnetoresistance. Application of electric currents could induce a remarkable reduction in resistivity, demonstrating a strong electroresistance effect. The ratio of the resistance variation, ER=[R(0)-R(I)]/R(I), is about 33% at metal-insulator transition temperature. Using a planar field effect configuration, significant field modulation of the metal-insulator transition was achieved. The observed field effects were discussed based on the strong interactions between carrier spins and localized spins in Mn ions, as well as the percolative mechanism of phase separation. © 2010 The Author(s).published_or_final_versionSpringer Open Choice, 31 May 201

Gao, J

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H. Y. Liu

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Electroresistance and field effects in epitaxial thin films of Pr0.7Sr0.3MnO3

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Title Electroresistance and field effects in epitaxial thin films of Pr0.7Sr0.3MnO3Author(s) Gao, J; Liu, HYCitation Applied Physics A: Materials Science & Processing, 2010, v. 101n. 4, p. 661-664Issued Date 2010URL http://hdl.handle.net/10722/133820Rights Creative Commons: Attribution 3.0 Hong Kong LicenseAppl Phys A (2010) 101: 661–664DOI 10.1007/s00339-010-5920-9Electroresistance and field effects in epitaxial thin filmsof Pr0.7Sr0.3MnO3J. Gao · H.Y. LiuReceived: 22 November 2009 / Accepted: 15 June 2010 / Published online: 29 July 2010© The Author(s) 2010. This article is published with open access at Springerlink.comAbstract Highly epitaxial thin films of Pr0.7Sr0.3MnO3were grown on (100) SrTiO3 single crystal substrates bylaser ablation. Similar to other manganite compounds, thesePr0.7Sr0.3MnO3 films exhibited remarkable magnetoresis-tance. Application of electric currents could induce a re-markable reduction in resistivity, demonstrating a strongelectroresistance effect. The ratio of the resistance variation,ER = [R(0) − R(I)]/R(I), is about 33% at metal-insulatortransition temperature. Using a planar field effect config-uration, significant field modulation of the metal-insulatortransition was achieved. The observed field effects were dis-cussed based on the strong interactions between carrier spinsand localized spins in Mn ions, as well as the percolativemechanism of phase separation.1 IntroductionPerovskite manganites R1−xAxMnO3 (where R and A arerare- and alkaline-earth ions, respectively) are known as atypical system with a strong electron correlation. In suchcompounds, spin, charge, lattice, and orbital are stronglycoupled, resulting in various ground states, such as the fer-romagnetic metallic state and charge/orbital ordering state,etc. These ground states have comparable energies so thatan external disturbance can easily drive the system fromone state to another state, leading to a dynamic coexistenceof phases with different magnetic and electronic proper-ties. Much research effort has been devoted to understandJ. Gao () · H.Y. LiuDepartment of Physics, The University of Hong Kong, PokfulamRoad, Hong Kong, Chinae-mail: jugao@hku.hkFax: +852-25599152the underlying physics and related phenomena like colos-sal magnetoresistance (CMR) and electroresistance (ER) ef-fects [1–3]. It is generally agreed that the double-exchangeinteraction via spin-polarized conduction electrons is themain cause. External magnetic fields can align the mag-netic moments of Mn ions, thereby enhancing the double-exchange transfer integral and decreasing the resistivity. Thestrong competition among the coexisted multiphase mightalso be responsible for their extremely rich phase diagramand may even lie at the core of the CMR and ER effectsfor manganites with different doping concentrations. Recentinvestigations proved that the equilibrium of multiphase co-existence can be influenced not only by a magnetic field butalso by an electric field or current bias [6].Since the magnetic field required for a significant MR isvery high, and usually exceeds several Tesla, the prospectfor a