- Email: [email protected]
ferromagnet based heterostructure
Physica C 497 (2014) 30–33
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Physica C journal homepage: www.elsevier.com/locate/physc
Electrical and magneto-transport properties of ferromagnet/ superconductor/ferromagnet based heterostructure Minaxi Sharma a, K.K. Sharma a,⇑, Ravi Kumar b,d, R.J. Choudhary c a
Department of Physics, National Institute of Technology, Hamirpur, (H.P.) 177 005, India Centre for Material Science and Engineering, National Institute of Technology, Hamirpur, (H.P.) 177 005, India c UGC-DAE Consortium for Scientific Research, Khandwa Road, Indore, (M.P.) 452 001, India d Beant College of Engineering and Technology, Gurdaspur, (Pb.) 143 521, India b
a r t i c l e
i n f o
Article history: Received 20 April 2013 Received in revised form 25 September 2013 Accepted 7 October 2013 Available online 26 October 2013 Keywords: High temperature superconductor CMR materials Heterostructures Magnetoresistance
a b s t r a c t La0.7Sr0.3MnO3/YBa2Cu3O7/La0.7Sr0.3MnO3 (LSMO/YBCO/LSMO) heterostructures were prepared by pulsed laser deposition technique. The resistivity–temperature (q–T) variation and magnetoresistance effects of these heterostructures have been studied and are found to be dependent on the thickness of superconducting spacer layer. We have observed that metal–insulator (TMI) transition get suppressed with increase in thickness of the spacer layer. The maximum magnetoresistance ratio 70% at 266 K and 65% at 254 K temperature are observed for 50 nm and 100 nm spacer layers respectively. In LSMO(200 nm)/YBCO(50 nm)/LSMO(200 nm) specimen the temperature coefficient of resistance (TCR) is 6.63% K 1 which can be useful for bolometric performances and temperature sensors. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction The advent of high-temperature superconductivity (HTS) in cuprates has rejuvenated the curiosity in magnetic oxides. Lanthanum manganite perovskite materials (LaMnO3), when doped with divalent metal ions such as Ba, Ca, Sr, Pb etc. at La site, it exhibits colossal magnetoresistance (CMR) [1,2]. The properties of CMR materials governed by the mechanism of double exchange (DE) interaction between manganese (Mn) ion spins via oxygen, indicates that the degree of spin polarization of charge carriers may be close to unity [3,4]. For ferromagnet (LSMO), the spin states of Mn ions fluctuate between trivalent (Mn3+) and tetravalent (Mn4+) ions. These spin fluctuating states of LSMO are analogous to the normal conducting state of superconductor (divalent and trivalent copper ions Cu2+ and Cu3+ ions), as far as the realization of antiferromagnetic correlation with the spin fluctuation is concerned [5]. The study of HTS/CMR heterostructures has gained substantially larger interest due to underlying new physics and discrete applications in spintronic devices [6]. Superconducting properties such as critical current (IC) and critical temperature (TC) get suppressed when spin polarized carriers from CMR materials are injected into high temperature superconductors [7,8]. It is apparent that these injected spin polarized carriers can be
⇑ Corresponding author. Tel.: +91 9418780275; fax: +91 1972 223834. E-mail address: [email protected] (K.K. Sharma). 0921-4534/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physc.2013.10.009
utilized as a powerful tool for probing the spin dependent properties of trilayered heterostructures, its experimental investigations would open the entry for viable wide class of superconducting devices. Chahara et al. [9] studied the magnetoresistance (MR) effects of La0.72Ca0.28MnOZ (LCMO) magnetic films and LCMO/YBa2Cu3Oy/ LCMO trilayered films. It has also been reported that the spin fluctuations in HTS/CMR system may be a key to understand the coupling and proximity effects [10]. Furthermore a large negative MR in La0.7Ca0.3MnO3/YBa2Cu3O7/La0.7Ca0.3MnO3 (LCMO/YBCO/LCMO) trilayers has been observed by Pena et al. [11] [12] and they have also reported the spin accumulation in YBCO spacer layer when the ferromagnetic (FM) layers get coupled antiferromagnetically. For exploring the possibility of new superconducting devices, the studies on manganite and high temperature superconductor heterostructures have initially focused on spin injection [13], suppression of critical temperature [14] and the effects of MR [15]. The proximity effects have also been studied in ferromagnet-insulator-superconductor heterostructures [7,13,14,16]. The studies of multilayered heterostructures have drawn a cogent attention towards understanding the physical phenomena occurring at the interface of CMR and HTS materials [6–8,16]. In such multilayered systems the experimental findings get affected by interface phenomena across the layers, due to possible scattering and disruption of either superconductivity or ferromagnetism. Moreover, the giant magnetoresistance (GMR) effects have also been observed in layered magnetic thin-film heterostructures consisting of alternating ferromagnetic and nonmagnetic layers
