Effect of double substitution on structural and magnetic properties of Y1−xCaxBa2(Cu1−yMgy)3O7−δ

Physica C 477 (2012) 36–42

Contents lists available at SciVerse ScienceDirect

Physica C journal homepage: www.elsevier.com/locate/physc

Review

Effect of double substitution on structural and magnetic properties of Y1xCaxBa2(Cu1yMgy)3O7d S. Attaf a,⇑, M.F. Mosbah a, R. Fittipaldi b, D. Zola b, S. Pace b, A. Vecchione b a b

Université Mentouri, Laboratoire de Couches Minces et Interfaces, Campus de Chaabet Erssas, 25017 Constantine, Algeria CNR-SPIN u.o.s. Salerno and Dipartimento di Fisica ‘‘E. R. Caianiello’’, Università di Salerno, I-84084 Fisciano, SA, Italy

a r t i c l e

i n f o

a b s t r a c t

Article history: Received 8 July 2011 Received in revised form 30 January 2012 Accepted 22 February 2012 Available online 3 March 2012

The effect of the double substitution of Ca and Mg on structural, compositional and magnetic properties in YBa2Cu3O7d polycrystalline compounds are investigated. All prepared samples were found to be single phase with small fraction of Ba-secondary phases. Substitution by more than 2% of magnesium causes an increase of spurious phases. Energy Dispersive Spectrometry (EDS) has been made to analyse the distribution of Ca and Mg in the samples. DC susceptibility measurements show that superconducting transition temperature Tc is reduced by Ca alone and much more when there is also Mg. These measurements have been analysed accurately in order to determine the variations, versus the content of Ca and Mg, of the width of the transition, the temperature of irreversibility Tirr and the difference between the ZFC and FC magnetisations. The critical current density Jc, deduced from the M(H) hysteresis loops, does not follow the same variation versus the content of Ca when the content of Mg is changed. Ca alone reduces Jc for x = 0.1; 0.2. Together with Ca, Mg seems to compensate the reduction of Jc and increasing its content near the solubility limit gives higher Jc than in the undoped sample. Ó 2012 Elsevier B.V. All rights reserved.

Keywords: Y based HTSC Ca/Mg co doping Tc variations Critical current

Contents 1. 2. 3. 4.

Introduction . . . . . . . . . Experimental details . . Results and discussion . Conclusion . . . . . . . . . . References . . . . . . . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

1. Introduction The common properties of nonisovalent substitution in YBa2Cu3O7d (YBCO) are an influence on the redistribution of the oxygen atoms in the lattice and, on the whole, the combination of two effects: influence of the impurity itself and change of the oxygen content. It is well known that the oxygen content affects the crystal structure, electronic transport and superconducting properties in YBCO. It is also realised that the superconducting transition temperature, Tc, sensitively depends on both the hole concentration in the CuO2 planes and the relative charge of the oxygen within the planes [1,2]. The level of this charge can be controlled either by manipulating the oxygen stoichiometry in the Cu–O chains, by application of pressure or by ionic substitution [3,4]. The ⇑ Corresponding author. Tel.: +213 31818833; fax: +213 31818872. E-mail address: [email protected] (S. Attaf). 0921-4534/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2012.02.031

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

36 37 37 42 42

substitution of Y3+ by Ca2+ ions with similar ionic radius but lower valence increases the hole concentration in CuO2 planes. In pure YBCO, Ca doping is able to revive the superconductivity of deoxygenated material while the structure remains tetragonal [5–8]. However, in the fully-oxygenated YBa2Cu3Oy (y = 6.93), Ca doping decreases Tc [10,11]. The reason is that, in YBCO compound [28,9], Ca doping is accompanied by a reduction in oxygen content which involves the Cu–O chains and, in the overdoped regime, the CuO2 planes. The resulting increase of the density of states leads, in the last regime, to a displacement of the Fermi level towards a metallic state, then a lowering of the condensation energy which lowers and suppresses Tc [12]. XRD analysis reveals that such substitution does not lead to changes in the structural symmetry of YBCO [11,13], but a decrease in the orthorhombicity of the system with increasing doping level of Ca is observed [10]. The study of non-magnetic cation substitution in high temperature superconducting cuprates has generated a thoroughly interest in the last years due to the