direct application of MR effect is not clear. In con-trast, the practical feasibility to use electric fields/currentsto modulate the insulator–metal (I–M) phase transition andtransport properties of manganites is of great merit [4–7].The compound Pr1−xSrxMnO3 is known being very sen-sitive to external disturbances, especially in the vicinity ofpercolation threshold [8–10]. It mimics the behavior of theLa1−xCaxMnO3 system. In this paper, we report a studyon the field/current effects in Pr0.7Sr0.3MnO3 (PSMO) thinfilms grown by laser ablation. From the phase diagram, theselected doping concentration x = 0.3 makes the PSMO be-having as a ferromagnetic metal.2 ExperimentalThe studied PSMO films were grown on (100) SrTiO3 sin-gle crystal substrates by laser ablation. The target was madeby standard solid state reaction from metallic oxides and662 J. Gao, H.Y. LiuFig. 1 Typical X-ray diffraction spectra for our PSMO films. The insetis the rocking curve of the (002) peak of PSMOcarbonates. The nominated composition of the target wasPr0.7Sr0.3MnO3. The deposition took place in oxygen of0.5 mbar. The beam energy was ∼380 mJ with the wave-length of 248 nm and pulse frequency of 2 Hz. The beamwas focused by a lens, which produced an energy density∼5 J/cm2 at the target. The distance between substrate andtarget was 6 cm. The substrate temperature was 750◦C asmeasured by a k-type thermocouple inserted into the heaterblock. The film thickness was between 150 to 300 nm, con-trolled by the deposition time. The electric measurementswere done by using the standard four-probe technique ina closed cycle cryostat. In order to apply a current withhigh density in the electric measurements, the films werepatterned into a microbridge with the width of 50 μm andlength of 200 μm using lithography technique. Field effectwas studied using a planar field effect configuration (FEC),which is formed on a monolayer of PSMO [11].3 Results and discussionsThe grown thin films of PSMO are of highly epitaxy as char-acterized by X-ray diffraction and rocking curve measure-ments. The spectra of X-ray diffraction always reveal thesharp peaks of the formed ABO3 phase with the c-axis per-pendicular to the substrate surface (Fig. 1). Besides the re-flections from substrate and the (00l) peaks of the PSMO,no peaks of secondary phases are visible. The value of fullwidth at half maximum for rocking curve of the PSMO(002) peak is less than 0.2 degree, implying very good epi-taxy and crystallinity formed in our films.As a typical feature of doped manganites, the grown filmsshow a clear phase transition from an insulator to a metalnear the Curie temperature. Such a phase transition can beaffected by oxygen deficiency. For the as-grown film, whichis suspected being oxygen deficient, the transition peak isfound T p ∼ 210 K. After annealed in 1 bar oxygen at 700◦Cfor 1 hour, the resistance of the film is reduced remarkablyFig. 2 The comparison of the R–T dependences of 300 nm PSMOfilm as grown and post annealed. The inset shows the magnetic fielddependence of the transport behaviorFig. 3 Temperature dependence of resistance for a 300 nm PSMO filmunder application of different electric currents from 50 μA to 5 mAand the I–M transition temperature increase to over 250 K(Fig. 2). The resistance of the sample also decreased from∼60 k to ∼7 k. The differences between as-grown andannealed films are attributed to the influence of oxygen de-fects on the phase separation. It is thought that the equi-librium between metallic phase and insulating phase in as-grown films is much unstable due to the oxygen deficiency.In addition, the resistance of as-grown films is one orderlarger than that of the annealed films. An external distur-bance to as-grown films could impact the equilibrium andmay result in a more significant reduction of the resistance.On the other hand, the transition temperature of the post-annealed PSMO films is slightly higher in comparing withthat of its bulk (∼245 K). This is probably due to the strainintroduced by the lattice mismatching between SrTiO3 sub-strate and grown PSMO layer [9].Electroresistance and field effects in PrSrMnO films 663When a magnetic field is applied, these PSMO filmsshowed a very strong magnetoresistance. As can be seen inFig. 