M. Sharma et al. / Physica C 497 (2014) 30–33
[7,8]. In present study, we have used LSMO as the ferromagnetic material which exhibits colossal magnetoresistance and YBCO as high temperature superconductor for fabrication of LSMO/YBCO/ LSMO heterostructure because of good matching in their lattice parameters. Due to structural similarity between HTS and CMR materials, the epitaxial growth is possible. These materials possess various magnetic states as they have very less chemical reaction at interfaces [17]. In this paper we report on the electrical and magneto-transport properties of LSMO/YBCO/LSMO based heterostructures grown on LaAlO3 (LAO) substrate by varying the thicknesses of superconducting spacer layer (50 nm, 100 nm, and 200 nm). It is observed that the transport properties depend on the thickness of superconducting spacer layer. 2. Experimental detail La0.7Sr0.3MnO3/YBa2Cu3O7/La0.7Sr0.3MnO3 heterostructures were deposited on one side polished [0 0 1] oriented LaAlO3 substrates. The schematic of considered heterostructures is shown in the inset of Fig. 2a. A multitarget pulsed laser deposition (PLD) technique using KrF (wavelength = 248 nm) excimer laser source was employed to deposit the single layers (YBCO, LSMO) and trilayers. To obtain good quality films and also to avoid any contamination, substrates were properly cleaned and the chamber was evacuated to a base pressure of 1.33 10 6 hPa. Deposition was carried out in oxygen environment with chamber pressure of 0.40 hPa at substrate temperature of 820 °C. During the deposition, the energy of laser beam was kept 220 mJ with repetition rate of 10 Hz. Keeping the thickness of top and bottom LSMO layers fixed at 200 nm, LSMO/YBCO/LSMO heterostructures were grown for different YBCO spacer layer thicknesses of 50 nm, 100 nm and 200 nm. After deposition, the films were cooled down to room temperature in ambient oxygen pressure. Single layered films of YBCO and LSMO
Fig. 1. X-ray diffraction patterns for (a) LSMO (200 nm)/YBCO (50 nm)/LSMO (200 nm), (b) LSMO (200 nm)/YBCO (100 nm)/LSMO (200 nm) trilayers, and (c) substrate LaAlO3 (LAO). Symbols ‘‘S’’, ‘‘Y’’ and ‘‘L’’ represent the peaks of LAO, YBCO and LSMO respectively.
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were also deposited on LaAlO3 substrates under the same experimental conditions to probe their electronic and magnetic properties. Deposited films were characterized by X-ray diffraction (XRD) using Bruker D8 Advance X-ray diffractometer with Cu Ka source (k = 1.54 Å). The measurements of resistivity and magnetoresistance in the temperature range of 5–300 K were carried out by standard four-probe method using Oxford He-cryostat. The electric contacts of top to bottom LSMO layers were made by indium. 3. Results and discussion Fig. 1 shows X-ray diffraction patterns of the LSMO/YBCO/LSMO heterostructures. It reveals highly c-axis oriented growth of both LSMO and YBCO layers with substrate (see Fig. 1). No impurity phase has been detected which confirms the single phase nature of grown LSMO and YBCO layers. Fig. 2 shows the temperature dependent electrical resistivity measurements for YBCO, LSMO single layers and LSMO/YBCO/LSMO trilayered films in 4 T and 0 T magnetic fields. The observed superconducting transition temperature of YBCO single layer is TC = 90 K (see the inset of Fig. 2b). The measurements of magnetization and resistivity of LSMO (200 nm) single layer in the temperature range of 300–365 K are shown in the inset of Fig. 2c. The magnetization data reveals that the ferromagnetic–paramagnetic transition at 350 K, coinciding with the metal–insulator (TMI) transition temperature. The resistivity of LSMO single layer at room temperature is very low (0.002 X cm) in 0 T field which is comparable with previously reported resistivity [18]. The magneto-electric properties of LSMO system are well explained on the basis of DE exchange theory [4], and it forms the basis for co-occurrence of metallicity and ferromagnetism in these CMR materials. The measured resistivity
Fig. 2. Temperature dependence of resistivity for LSMO/YBCO/LSMO trilayers with (a) 50 nm, (b) 100 nm AND (c) 200 nm thick superconducting spacer layers in 0 T and 4 T fields. Insets showing (a) the measurement geometry for trilayers, (b) resistivity data of YBCO single layer, and (c) the resistivity and magnetization data of LSMO thin film of single layer of thickness 200 nm in the temperature range of 300–365 K.