S. Attaf et al. / Physica C 477 (2012) 36–42

observation of a very high efficiency in depressing Tc in these materials. This is now known to be an indication primarily of the d-wave symmetry of the superconducting order parameter [14,15]. The most interesting substitution effects are those where the impurities occupy the CuO2 planes. In that case the spinless impurities such as Zn2+ create a strong elastic scattering simultaneously to the formation of a localised magnetic moment. This is indeed the origin of the strong coupling between charge and spin dynamics in these systems [16]. Hole density control is yet a problem in Ca doped YBCO [17]. The substitution by Zn2+ is isovalente and consequently, contrary to a non isovalente substitution, the balance charge and, in first approximation, the concentrations of charge carriers in the CuO2 planes are not changed. Substitution with Mg2+ gives similar effects on Tc suppression [18]. Mg2+ is also spinless but with an outer electronic shell 2s22p6 while the Zn2+ one is 3d10. The double substitution (Ca in Y site and Mg in the place of Cu) is of great interest to see how the effect of Ca on reduction of oxygen content and increase of whole density in CuO2 planes can compensate the perturbing role of Mg. The co doping with Ca and Zn (for fixed value of Zn) in [18] shows that Ca gives a noticeable contribution to the density of states leading to a strong rise in its value at the Fermi level which results in a restoration or improvement of the superconducting properties. At our knowledge, the case of co doping with Ca and Mg has not been studied and so its investigation may bring further understanding of the microscopic mechanisms of high temperature superconductivity. In the present work we investigated the effect of partial substitution of Ca for Y with various Ca content (indicated by x) and fixed values of Mg content for low doping level (indicated by y) substituting for Cu, on structural and magnetic properties of YBa2Cu3O7d compounds. 2. Experimental details Y1xCaxBa2Cu(Cu1yMgy)3O7d samples were synthesized by standard solid state reaction of stoichiometric mixture of Y2O3, CaCO3, BaCO3, MgO and CuO at 750 °C and 900 °C for 5 h. This step was followed by sintering in air at 915 °C, 930 °C and 940 °C for 24 h (the samples were reground and shaped as pellet before each sintering). The samples were then heated at 940 °C in flowing O2 at 1.5 bar for 6 h followed by rapid cooling to 400 °C, then 400 °C for 24 h and finally furnace cooled down to room temperature. The structure and phase purity of samples were examined by the X-ray powder diffraction technique (XRD) performed by means of a D8 Advance Bruker diffractometer with CuKa radiation. The diffraction data were collected over the diffraction angle range of 15–70° by scanning with a step of 0.02°. The grain morphology of surface of the samples was analysed by scanning electron microscopy (SEM). Compositional analysis of grains was determined by Energy Dispersive Spectroscopy (EDS) using an INCA Oxford analyser. The magnetic properties were obtained from Zero Field Cooled (ZFC) and Field Cooled (FC) M(T) DC magnetisation measurements, and M(H), using a Physical Properties of Materials System (PPMS) of Quantum Design working in a vibrating samples magnetometer (VSM) mode. 3. Results and discussion XRD patterns of Ca and Mg (kept to y = 0.02) doped YBCO samples are displayed in Figs. 1a and 1b with the Miller indices of the corresponding diffracting planes. The patterns of pure system and of the sample with concentration x = 0.05 and 0.10 of Ca show the presence of certain degree of orientation in significant regions of samples detected by the high relative intensity of (0 0 l) reflections

