3MnO3 heterostructures

Physica C 449 (2006) 87–90 www.elsevier.com/locate/physc

Different behavior in transport measurements on planar-type YBa2Cu3O7 d/La1/3Ca2/3MnO3/La2/3Ca1/3MnO3 heterostructures W. Saldarriaga b

a,*

, O. Mora´n b, E. Baca

a

a Departamento de Fı´sica, Universidad del Valle, A. A. 25360 Cali, Colombia Escuela de Fı´sica, Universidad Nacional de Colombia, sede Medellı´n AA 3840, Medellı´n, Colombia

Received 26 April 2006; received in revised form 7 July 2006; accepted 11 July 2006

Abstract YBa2Cu3O7 d(YBCO)/La1/3Ca2/3MnO3(LCMO-AF)/La2/3Ca1/3MnO3(LCMO-F) junctions with nominal layer thicknesses of 100, 7, and 80 nm, respectively, were fabricated using a dc sputtering technique. The junctions were structured to transistor dimensions by standard UV-photolithography and chemical etching process. Electrical measurements were carried out both on the superconducting micro-bridges and the antiferromagnetic (AF) barriers. The AF barriers displayed different electrical behavior for the same nominal barrier thickness. Thus, for junctions with a barrier resistance of around 60 X (V = 0), a clear superconducting gap-like structure was observed. On the contrary, junctions with a barrier resistance of around 30 X (V = 0) featured a pronounced, temperature dependent zero-bias conductance peak G(V = 0). The value of G(V = 0) decreased as the temperature was increased, which was an indication of metallic-like behavior associated to the presence of pin-holes in the junction area. On the other hand, spin-polarized carrier injection from the ferromagnetic layer into the strip-like YBa2Cu3O7 d film provoked a strong reduction of IC with an efficiency K(K = DIC/DIg) as large as 1.4 at 15 K.  2006 Elsevier B.V. All rights reserved. Keywords: Elastic tunneling; Ferromagnetic; Spin-polarized carrier; Superconducting; Zero-bias conductance

1. Introduction In recent years, the control of superconductivity by spinpolarized carrier injection has been a subject of great interest and controversy [1–3]. Intensive work on injection of charge carriers from conventional ferromagnetic and novel manganite electrodes has mainly been performed on permalloy/Au/YBa2Cu3O7 d (YBCO), YBCO/(SrTiO3, MgO, LaAlO3)/L2/3Sr1/3MnO3, as well as in YBCO/L1/3Ca2/3MnO3 (LCMO-AF)/L2/3Ca1/3MnO3 (LCMO-F) heterostructures [1–7]. Advances in thin film deposition techniques have allowed the manufacture of heteroepitaxial structures involving high-temperature superconductors and perovskite-type magnetic oxides like manganites [3–6]. However,

*

Corresponding author. Tel.: +57 2 3394610; fax: +57 2 3393237. E-mail address: [email protected] (W. Saldarriaga).

0921-4534/$ - see front matter  2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2006.07.003

growing of artificial layers used as barriers in superconducting or ferromagnetic layer-based heterostructures is indeed complicated. In this sense, a very good matching between the lattice parameters of the components, which provides for sharp interfaces, should be assured. An optimum injection process demands that the previous requirements is fulfilled as the particles should cross the barrier without energy loss (elastic tunnelling) [8,9]. Intrinsic roughness of the ceramic constituents of the device represents a serious hurdle to realize junctions with sharp interfaces. In fact atomic force microscopy (AFM) analysis on the surface of YBCO, LCMO-AF, and LCMO-F films reveal root-meansquare surface roughness of 2 nm, which may hamper a reliable and reproducible growing of the artificial barriers [10]. As a consequence, the ultra-thin barriers may be affected by the presence of a high pinhole-concentration, which raises the probability of observing giant zero-bias conductance peaks instead of a gap-like structure as expected from an elastic tunneling process.