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M. Sharma et al. / Physica C 497 (2014) 30–33
values of LSMO/YBCO/LSMO trilayered films with 50 nm, 100 nm and 200 nm thick YBCO spacer layers at room temperature are 0.04 X cm, 0.02 X cm and 0.004 X cm respectively. These resistivity values of trilayered films are more than that of LSMO single film due to interface scattering process. We have observed that the TMI transition temperature for single LSMO film is 350 K, which gets reduced to 269 K (H = 0 T) for 50 nm thick YBCO spacer layer specimen and tends to get further suppressed for 100 nm thick spacer layer (see Fig. 2b). Moreover, the trilayered film with 200 nm thick spacer layer remains metallic from room temperature down to 5 K (see in Fig. 2c). The above observed variation in q–T behavior may be due to an interaction between top and bottom LSMO layers through superconducting spacer layer, as discussed later. Hence it is observed that under the same experimental conditions the electrical transport properties of these trilayered films depends on the thicknesses of spacer layer. Fig. 3 shows the magnetoresistance as a function of temperature (5–300 K) for 50 nm and 100 nm spacer trilayered films in the applied magnetic field of 4 T. The MR ratios are obtained using the expression of MR = (RH R0) 100/R0, where RH and R0 are the resistances with the applied magnetic field (H = 4 T) and without field respectively. During the measurements the applied field was parallel to the current direction (H||I). In trilayered films the maximum MR ratio is 70% at 266 K (19% at 300 K, see Fig 3a) and 65% at 254 K (13% at 300 K, see Fig. 3b) for 50 nm and 100 nm thick YBCO spacer layers, respectively. This observed decrease in MR magnitude with increase in thickness of spacer layer in the trilayered films may be due to the reduced magnetic interaction between top and bottom ferromagnetic layers through YBCO layer. Due to this reduced magnetic interaction YBCO and LSMO would act like parallel electrical circuits [9]. Moreover, these magnetic interactions may be because of novel proximity effects between
Fig. 4. Variation of temperature coefficient of resistance with temperature for LSMO (200 nm)/YBCO (50 nm, 100 nm)/LSMO (200 nm) trilayered films.
neighboring Cu–O and Mn–O conduction planes [10]. To better understand the magnetic nature of trilayered films, we have also carried out field cooled (FC) magnetic measurements with respect to temperature in the external magnetic field of 0.01 T for 200 nm thick spacer layer A 7 Tesla SQUID-VSM (Quantum Design). These magnetization trends reveal anomaly above 190 K (see in the inset of Fig. 3b) and apart from this anomaly the behavior of trilayered films is like LSMO layer having ferromagnetic to paramagnetic transitions above 350 K. 3.1. Temperature coefficient of resistance study Fig. 4 shows the variation of temperature coefficient of resistance [TCR = (1/R)(dR/dT) 100] of trilayered films with temperature. Here, the data values are obtained by point-by-point differentiation of Fig. 2a and b. For the 50 nm thick spacer trilayered film the observed maximum TCR is 6.63% K 1 at 244 K and at room temperature (300 K) it becomes negative ( 0.41% K 1), 100 nm thick spacer trilayered film shows the maximum TCR 6.19% K 1 at 233 K and negative TCR 0.01% K 1 at 277 K. It has been observed that TCR also decreases with increasing spacer layer thickness which again reveals that the interaction of top and bottom LSMO layers play an important role as explained earlier. Choudhary et al. [19] reported the maximum TCR 3% K 1 at 262 K in La0.7Ce0.3MnO3 thin films. In present trilayered study the TCR value almost get doubled and shifts toward the lower temperature side. It is also worth mentioning here that the commercially available vanadium oxide (VOx) or semiconducting YBCO, at room temperature have TCR values lying between 1.5% K 1 and 3% K 1 [20–22]. It is mainly observed that the TCR value get enhanced significantly in the presence of YBCO spacer layer which can be interesting from physics as well as application (bolometric performances and temperature sensors) point of view.
Fig. 3. Magnetoresistance ratios versus temperature curve for LSMO/YBCO/LSMO trilayers with (50 nm), (b) 100 nm (c) 200 nm thick superconducting spacer layers in 4 T field. Inset shows the temperature dependence of the field cooled magnetization of LSMO(200 nm)/YBCO(200 nm/LSMO(200 nm) trilayers in the presence of magnetic field (0.01 T). In all the measurements, the direction of magnetic field is parallel with respect to current.