37

compared to other reflections, witch may be due to stress induced by the thermal cycle, likely promoted by some kind of seeding, or alternatively, to the pelletization and polishing procedure of the powders. Very small fraction of BaCuO2 secondary phase is identified at 2h between 28° and 31° in all doped samples. The peaks of these impurity phases cannot be seen in the presented patterns because of the chosen scale. The BaCuO2 impurity indicates a partial substitution of Ca at the Ba sites. The creation of Cu vacancies is also possible as reported for boron doped YBCO system study in [20]. All spectra were indexed in orthorhombic phase. The lattice parameters were determined from the position of the peaks using the Wolfram Mathematical 6 program. The results are shown in Figs. 2 and 3 and listed in Table 1 (where values of x = 0 correspond to the pure system y = 0). It may be noticed from the figures that the lattices parameters a, b, c do not change much and orthorhombic strain (a  b)/(a + b) slightly increases for samples with a fixed value of Mg 1% changing the Ca content. The substitution with 2% of Mg leads to a slight decreasing of the lattice parameter a, b, c, orthorhombicity (a  b)/(a + b) and in the volume of unit cell. This decrease in orthorhombicity with increasing Mg doping level from 1% to 2% can be visualised with the help of characteristic orthorhombic splitting of the main reflections (0 1 3) and (1 0 3). The merging of these peaks is more significant as Mg and Ca content increases (Fig. 1b). The substitution of Ca in Y site slightly decreases difference between parameters b and a thus reduces the orthorhombic distortion [13]. On the other hand, the contraction on the crystallographic c axis as suggested in [18,21] could be linked to the reduction in the local Jahn Teller distortion of the oxygen octahedron around Cu+2 and can also be attributed to the dif-0 ference in the size of the ionic radii of Mg+2 and Cu+2(rMg+2 = 0.65 Å A 0 and rCu+2 = 0.73 Å A) [18] or may be due to the loss of oxygen which is known to affect the c parameter [22]. From Table 1 one 0 can see that 0 the lattice parameters, 0 a = 3.8159 Å A, b = 3.8789 Å A and c = 11.669 Å A, for the pure YBCO phase are in agreement with those reported in the literature [20,23,24]. SEM images of the prepared samples revealed very homogeneous grains. Fig. 4a shows grains with different shape (generally flat tablets) and small fraction of grains having a rectangular shape (indicated by white arrows in the figure). With the increase of the concentration of Ca the average grain size decreases and the fraction of grains with rectangular shape increases (Fig. 4b–d). The chemical doping with Ca and Mg induces a finer, best connected and closely packed grains. This can improve the percolation path of the superconducting current. The finer grain would bring more disorder in the grain boundary region, which results in an increase of vortex pinning centres and subsequently enhances the critical current [25]. Fig. 5 shows the EDS spectrum of samples with content of Mg y = 0.02 and of Ca x = 0.05 and x = 0.2. The analysis by means of EDS on various regions and areas of the same sample confirmed, for all the samples, the incorporation of Ca and Mg into the grains and was consistent with XRD results. The measured atomic percentage of Y, Ca and Cu, Mg is in agreement with the nominal ones (Table 2). The EDS analysis reveals also that the distribution of Ca in the sample is quite homogenous while the distribution of the Mg particles seems to be inhomogeneous. This result is in disagreement with the one reported by Ref. [25] for Ca. Fig. 6 shows, for the various contents of Mg and Ca, the ZFC and FC DC susceptibility measured with an applied magnetic field of 10 Oe. All the doped samples show a broad transition. The same behaviour is observed for the absolute value of the susceptibility at low temperature: when the ZFC one increases with the content of Ca, the corresponding FC one decreases. The result is the increase of the low temperature values of DM = MFC  MZFC where MFC and MZFC are the magnetisations measured in FC and ZFC mode respectively. This suggests that doping with Ca promotes an enhancement of flux trapping capability.