88

W. Saldarriaga et al. / Physica C 449 (2006) 87–90

In this work, electrical measurements on planar-type YBa2Cu3O7 d/La1/3Ca2/3MnO3/La2/3Ca1/3MnO3 heterojunctions are reported and relevant parameters corresponding both to the tunneling of spin-polarized carriers through the LCMO-AF barrier and to the injection of these particles into the strip-like YBCO film are determined. 2. Experimental The heterostructures were grown on (0 0 1)-SrTiO3(STO) substrates using dc-sputtering at high oxygen pressure. Details of the fabrication process have been indicated elsewhere [5,6]. Briefly, a 100 nm thick base LCMO-F ferromagnetic layer was grown onto a STO substrate. This basis LCMO-F layer was then patterned to 40 lm wide micro-bridge by using a H2SO4-solution. Subsequently, layers of LCMO-AF and YBCO with thicknesses of 7 and 80 nm, respectively, were grown on this strip. After slow cooling down to 300 K, the sample was taken out of the chamber and then the junction was completed by patterning a 20 lm wide strip on the top electrode (YBCO layer) by means of an H3PO4-solution. As the H3PO4-solution solely attacks the superconducting material without affecting the LCMO-AF layer, the presence of an YBCO/LCMOAF/LCMO-F junction with an area of 20 · 40 lm2 is guarantied. The so patterned junctions were electrically characterized by conventional four-terminal configuration. Temperature dependent measurements were performed in a He closed cycle cryostat. 3. Results and discussion In this work we present the results of electrical characterization for two junctions (A and B junctions) patterned on the same YBCO/LCMO-AF/LCMO-F heterostructure. I–V characteristics of A junction with a nominally 7 nm thick LCMO-AF barrier layer measured in the temperature range 15–120 K, are shown in Fig. 1(a). The electrical behavior is perfectly ohmic throughout the voltage range for T > 90 K. A non-linear behavior, accompanied by a slight reduction in the resistance R(V = 0), becomes apparent at temperatures below 90 K. In turn, the normalized conductance, G  (dI/dV)/(dI/dV)N, versus voltage (see Fig. 1(b)), displays a sharp peak at zero-bias where the height is strongly reduced by increasing temperature and disappears at TC  90 K, where TC is the critical temperature of YBCO layer. Moreover, at a given temperature, the height of this conductance peak decreases abruptly as the bias departs from zero. The decreasing of the conductance peak as the temperature increases points out the dominance of a metallic behavior in the junction, which is incompatible with a pinhole-free barrier. As previously stated, the presence of pinholes in the structure might be due to inhomogeneity in the YBCO/LCMO-AF and LCMO-AF/LCMO-F interfaces associated with roughness of the YBCO- and/or LCMOsurfaces. In contrast, the I–V characteristic measured on

Fig. 1. (a) I–V characteristics of a YBCO/LCMO-AF/LCMO-F junction (junction A) with a nominal LCMO-AF barrier thickness of 7 nm, recorded at various temperatures. (b) Normalized conductance, GN  (dI/ dV)/(dI/dV)N, as a function of the voltage. The curves were displaced respect to the curve measured at 15 K.

B junction with the same nominal barrier thickness at T = 25 K features a low current value within the region around V = 0, an abrupt increasing of the absolute current value for V  ±10 mV, and a linear I–V-dependence in the high voltage region as is shown in the Fig. 2(a). These findings suggest that the main transport mechanism in this junction is elastic tunneling through the LCMO-AF barrier. In turn, the conductance G versus voltage reflects the shape of the superconducting density of states (see Fig. 2(a)). Two well defined maxima are observed at  18 and 13 meV, indicating the presence of superconducting energy gap ±D. The asymmetry observed in the I–V and G–V curves of Fig. 2(a) might arise from an asymmetry tunnel barrier due to the existence of not equivalent interfaces. The difference in the carriers’ concentration and difference of states density of the superconducting and ferromagnetic electrodes is determinant in the asymmetrical behavior of the conductance measurements [11]. I–V characteristics together with their dynamic conductance G = dI/dV versus V at 15, 35, 45, 55, 65, and 90 K for

W. Saldarriaga et al. / Physica C 449 (2006) 87–90

Fig. 2. (a) I–V and G–V characteristics of a c-axis oriented YBCO/ LCMO-AF/LCMO-F tunnel junction (junction B), measured at 25 K. The nominal thickness of the LCMO-AF barrier amounted to 7 nm. (b) Temperature dependence of the zero-bias normalized resistance for two YBCO/LCMO-AF/LCMO-F junctions with the same nominal barrier thickness (7 nm) but with different barrier quality.

the same junction were also measured. The presence of the gaplike structure, as well as the asymmetry in these curves was also evidenced. The energy gap of superconducting electrode decrease with an increase of temperature as usually occurs in superconducting materials. Finally, the particularly low G values observed for this junction at very high voltages will be further analyzed and discussed in a forthcoming publication. For the sake of completeness, resistance values of the LCMO-AF barriers as calculated from the I–V characteristics of the A (open circles) and B (solid circles) junctions at V = 0 and at various temperatures are plotted in Fig. 2(b). The increase in the zero-bias resistance values as the temperature increases in the junction A is characteristic of a metallic-like behavior and in the case of a sandwich-type structure an indication that the barrier is seriously affected by a high density of pinholes. The insulating behavior of the junction B is clearly shown by the close circles in Fig. 2(b). The contrasting behavior of these two junctions indicates that the most