4. Conclusion In summary, we have studied the transport properties of highly oriented pulsed-laser-deposited YBCO, LSMO single layers and
M. Sharma et al. / Physica C 497 (2014) 30–33
LSMO/YBCO/LSMO heterostructures on LAO substrates. For LSMO/ YBCO(50 nm)/LSMO trilayered film the observed MR ratio is 70% at 266 K (H = 4 T) and TCR 6.63% K 1 at 244 K and both (MR ratio & TCR) are found to decrease with increasing spacer layer thickness. This decrease in magneto-transport parameters may be due to the reduced magnetic interaction between top and bottom ferromagnetic layers through YBCO layer. In conclusion, we would also like to mention here that by controlling the thicknesses of spacer layer, one can tailor the electrical and magneto-transport properties of LSMO/YBCO/LSMO trilayered films at desire temperature range as well as magnetic field of interest. Acknowledgments We gratefully acknowledge Dr. M. Gupta and Dr. R. Rawat for Xray diffraction measurements and magneto-transport measurements, respectively at UGC-DAE CSR, Indore (M.P.) India References [1] K. Chahara, T. Ohno, M. Kasai, Y. Kozono, Magnetoresistance in magnetic manganese oxide with intrinsic antiferromagnetic spin structure, Appl. Phys. Lett. 63 (1993) 1990–1992. [2] R. Von Helmolt, J. Weckerg, B. Holzapfel, L. Schultz, K. Samwer, Giant negative magnetoresistance in Perovskitelike La2/3Ba1/3MnOx ferromagnetic films, Phys. Rev. Lett. 71 (1993) 2331–2333. [3] C. Zener, Interaction between the d-shells in the transition metals. II. Ferromagnetic compounds of manganese with Perovskite structure, Phys. Rev. 82 (1951) 403–405. [4] P.G. de Gennes, Effects of Double Exchange in Magnetic Crystals, Phys. Rev. 118 (1960) 141–154. [5] J.R. Mignod, L.P. Regnault, C. Vettier, P. Bourges, P. Burlet, J. Bossy, J.Y. Henry, G. Lapertot, Neutron scattering study of the YBa2Cu3O6+x system, Physica C 185– 189 (1991) 86–92. [6] S.A. Wolf, D.D. Awschalom, R.A. Buhrman, J.M. Daughton, S. von Molnar, M.L. Roukes, A.Y. Chtchelkanova, D.M. Treger, Spintronics: a spin-based electronics vision for the future, Science 294 (2001) 1488–1495. [7] V.A. Vasko, V.A. Larkin, P.A. Kraus, K.R. Nikolaev, D.E. Grupp, C.A. Nordman, A.M. Goldman, Critical current suppression in a superconductor by injection of spin-polarized carriers from a ferromagnet, Phys. Rev. Lett. 78 (1997) 1134– 1137.
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[8] Z.W. Dong, R. Ramesh, T. Venkatesan, M. Johnson, Z.Y. Chen, S.P. Pai, V. Talyansky, R.P. Sharma, R. Shreekala, C.J. Lobb, R.L. Greene, Spin-polarized quasiparticle injection devices using Au/YBa2Cu3O7/LaAlO3/Nd0.7Sr0.3MnO3 heterostructure, Appl. Phys. Lett. 71 (1997) 1718–1720. [9] K. Chahara, T. Ohno, M. Kasai, Y. Kanke, Y. Kozono, Magnetoresistance effect of La0.72Ca0.25Mn0z/YBa2Cu3Oy/La0 72Ca0.25MnOz trilayered films, Appl. Phys. Lett. 62 (1993) 780–782. [10] M. Kasai, T. Ohno, Y. Kanke, Y. Kozono, M. Hanazono, Y. Sugita, Current– Voltage characteristics of YBa2Cu3Oy/La0.7Ca0.3MnOz/YBa2Cu3Oy trilayeredtype junctions, Jpn. J. Appl. Phys. 29 (1990) 2219. [11] V. Pena, Z. Sefrioui, D. Arias, C. Leon, J. Santamaria, J.L. Martinez, S.G.E. te Velthuis, A. Hoffmann, Giant magnetoresistance in ferromagnet/ superconductor superlattices, Phys. Rev. Lett. 94 (2005) 057002–57004. [12] C. Visani, V. Peña, J. Garcia- Barriocanal, D. Arias, Z. Sefrioui, C. Leon, J. Santamaria, N.M. Nemes, M. Garcia-Hernandez, J.L. Martinez, S.G.E. Te Velthuis, A. Hoffmann, Spin-dependent magnetoresistance of ferromagnet/ superconductor /ferromagnet La0.7Ca0.3MnO3/YBa2Cu3O7 d/La0.7Ca0.3MnO3 trilayers, Phys. Rev. B 75 (2007) 054501–54505. [13] N.C. Yeh, R.P. Vasquez, C.C. Fu, A.V. Samoilov, Y. Li, K. Vakili, Nonequilibrium superconductivity under spin-polarized quasiparticle currents in perovskite ferromagnet–insulator–superconductor heterostructures, Phys. Rev. B 60 (1999) 10522–10526. [14] Z. Sefrioui, D. Arias, V. Peña, J.E. Villegas, M. Varela, P. Prieto, C. Leon, J.L. Martinez, J. Santamaria, Ferromagnetic/superconducting proximity effect in La0.7Ca0.3MnO3/YBa2Cu3O7 d superlattices, Phys. Rev. B 67 (2003) 214511– 214515. [15] G. Jakob, V.V. Moshchalkov, Y. Bruynseraede, Superconductivity and gaint negative magnetoresistance in