2, a magnetic field of 1.0 Tesla can induce a remark-able MR effect. The ratio of the resistance variation {MR =[R(0) − R(H)]/R(0)} is found ∼57%, which is more sig-nificant in comparing with some other CMR compounds[5, 6, 8]. The ER effect induced by electric field/current isknown as a common feature for thin films of doped man-ganites [6, 7, 11]. The influence of the electric field/currenton the transport properties of our PSMO films has also beenexplored. Figure 3 shows the temperature dependences ofresistance for a 300 nm film of PSMO with different cur-rents applied. It is found that the transition temperaturepeak, T p, remains nearly unchanged, whereas the resis-tance, especially the peak resistance, is modulated by theapplied currents. When the current increases from 1 μAto 50 μA (the current density ∼3.4 × 102 A/cm2), thepeak resistance drops remarkably from ∼105 k to 70 k.The changing ratio of the peak resistance (defined as ER =[R(0)−R(I)]/R(0)) reached 33% under a current of 50 μA,which is also much pronounced than what we observed onLa0.8Ca0.2MnO3 and La0.9Ba0.1MnO3 thin films [6]. To en-sure that the obtained results are reliable, similar measure-ments were performed on a number of samples and all theresults were consistent.Such an ER effect is fully reversible as the transitioncurve could return to the initial state if the applied currentis removed. Obviously, the observed ER effect cannot becaused by the loss of oxygen. On the other hand, the selfmagnetic field generated by the applied current also can-not be the main cause for the observed ER effect. Suppos-ing the film is an infinitely large plane carrying a uniformcurrent with a surface current density ∼1 × 103 A/cm. Themagnetic field produced by the applied current is estimated∼10−2 T. Such a value of the self-field is far below the mag-netic field required to cause a MR equivalent to 33%.The influence of a static electric field on the I–M tran-sition has been studied as well. In our work, a very simpleplanar FEC formed on a monolayer film is employed (seethe inset of Fig. 4). As such a FEC is formed in a mono-layer, a transverse electric field is applied to the microbridgeof PSMO. Significant modulation of the I–M transition wasachieved. The modulation of the resistance for the PSMOchannel upon a bias of +350 V (+1.75×105 V/cm) reaches44%, whereas the modulation caused by a negative bias of−200 V (−1.0 × 105 V/cm) is −18%. The results of fieldeffect for PSMO channel are similar to that reported by Wuet al. and Gao et al. [12, 13]. The gate voltage applied toour planar FEC is remarkably larger in comparing with theresults obtained on multilayer FEC. Actually we find thatthe field gradient, rather than the absolute field value, playsa more important role for the field effect. In our experi-ments, the bias applied to the sample is a transverse fieldFig. 4 Temperature dependence of resistance for a 150 nm PSMOas grown film under application of different electric fields. The insetshows the planar field effect configurationwith a spacing ∼20 μm. The field gradient is therefore muchsmaller. Moreover, it is material dependent, e.g., the neededgate voltage is ∼80 V for a La0.7Ca0.3MnO3 channel [11].The sign of the field modulation is found depending onthe bias direction. Similar to what reported on multilayerFEC [12], the resistance enhances when a positive gate volt-age is applied and decreases with a negative gate voltage.Also, the magnitude of modulation is asymmetric with thebias polarity. When the gate potential sweeps between pos-itive and negative directions, nonlinear dependence appearsand no saturation trend is revealed. All measured films couldreturn to their pristine state completely if the bias was re-moved.At this moment, the underlying physics