89

important problem in fabricating YBCO/LCMO heterostructures is obtaining very smooth surfaces and sharp interfaces. The presence of a high density of defects on the surface leads to a high density of localized states, which in turn dominates current transport through the interface [3]. Nevertheless, the present study shows that LCMO-AF manganites may act as artificial tunneling barriers, provided that the preparation conditions are optimized. The spin-injection mechanism relies on the fact that the large spin relaxation time expected in superconducting oxides would prevent polarized quasiparticles from recombining into pairs, therefore depressing the order parameter. This mechanism is basically a non-equilibrium effect where the spin polarization simply increases the relaxation time [10]. In order to verify the effectiveness of LCMO-AF manganites as potential tunneling barriers, spin-polarized currents Ig were injected from the LCMO-F layer to the overlying superconducting electrode via the LCMO-AF barrier with a resistance of 60 X at 15 K, which correspond to the junction B (see the Fig. 2(a)). Shown in Fig. 3 are the I–V characteristics of the YBCO bar, measured at 15 K under injection current values of 1, 3, 5 and 7 mA. The inset presents the dependence of IC on Ig at 15 K, where, IC has been defined employing a ±1 lV criterion. A drastic reduction of IC with Ig is clearly evidenced in this case. The relatively small negative current gain (K = DIC/DIg  1.4) estimated for junction B may be explained by the presence of effects other than the actually elastic tunneling. For instance, the barrier might be affected by a non-negligible pin-hole concentration as the injection area is still very large. In fact, the I–V characteristic measured on the junction B does not mimic an ideal tunnel barrier as the conductance at V = 0 is not vanishing. Thus, the

Fig. 3. I–V characteristics of the YBCO micro-bridge (junction B), measured by passing a 0, 1, 3, 5, and 7 mA spin-polarized current at 15 K. Inset: variation of IC as a function of Iinj at 15 K.

90

W. Saldarriaga et al. / Physica C 449 (2006) 87–90

barrier would have localized states, which play an important role in the electronic conduction processes [12]. Apart from these results, it is evident that concerning the performance of ‘‘spintronic’’ high temperature superconductor devices as the geometry of the injection or the heating in the manganite electrode due to the dissipation of the polarized current should be analyzed with great care. This study is in progress.

Acknowledgements The authors acknowledge the valuable collaboration of the Thin Films Group at Universidad del Valle allowing the growth process and characterization of the samples. Two of the authors (W. Saldarriaga and O. Mora´n) acknowledges the support of the Colombian Institute for the Development of Science and Technology (COLCIENCIAS).

4. Conclusions References In summary, electrical measurements on planar-type YBa2Cu3O7 d/La1/3Ca2/3MnO3/La2/3Ca1/3MnO3 heterojunctions were carried out. LCMO-AF barriers depending on the barrier quality different electrical behaviors were observed for YBCO/LCMO-AF/LCMO-F junctions with a nominal 7 nm thick LCMO-AF layer. A metallic-like transport mechanism was evidenced for a junction with a barrier resistance of 30 X (V = 0), which pointed to a barrier with a high concentration of pinholes. In contrast, the I–V characteristic of a junction with barrier resistance 60 X (V = 0) indicated that the main transport mechanism was probably elastic tunneling. The injection of spin-polarized carriers through the high quality barrier provokes a strong reduction of IC of the YBCO bar with an efficiency K  1.4 at 15 K, which is still too low for reliable applications. Beyond doubt, intensive theoretical experimental and work should be done in order to understand the role of the spin-polarized carrier injection in order parameters of YBCO layers and extent the field of applications that could be opened by a truly ‘‘spintronic’’ high-TC device.

[1] Y. Gim, A.W. Kleinsasser, J.B. Barner, J. Appl. Phys. 90 (2001) 4063. [2] V.A. Vas’ko, V.A. Larkin, P.A. Karuss, K.R. Nikolaev, D.E. Grupp, C.A. Nordman, A.M. Goldman, Phys. Rev. Lett. 78 (1997) 1134. [3] O. Mora´n, R. Hott, R. Schneider, W. Wu¨hl, J. Halbritter, J. Appl. Phys. 94 (2003) 667. [4] D.B. Chrisey, G.K. Hubler, Pulsed Laser Deposition of Thin Films, Wiley, New York, 1994. [5] E. Baca, W. Saldarriaga, J. Osorio, G. Campillo, M.E. Go´mez, P. Prieto, J. Appl. Phys. 93 (2003) 8206. [6] E. Baca, W. Saldarriaga, P. Prieto, O. Mora´n, R. Hott, K. Grube, D. Fuchs, R. Schneider, J. Appl. Phys. 97 (2005) 10C922. [7] T. Nojima, M. Iwata, T. Hyodo, S. Nakamura, N. Kobayashi, Physica C 412–414 (2004) 147. [8] B.J. Jonsson-Akerman, R. Escudero, C. Leighton, S. Kim, I.K. Schuller, D.A. Rabson, Appl. Phys. Lett. 77 (2000) 1870. [9] D.A. Rabson, B.J. Jonsson-Akerman, A.H. Romero, R. Escudero, C. Leighton, S. Kim, I.K. Schuller, J. Appl. Phys. 89 (2001) 2786. [10] J. Dumont, M. Morague`s, B. Leridon, J. Lesuer, J.P. Contour, Eur. Phys. J. B 35 (2003) 331. [11] S. Kashiwaya, Y. Tanaka, N. Yoshida, M.R. Beasley, Phys. Rev. B 60 (1999) 3572. [12] Z.W. Dong, S.P. Pai, R. Ramesh, T. Venkatessan, M. Johnson, Z.Y. Chen, A. Cavanaugh, Y.G. Zhao, X.L. Jan, R.P. Sharma, S. Ogale, R.L. Greene, J. Appl. Phys. 83 (1998) 6780.