YBa2Cu3O7/La0.67Ba0.33MnO3 superlattices, Appl. Phys. Lett. 66 (1995) 2564–2566. [16] A.M. Goldman, V. Vasko, P. Kraus, K. Nikolaev, V.A. Larkin, Cuprate/manganite heterostructures, J. Magn. Magn. Matter 200 (1999) 69–82. [17] T. Ohno, M. Kasai, Y. Kanke, Y. Kozono, M. Hanazono, Y. Sugita, Fabrication of Y–Ba–Cu–O superconductor/La–Ca–Mn–O oxide magnetic material layered films, Jpn. J. Appl. Phys. 29 (1990) L607–L610. [18] K. Senapati, R.C. Budhani, Appearance of an inhomogeneous superconducting state in La0.67Sr0.33MnO3–YBa2Cu3O7–La0.67Sr0.33MnO3 trilayers, Pramana J. Phys. 69 (2007) 267–275. [19] R.J. Choudhary, Ravi Kumar, S.I. Patil, Shahid Husain, J.P. Srivastava, S.K. Malik, Enhanced magnetic and bolometric sensitivity of La0.7Ce0.3MnO3 thin films due to 200 MeV Ag ion irradiation, Appl. Phys. Lett. 86 (2005) 222501–222503. [20] C. Chen, X. Yi, X. Zhao, B. Xiong, Characterizations of VO2- based uncooled microbolometer linear array, Sens. Actuators A 90 (2001) 212–214. [21] J.C. Brasunas, S.H. Moseley, B. Lakew, R.H. Ono, D.G. McDonald, J.A. Beall, J.E. Sauvageau, Construction and performance of a high-temperaturesuperconductor composite bolometer, J. Appl. Phys. 66 (1989) 4551–4554. [22] J.C. Brasunas, B. Lakew, High-Tc superconductor bolometer with record performance, Appl. Phys. Lett. 64 (1994) 777–778.
Contents lists available at ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Electrical and magneto-transport properties of ferromagnet/ superconductor/ferromagnet based heterostructure Minaxi Sharma a, K.K. Sharma a,⇑, Ravi Kumar b,d, R.J. Choudhary c a
Department of Physics, National Institute of Technology, Hamirpur, (H.P.) 177 005, India Centre for Material Science and Engineering, National Institute of Technology, Hamirpur, (H.P.) 177 005, India c UGC-DAE Consortium for Scientific Research, Khandwa Road, Indore, (M.P.) 452 001, India d Beant College of Engineering and Technology, Gurdaspur, (Pb.) 143 521, India b
a r t i c l e
i n f o
Article history: Received 20 April 2013 Received in revised form 25 September 2013 Accepted 7 October 2013 Available online 26 October 2013 Keywords: High temperature superconductor CMR materials Heterostructures Magnetoresistance
a b s t r a c t La0.7Sr0.3MnO3/YBa2Cu3O7/La0.7Sr0.3MnO3 (LSMO/YBCO/LSMO) heterostructures were prepared by pulsed laser deposition technique. The resistivity–temperature (q–T) variation and magnetoresistance effects of these heterostructures have been studied and are found to be dependent on the thickness of superconducting spacer layer. We have observed that metal–insulator (TMI) transition get suppressed with increase in thickness of the spacer layer. The maximum magnetoresistance ratio 70% at 266 K and 65% at 254 K temperature are observed for 50 nm and 100 nm spacer layers respectively. In LSMO(200 nm)/YBCO(50 nm)/LSMO(200 nm) specimen the temperature coefficient of resistance (TCR) is 6.63% K 1 which can be useful for bolometric performances and temperature sensors. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction The advent of high-temperature superconductivity (HTS) in cuprates has rejuvenated the curiosity in magnetic oxides. Lanthanum manganite perovskite materials (LaMnO3), when doped with divalent metal ions such as Ba, Ca, Sr, Pb etc. at La site, it exhibits colossal magnetoresistance (CMR) [1,2]. The properties of CMR materials governed by the mechanism of double exchange (DE) interaction between manganese (Mn) ion spins via oxygen, indicates that the degree of spin polarization of charge carriers may be close to unity [3,4]. For ferromagnet (LSMO), the spin states of Mn ions fluctuate between trivalent (Mn3+) and tetravalent (Mn4+) ions. These spin fluctuating states of LSMO are analogous to the normal conducting state of superconductor (divalent and trivalent copper ions Cu2+ and Cu3+ ions), as far as the realization of antiferromagnetic correlation with the spin fluctuation is concerned [5]. The study of HTS/CMR heterostructures has gained substantially larger interest due to underlying new physics and discrete applications in spintronic devices [6]. Superconducting properties such as critical current (IC) and critical temperature (TC) get suppressed when spin polarized carriers from CMR materials are injected into high temperature superconductors [7,8]. It is apparent that these injected spin polarized carriers can be
