Angular dependence of the resistivity of YBa2Cu3O7 superconducting a-axis oriented thin films

ELSEVIER

Physica C 288 (1997) 243-248

Angular dependence of the resistivity of YBa2Cu307 superconducting a-axis oriented thin films C. Prouteau, F. Warmont, Ch. Goupil, J.F. Hamet, Ch. Simon * Laboratoire Crismat, UMR CNRS 6508, lSMRa-Universit~ de Caen, 14050 Caen Cedex, France Received 9 April 1997; revised 14 June 1997; accepted 23 June 1997

Abstract Thin films of Y B a 2 C u 3 0 7, a-axis oriented, were grown by laser ablation. The angular dependence of the resistivity was studied in the superconducting phase. The I - V curves were also studied close to the vortex lattice melting transition. This transition line, the critical currents and the pinning energies are compared to that of single crystals and to c-axis oriented films. These values appear much lower in a-axis oriented films. This can be compared to the decrease of the resistive transition temperature in zero applied field, suggesting an important role of the carrier deficiency which exists in such films. The origin of this deficiency seems not to be in the grains themselves but confined in the grain boundaries. There is also a mmimum of resistivity when the field is applied parallel to the substrate. This can be due to the vortex pinning by the surfaces of the film, or by the grain boundaries which exist in parallel to these surfaces. In these films, the presence of the grain boundaries induces a vortex pinning, but also a decrease of Tc and a correlative decrease of the vortex pinning ability. This decrease dominates the critical current properties. © 1997 Elsevier Science B.V. Keywords: Superconducting; a-axis films; YBaCuO; Resistivity

1. Introduction The growth control of a-axis oriented thin films of Y B a 2 C u 3 0 7 is very important for many possible applications such as Josephson junctions and SQUIDs [1]. Such films present a very different microscopic structure from that of the c-axis oriented films, due to their different growth mechanisms [2,3]. The aaxis oriented films are built with small a-axis 90 ° misoriented domains, with every domain being oriented with its a-axis perpendicular to the substrate

* Corresponding author. Fax: + 3 3 2 3195 1600; e-mail: simon @crismat.ismra.fr.

plane. For this reason, there are a lot of grain boundaries including those which are not perpendicular to the substrate surface within the film thickness. These grain boundaries could play the role of good pinning centers and should increase the critical currents, but they also can be an obstacle for the transport of current since they can be considered as weak links for the superconductive properties. The presence of these grain boundaries can also be at the origin of a decrease of the average cartier density at the scale of the London length h and drives to a decrease of Tc, even when the grains themselves are fully oxygenated. As it was previously shown [4], this increase of A decreases the irreversibility line very rapidly. So it appears very important to study the respective role of the increase of the defect

0921-4534/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S 0 9 2 1 - 4 5 3 4 ( 9 7 ) 0 1 5 6 4 - 5

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C. Prouteau et al. / Physica C 288 (1997) 243-248

density and of the decrease of the pinning ability upon the transport properties of such films.

2. E x p e r i m e n t a l

The a-axis oriented YBa2Cu30 7 (YBCO) films were prepared by in-situ pulsed laser deposition technique, using a KrF excimer laser ( h = 248 nm) [5]. During film deposition, the oxygen pressure was kept at about 0.3 mbar. The films were deposited on SrTiO 3 (STO) buffered MgO (100) substrate following the template method with a P r B % C u 3 0 7 (PBCO) intermediate layer. The deposition of a 250 ,~ STO buffer layer was performed at 720°C. It has been already shown [6] that STO deposited by this method on MgO is well crystallized with a cubic structure having a parameter close to 3.9 ,~ which is consistent with that of the bulk material. Moreover, the a- and b-axes of the STO buffer layer are parallel to those of the MgO substrate. In the case of the growth of c-axis oriented YBa2Cu30 v films, this procedure permits the attainment of similar superconducting properties to those of films directly grown on STO substrates. After the deposition of the STO buffer layer, the temperature is decreased to 680°C in order to depose the a-axis oriented PBCO template layer. The deposition of YBCO is also started at 680°C, but the temperature is increased to 720°C without interruption of the deposition process, in order to enhance the oxygen content. At the end of the deposition, the oxygen pressure was increased up to 200 mbar and the film was cooled down to room temperature. The thickness was typically 3000 ,~. The film structure was characterized by 0 - 2 0 X-ray diffraction and transmission electron microscopy (TEM). The plane view of Fig. 1 exhibits a typical contrast with 90 ° misoriented domains [2,7] characteristic of a-axis oriented films. Moreover, the Moir6 fringes localized in the A region of Fig. 1 attest of the superposition of two similar lattices with a misorientation of 90 °. This means that existing in such films are two types of grain boundaries: the first ones are perpendicular to the substrate and the second ones separate two superimposed domains and are not perpendicular to the substrate. Tc onset measured on this a-axis film is about 90 K, but the resistive transition is quite broad in zero