behind the ob-served field effects has not been well understood yet. In ouropinion, such effects induced by the applied dc-current/fieldshould be related to the percolative mechanism of phase sep-aration [11]. A dynamic coexistence of phases with differ-ent magnetic and electronic properties is known as one ofthe most characteristic features of doped manganites. Thesecoexisted phases have comparable energies so that externaldisturbances can easily drive the system transiting from onestate to another. Application of an electric field/current insuch an inhomogeneous system may change, by accumula-tion of charge, the relative volume fractions of the metal-lic and insulating phases. The boundary between metallicand insulating phases can be therefore moved. The direc-tion of such interface shift is controlled by the polarity ofthe applied field. Application of a negative field shifts theI–M phase boundary to the insulating phase. It increasesthe fraction of metallic phase, hence reduces the resistance.Whereas by moving the I–M interface to the metallic site, apositive field may enhance the resistance. More clear under-standing of the observed effects needs more detailed studies.664 J. Gao, H.Y. Liu4 ConclusionThe effects of electric fields/currents on the I–M phase tran-sition and transport properties of Pr0.7Sr0.3MnO3 epitaxialthin films have been investigated. These films are epitaxi-ally grown with c-axis perpendicular to the film surface andshow a good crystallinity, as revealed by X-ray diffractionand rocking curve measurements. Application of electriccurrents to these PSMO films causes remarkable changesin their resistance, demonstrating a strong ER effect. Theobserved ER ratio of the peak resistance reached 33% witha current of 50 μA, which is much pronounced than whatwe observed on La0.8Ca0.2MnO3 and La0.9Ba0.1MnO3 thinfilms. Significant modulation on the I–M transition peak bya static electric field was obtained using a simple planarFEC. The sign of the field modulation changes with the po-larity of gate voltage, which is similar to what reported onmultilayer FEC. Such ER and field effects might be relatedto the percolative mechanism of phase separation.Acknowledgement This work has been supported by a grant of theResearch Grant Council of Hong Kong (Project No. HKU 7024/07P).Open Access This article is distributed under the terms of the Cre-ative Commons Attribution Noncommercial License which permitsany noncommercial use, distribution, and reproduction in any medium,provided the original author(s) and source are credited.References1. P.W. Anderson, H. Hasegawa, Phys. Rev. 100, 675 (1955)2. R. von Helmolt, J. Wecker, B. Holzapfel, L. Schultz, K. Samwer,Phys. Rev. Lett. 71, 2331 (1993)3. S. Jin, T.H. Tiefel, M. McCormack, R.A. Fastnacht, R. Ramesh,L.H. Chen, Science 264, 413 (1994)4. C. Mitra, P. Raychaudhuri, K. Dörr, K.-H. Müller, L. Schultz,P.M. Oppeneer, S. Wirth, Phys. Rev. Lett. 90, 017202 (2003)5. H. Tanaka, J. Zhang, T. Kawai, Phys. Rev. Lett. 88, 027204 (2002)6. J. Gao, S.Q. Shen, T.K. Li, J.R. Sun, Appl. Phys. Lett. 82, 4732(2003)7. J. Gao, F.X. Hu, Appl. Phys. Lett. 86, 092504 (2005)8. G.X. Xiong, B. Zhang, S.C. Wu, Z.X. Lu, G.J. Lian, D.S. Dai,Solid State Commun. 97, 777 (1996)9. W. Boujelben, A. Cheikh-Rouhou, M. Ellouze, J.C. Joubert, Phys.Stat. Sol. 177, 503 (2000)10. V. Markovich, I. Fita, R. Puzniak, A. Wisniewski, K. Suzuki,J.W. Cochrane, Y. Yuzhelevskii, Ya.M. Mukovskii, G. Gorodet-sky, Phys. Rev. B 71, 224409 (2005)11. F.X. Hu, J. Gao, Appl. Phys. Lett. 88, 132502 (2006)12. T. Wu, S.B. Ogale, J.E. Garrison, B. Nagaraj, A. Biswas, Z. Chen,R.L. Greene, R. Ramesh, T. Venkatesan, A.J. Millis, Phys. Rev.Lett. 86, 5998 (2001)13. J. Gao, F.X. Hu, IEEE Trans. Magn. 43, 3058 (2007)

Electroresistance and field effects in epitaxial thin films of Pr 0.7Sr0.3MnO3

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Electroresistance and field effects in epitaxial thin films of Pr 0.7Sr0.3MnO3

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