⇑ Corresponding author. Tel.: +91 9418780275; fax: +91 1972 223834. E-mail address: [email protected] (K.K. Sharma). 0921-4534/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physc.2013.10.009
utilized as a powerful tool for probing the spin dependent properties of trilayered heterostructures, its experimental investigations would open the entry for viable wide class of superconducting devices. Chahara et al. [9] studied the magnetoresistance (MR) effects of La0.72Ca0.28MnOZ (LCMO) magnetic films and LCMO/YBa2Cu3Oy/ LCMO trilayered films. It has also been reported that the spin fluctuations in HTS/CMR system may be a key to understand the coupling and proximity effects [10]. Furthermore a large negative MR in La0.7Ca0.3MnO3/YBa2Cu3O7/La0.7Ca0.3MnO3 (LCMO/YBCO/LCMO) trilayers has been observed by Pena et al. [11] [12] and they have also reported the spin accumulation in YBCO spacer layer when the ferromagnetic (FM) layers get coupled antiferromagnetically. For exploring the possibility of new superconducting devices, the studies on manganite and high temperature superconductor heterostructures have initially focused on spin injection [13], suppression of critical temperature [14] and the effects of MR [15]. The proximity effects have also been studied in ferromagnet-insulator-superconductor heterostructures [7,13,14,16]. The studies of multilayered heterostructures have drawn a cogent attention towards understanding the physical phenomena occurring at the interface of CMR and HTS materials [6–8,16]. In such multilayered systems the experimental findings get affected by interface phenomena across the layers, due to possible scattering and disruption of either superconductivity or ferromagnetism. Moreover, the giant magnetoresistance (GMR) effects have also been observed in layered magnetic thin-film heterostructures consisting of alternating ferromagnetic and nonmagnetic layers
M. Sharma et al. / Physica C 497 (2014) 30–33
[7,8]. In present study, we have used LSMO as the ferromagnetic material which exhibits colossal magnetoresistance and YBCO as high temperature superconductor for fabrication of LSMO/YBCO/ LSMO heterostructure because of good matching in their lattice parameters. Due to structural similarity between HTS and CMR materials, the epitaxial growth is possible. These materials possess various magnetic states as they have very less chemical reaction at interfaces [17]. In this paper we report on the electrical and magneto-transport properties of LSMO/YBCO/LSMO based heterostructures grown on LaAlO3 (LAO) substrate by varying the thicknesses of superconducting spacer layer (50 nm, 100 nm, and 200 nm). It is observed that the transport properties depend on the thickness of superconducting spacer layer. 2. Experimental detail La0.7Sr0.3MnO3/YBa2Cu3O7/La0.7Sr0.3MnO3 heterostructures were deposited on one side polished [0 0 1] oriented LaAlO3 substrates. The schematic of considered heterostructures is shown in the inset of Fig. 2a. A multitarget pulsed laser deposition (PLD) technique using KrF (wavelength = 248 nm) excimer laser source was employed to deposit the single layers (YBCO, LSMO) and trilayers. To obtain good quality films and also to avoid any contamination, substrates were properly cleaned and the chamber was evacuated to a base pressure of 1.33 10 6 hPa. Deposition was carried out in oxygen environment with chamber pressure of 0.40 hPa at substrate temperature of 820 °C. During the deposition, the energy of laser beam was kept 220 mJ with repetition rate of 10 Hz. Keeping the thickness of top and bottom LSMO layers fixed at 200 nm, LSMO/YBCO/LSMO heterostructures were grown for different YBCO spacer layer thicknesses of 50 nm, 100 nm and 200 nm. After deposition, the films were cooled down to room temperature in ambient oxygen pressure. Single layered films of YBCO and LSMO
Fig. 1. X-ray diffraction patterns for (a) LSMO (200 nm)/YBCO (50 nm)/LSMO (200 nm), (b) LSMO (200 nm)/YBCO (100 nm)/LSMO (200 nm) trilayers, and (c) substrate LaAlO3 (LAO). Symbols ‘‘S’’, ‘‘Y’’ and ‘‘L’’ represent the peaks of LAO, YBCO and LSMO respectively.