Fig. 1. High resolution electron microscopyof an a-axis oriented film showing the two orientations of the grains and the existence of grain boundaries within the thickness of the film (marked by an arrow).

applied field, though no oxygen treatment was able to improve it. No oxygen deficiency was evidenced by any technique (cell parameter or orthorhombic distortion) in the bulk of the grains. Resistive measurements were performed on microbridges of typically 500 × 20 ixm 2 with the four-probe method. The axis of the microbridge is either parallel or perpendicular to the c-axis (Fig. 2). The angular dependence was studied with an horizontal rotating sample holder put in a split horizontal 6 T magnet. 0o 0

,4

90°

B /

/ / J

Fig. 2. Schematic drawing of the microbridge indicating the two types of grains in an a-axis oriented film.

C. Prouteau et al. /Physica C 288 (1997) 243-248

The angular resolution is then about 0.1 °. The current was reversed every 10 s and the resulting resistivities are averaged. Magnetic measurements were performed in a DC and AC SQUID magnetometer with the a-axis oriented along the AC and the DC fields with an angular resolution of about 3 ° . From the present study of the angular dependence of the resistivity, it will be shown that this resolution is adequate for the measurement. From DC measurements, critical current densities J~ were extracted using the Bean model: A M = J~R, where A M is the width of the magnetic hysteresis and R the typical radius of the sample, about 2 m m in our case. The irreversibility lines have been extracted from AC measurements using the criterion as follows: 10 -l~ 1-1 m for the resistivity at the frequency f = 80 Hz and Jc of 103 A / c m 2. The resistivity is given by the following relationship: p = 0.18/x0fRd, where d is the film thickness.

3. R e s u l t s

3.1. Angular dependence of the resistivity The resistivity was measured with a current of 50 ~zA which corresponds to a current density of J = 1 0 3 A / c m 2. Fig. 3 shows a typical result obtained at 6 T as a function of the angle between the applied magnetic field and the a-axis. The film has been patterned in order to create a microbridge which is aligned to b- and c-axis directions of both kinds of YBCO domains and the main difficulty is here that the current flows in a mixing of c and b directions (Fig. 2). Fig. 3 shows that a minimum of the resistivity is observed at 0 = 0 and corresponds to the intrinsic anisotropy of the material similarly to that is observed in c-axis oriented YBCO films and single crystals. More surprisingly is the presence of another minimum at 0 = 90 ° at high temperature and magnetic fields which do not exist in c-axis oriented films with these conditions (Fig. 4). This behaviour has already been recorded by Velez et al. [8] in the case of a-axis oriented Y B C O / P B C O superlattices. Moreover, one can note that this minimum disappears at low temperatures and low magnetic fields. In Fig. 5, we have plotted the measured points

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according to the fact that there exists a minimum (circles) or not (triangles). This permits the definiton of two domains separated by a crossover line in the B-T diagram as shown at Fig. 5. The existence of the minimum of resistivity when the applied magnetic field is parallel to the substrate can be interpreted as the pinning by the defects which are parallel to the substrate plane. This is the case of the surfaces of the film and also of the grain boundaries which are parallel to the substrate and which do not exist in c-axis oriented films. This interpretation is consistent with the results reported by Velez et al. [8] where it has been shown that the amplitude of this minimum was depending of the

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C Prouteau et a L / Physica C 288 (1997) 243-248

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fields along the a-axis (Fig. 6a) or perpendicular to it (Fig. 6b). Following [10], assuming that p = p 0 e x p ( - U(T)/T), and U = U0(l - t) (t = T/L), one extracts the activation energy Uo(B) which is given in Fig. 7. It follows a law in - In (B), which is usually interpreted by the interactions of dislocations in the 2D vortex lattice [10]. The corresponding value of U0(1) is much smaller (20 times less) than that of the c-axis oriented films for both directions of the applied magnetic field i.e. along the a-axis and perpendicular to it. The critical current density was also determined by magnetic measurements (AC and DC). The critical current density is presented in Fig. 8 as a function of temperature at various magnetic