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were also deposited on LaAlO3 substrates under the same experimental conditions to probe their electronic and magnetic properties. Deposited films were characterized by X-ray diffraction (XRD) using Bruker D8 Advance X-ray diffractometer with Cu Ka source (k = 1.54 Å). The measurements of resistivity and magnetoresistance in the temperature range of 5–300 K were carried out by standard four-probe method using Oxford He-cryostat. The electric contacts of top to bottom LSMO layers were made by indium. 3. Results and discussion Fig. 1 shows X-ray diffraction patterns of the LSMO/YBCO/LSMO heterostructures. It reveals highly c-axis oriented growth of both LSMO and YBCO layers with substrate (see Fig. 1). No impurity phase has been detected which confirms the single phase nature of grown LSMO and YBCO layers. Fig. 2 shows the temperature dependent electrical resistivity measurements for YBCO, LSMO single layers and LSMO/YBCO/LSMO trilayered films in 4 T and 0 T magnetic fields. The observed superconducting transition temperature of YBCO single layer is TC = 90 K (see the inset of Fig. 2b). The measurements of magnetization and resistivity of LSMO (200 nm) single layer in the temperature range of 300–365 K are shown in the inset of Fig. 2c. The magnetization data reveals that the ferromagnetic–paramagnetic transition at 350 K, coinciding with the metal–insulator (TMI) transition temperature. The resistivity of LSMO single layer at room temperature is very low (0.002 X cm) in 0 T field which is comparable with previously reported resistivity [18]. The magneto-electric properties of LSMO system are well explained on the basis of DE exchange theory [4], and it forms the basis for co-occurrence of metallicity and ferromagnetism in these CMR materials. The measured resistivity
Fig. 2. Temperature dependence of resistivity for LSMO/YBCO/LSMO trilayers with (a) 50 nm, (b) 100 nm AND (c) 200 nm thick superconducting spacer layers in 0 T and 4 T fields. Insets showing (a) the measurement geometry for trilayers, (b) resistivity data of YBCO single layer, and (c) the resistivity and magnetization data of LSMO thin film of single layer of thickness 200 nm in the temperature range of 300–365 K.
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values of LSMO/YBCO/LSMO trilayered films with 50 nm, 100 nm and 200 nm thick YBCO spacer layers at room temperature are 0.04 X cm, 0.02 X cm and 0.004 X cm respectively. These resistivity values of trilayered films are more than that of LSMO single film due to interface scattering process. We have observed that the TMI transition temperature for single LSMO film is 350 K, which gets reduced to 269 K (H = 0 T) for 50 nm thick YBCO spacer layer specimen and tends to get further suppressed for 100 nm thick spacer layer (see Fig. 2b). Moreover, the trilayered film with 200 nm thick spacer layer remains metallic from room temperature down to 5 K (see in Fig. 2c). The above observed variation in q–T behavior may be due to an interaction between top and bottom LSMO layers through superconducting spacer layer, as discussed later. Hence it is observed that under the same experimental conditions the electrical transport properties of these trilayered films depends on the thicknesses of spacer layer. Fig. 3 shows the magnetoresistance as a function of temperature (5–300 K) for 50 nm and 100 nm spacer trilayered films in the applied magnetic field of 4 T. The MR ratios are obtained using the expression of MR = (RH R0) 100/R0, where RH and R0 are the resistances with the applied magnetic field (H = 4 T) and without field respectively. During the measurements the applied field was parallel to the current direction (H||I). In trilayered films the maximum MR ratio is 70% at 266 K (19% at 300 K, see Fig 3a) and 65% at 254 K (13% at 300 K, see Fig. 3b) for 50 nm and 100 nm thick YBCO spacer layers, respectively. This observed decrease in MR magnitude with increase in thickness of spacer layer in the trilayered films may be due to the reduced magnetic interaction between top and bottom ferromagnetic layers through YBCO layer. Due to this reduced magnetic interaction YBCO and LSMO would act like parallel electrical circuits [9]. Moreover, these magnetic interactions may be because of novel proximity effects between
Fig. 4. Variation of temperature coefficient of resistance with temperature for LSMO (200 nm)/YBCO (50 nm, 100 nm)/LSMO (200 nm) trilayered films.
neighboring Cu–O and Mn–O conduction planes [10]. To better understand the magnetic nature of trilayered films, we have also carried out field cooled (FC) magnetic measurements with respect to temperature in the external magnetic field of 0.01 T for 200 nm thick spacer layer A 7 Tesla SQUID-VSM (Quantum Design). These magnetization trends reveal anomaly above 190 K (see in the inset of Fig. 3b) and apart from this anomaly the behavior of trilayered films is like LSMO layer having ferromagnetic to paramagnetic transitions above 350 K. 3.1. Temperature coefficient of resistance study Fig. 4 shows the variation of temperature coefficient of resistance [TCR = (1/R)(dR/dT) 100] of trilayered films with temperature. Here, the data values are obtained by point-by-point differentiation of Fig. 2a and b. For the 50 nm thick spacer trilayered film the observed maximum TCR is 6.63% K 1 at 244 K and at room temperature (300 K) it becomes negative ( 0.41% K 1), 100 nm thick spacer trilayered film shows the maximum TCR 6.19% K 1 at 233 K and negative TCR 0.01% K 1 at 277 K. It has been observed that TCR also decreases with increasing spacer layer thickness which again reveals that the interaction of top and bottom LSMO layers play an important role as explained earlier. Choudhary et al. [19] reported the maximum TCR 3% K 1 at 262 K in La0.7Ce0.3MnO3 thin films. In present trilayered study the TCR value almost get doubled and shifts toward the lower temperature side. It is also worth mentioning here that the commercially available vanadium oxide (VOx) or semiconducting YBCO, at room temperature have TCR values lying between 1.5% K 1 and 3% K 1 [20–22]. It is mainly observed that the TCR value get enhanced significantly in the presence of YBCO spacer layer which can be interesting from physics as well as application (bolometric performances and temperature sensors) point of view.
Fig. 3. Magnetoresistance ratios versus temperature curve for LSMO/YBCO/LSMO trilayers with (50 nm), (b) 100 nm (c) 200 nm thick superconducting spacer layers in 4 T field. Inset shows the temperature dependence of the field cooled magnetization of LSMO(200 nm)/YBCO(200 nm/LSMO(200 nm) trilayers in the presence of magnetic field (0.01 T). In all the measurements, the direction of magnetic field is parallel with respect to current.
4. Conclusion In summary, we have studied the transport properties of highly oriented pulsed-laser-deposited YBCO, LSMO single layers and
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LSMO/YBCO/LSMO heterostructures on LAO substrates. For LSMO/ YBCO(50 nm)/LSMO trilayered film the observed MR ratio is 70% at 266 K (H = 4 T) and TCR 6.63% K 1 at 244 K and both (MR ratio & TCR) are found to decrease with increasing spacer layer thickness. This decrease in magneto-transport parameters may be due to the reduced magnetic interaction between top and bottom ferromagnetic layers through YBCO layer. In conclusion, we would also like to mention here that by controlling the thicknesses of spacer layer, one can tailor the electrical and magneto-transport properties of LSMO/YBCO/LSMO trilayered films at desire temperature range as well as magnetic field of interest. Acknowledgments We gratefully acknowledge Dr. M. Gupta and Dr. R. Rawat for Xray diffraction measurements and magneto-transport measurements, respectively at UGC-DAE CSR, Indore (M.P.) India References [1] K. Chahara, T. Ohno, M. Kasai, Y. Kozono, Magnetoresistance in magnetic manganese oxide with intrinsic antiferromagnetic spin structure, Appl. Phys. Lett. 63 (1993) 1990–1992. [2] R. Von Helmolt, J. Weckerg, B. Holzapfel, L. Schultz, K. Samwer, Giant negative magnetoresistance in Perovskitelike La2/3Ba1/3MnOx ferromagnetic films, Phys. Rev. Lett. 71 (1993) 2331–2333. [3] C. Zener, Interaction between the d-shells in the transition metals. II. Ferromagnetic compounds of manganese with Perovskite structure, Phys. Rev. 82 (1951) 403–405. [4] P.G. de Gennes, Effects of Double Exchange in Magnetic Crystals, Phys. Rev. 118 (1960) 141–154. [5] J.R. Mignod, L.P. Regnault, C. Vettier, P. Bourges, P. Burlet, J. Bossy, J.Y. Henry, G. Lapertot, Neutron scattering study of the YBa2Cu3O6+x system, Physica C 185– 189 (1991) 86–92. [6] S.A. Wolf, D.D. Awschalom, R.A. Buhrman, J.M. Daughton, S. von Molnar, M.L. Roukes, A.Y. Chtchelkanova, D.M. Treger, Spintronics: a spin-based electronics vision for the future, Science 294 (2001) 1488–1495. [7] V.A. Vasko, V.A. Larkin, P.A. Kraus, K.R. Nikolaev, D.E. Grupp, C.A. Nordman, A.M. Goldman, Critical current suppression in a superconductor by injection of spin-polarized carriers from a ferromagnet, Phys. Rev. Lett. 78 (1997) 1134– 1137.
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