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Temperature (K) Fig. 5. The limit of the observation of the m i n i m u m of resistivity at 90 ° . (The triangles are the measurements without any m i n i m u m at 90 °, the circles the measurements with a minimum). 2

n u m b e r of interfaces. A s s u m i n g that the Y B C O / P B C O interfaces are similar to grain boundaries parallel to the substrate plane, this supports the present interpretation. Since the average distance between the grain boundaries is here about 500 ,~ and the distance between the surfaces of the film is o about 3000 A, one can see that it is very difficult to choose between the surfaces and the grain boundaries for a dominant role here. The disappearance of the minimum at low temperatures can be due to the transition of the vortex lattice induced by the planar defects, as it was described in a similar situation in the case of the twin boundaries [9], but certainly is not due to the disappearance of the pinning efficiency of the defects at low temperature. However, the presence of different orientations of the grains (b- and c-axis are mixed), makes difficult a precise analysis of the minimum. In conclusion of this part, the presence of the grain boundaries induces a vortex pinning which is very efficient even at high temperature and high magnetic field.

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C. Prouteau et al. / Physica C 288 (1997) 243-248

247

3.3. The irreversibility line

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In order to analyse in more detail the role of the pinning in the case of a-axis oriented films, the current dependence of the resistivity was studied with the magnetic field applied along the a-axis in the neighbourhood of the irreversibility line. Fig. 9 presents the results at 0.1 T. In Fig. 9b, we have plotted the same data in a reduced plot which corresponds to a scaling of the resistivity at the phase transition with the critical exponents a = - 6 . 8 8 , /3 = - 2 . 1 [9], which are typical of a Bose glass transition [12]. This confirms that the pinning is due to extended defects [9], such as the grain boundaries. In Fig. 10, we have reported the corresponding phase transition temperature Tg in the B,T plane. We have also reported on the same figure, the ~ lines of 10-1 .

fields. The values are also 20 times smaller than the corresponding values in c-axis films. Since U = ~od/(41rlxoA2)ln(B/Bc2)where d is the film thickness [11], and since A -2 = txoe2ns/m (ns is the carrier density and m their effective mass) [9], it is clear that a decrease of n~ induces a decrease of the activation energy U. This decrease seems to be more important than the gain which should exist between a- and c-axis films due to the effective mass anisotropy which is about 5 in this material.

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Fig. 9. (a) I - V curves obtained at 0.1 T at different temperatures ranging from 79.5 to 83 K. (b) The corresponding scaling of the data in the framework of a critical scaling analysis of the phase transition.

248

C. Prouteau et al. /Physica C 288 (1997) 243-248

a-orientedfilm c-orientedfilm cry~tal c-orientedfilm 5= 0.12 10.00 .......... , ......... , ......... , ......... , ......... , ....... ~irr ¢ ~Tg

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different orientations of a single crystal and that of a c-axis oriented film. For some of them, since no Tg data were available, we have reported the irreversibility line determined by a criterion of 10-15 m. The irreversibility line of the corresponding direction of the field is slightly shifted from the Tg line. Finally, the irreversibility line of an oxygen deficient c-axis film from Ref. [4] was also reported. It is very clear in Fig. 10 that the transition line is always higher if the magnetic field is applied along the a-axis. It is also very clear that the transition line is higher in single crystals and c-axis films than in the a-axis films. One can notice that the critical temperature of the a-axis films is also lower than that of the c-axis. This behaviour leading to a decrease of the value of critical temperature looks like that of a deoxygenated c-axis film [4], but lattice parameters are consistent with an oxygen content close the YBa2Cu30 v stoichiometry.

4. Conclusion In this study a-axis oriented thin films of YBa2Cu307 were grown by laser ablation. The angular dependence of the resistivity was studied in the

superconducting phase. The I - V curves were also studied close to the vortex lattice melting transition. This transition line, the critical currents and the pinning energies are compared to those of single crystals and to c-axis oriented films: they are much lower in a-axis oriented films. This can be compared to the decrease of the resistive transition temperature in zero applied field, suggesting an important role of the carrier deficiency which exists in such films. The origin of this deficiency appears not to be in the grains themselves but in the grain boundaries. There is also a minimum of resistivity when the field is applied parallel to the substrate. This can be due to the vortex pinning by the surfaces of the film or by the grain boundaries which exist parallel to these surfaces. A transition in the behaviour of the vortices is observed, leading to a crossover in the accommodation of the vortices to the defects. In these films, the presence of the grain boundaries induces a vortex pinning, but also induces a decrease of Tc and an increase of the A value. These two parameters dominate the critical current properties which are in fact degraded.

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