38

S. Attaf et al. / Physica C 477 (2012) 36–42

♦ΒaCuO2

3.828

YMg =1% YMg =2%

3.824

(e)

3.820

Intensity (arbit.units)

3.816

a(Å)

(d)

3.812 3.808

(c)

3.804 3.800

(b)

3.90

(006)

3.89

b(Å)

(026) (220)

(123) (213)

(115) (023) (210) (007)

(a) (200)

(113)

(012) (102) (004) (013) (103)

(002)

(003)

(005)

♦♦

3.88 3.87 3.86

15

20

25

30

35

40

45

50

55

60

65

70

3.85

2θ (°)

11.70

Fig. 1a. XRD pattern for Y1xCaxBa2(Cu1yMgy)3O7d for y = 0.02: (a) undoped system, (b) x = 0.05, (c) x = 0.1, (d) x = 0.15 and (e) x = 0.2.

c(Å)

11.68 11.66 11.64

x=0.2

11.62 11.60

Intensity (arbit.units)

x=0.15

0.00

0.05

0.10

0.15

0.20

x (Ca content)

(013)

(103)

x=0.1

Fig. 2. Variation of a, b and c lattices parameters as function of x and y of Y1xCaxBa2(Cu1yMgy)3O7d (values of x = 0 correspond to the pure system y = 0).

0.010

x=0.05

32

33

34

2θ (°) Fig. 1b. Variations in the position and the merging of (0 1 3) and (1 0 3) reflections of Y1xCaxBa2(Cu1yMgy)3O7d for y = 0.02.

Another feature of these curves may be noticed after the beginning of the transition: a change in the slope in the ZFC curves and not in the FC ones; this change corresponds approximately to the ending of the FC transition. It varies from sample to sample by the level at which it occurs. This effect, not observed in the sample with 0.01 Mg and 0.2 Ca, may be due to a lack of homogeneity in the samples. A secondary superconducting phase would have the same effect on the ZFC and FC transitions. Without Mg (Fig. 6a), the effect of doping with Ca is to lower the Tc. This decrease is significant (between 5.3 and 6.4 K) with a small variation when the x content of Ca change from 0.1 to 0.2 but the width of the transition is increased by about 50%. The effect of a higher content of Ca (x = 0.2) is to broaden more the transition after the change of slope. The same figure shows for the highest content of Ca, after the end of the transition, a step like feature which may be attributed to the presence of weak links. This effect is less evident for the other

Orthorhombicity (b-a) / (b+a)

yMg =0.01 y Mg =0.02 0.009

0.008

0.007

0.006

0.005 0.00

0.05

0.10

0.15

0.20

0.25

x (Ca content) Fig. 3. Variation of orthorhombic distortion (a  b)/(a + b) as function of Mg and Ca content of Y1xCaxBa2(Cu1yMgy)3O7d (values of x = 0 correspond to the pure system y = 0).

contents of Ca and Mg (Fig. 6b and c). Fig. 6b shows a broadening of the transition for the samples containing 0.01 Mg. The lowering of Tc is higher and lies between 11.5 and 16 K. The width of the transition has a great increase, nearly twice as it is seen for the samples with 0.2 Ca. For the highest content of Ca (x = 0.2), the transition has a width of more than 30 K. Increasing the

39

S. Attaf et al. / Physica C 477 (2012) 36–42 Table 1 Lattice parameters, units-cell volumes and orthorhombicity of Y1xCaxBa2(Cu1yMgy)3O7d for fixed value y = 0.01 of series A and y = 0.02 of series B. Code samples

Ca doping level

Mg doping level

YP00 YPCM11A YPCM21A YPCM31A YPCM41A YPCM11B YPCM21B YPCM31B YPCM41B

0.00 0.05 0.10 0.15 0.20 0.05 0.10 0.15 0.20

0.00 0.01 0.01 0.01 0.01 0.02 0.02 0.02 0.02

0

0

0

0

a (Å A)

b (Å A)

c (Å A)

V (Å A3)

|a  b|/(a + b)

Tconset

3.8159(4) 3.8138(4) 3.8111(4) 3.8131(4) 3.8132(4) 3.8179(4) 3.8179(4) 3.8095(4) 3.8068(4)

3.8789(6) 3.8771(6) 3.8720(6) 3.8724(6) 3.8749(6) 3.8856(5) 3.8789(5) 3.8522(5) 3.8542(5)

11.669(2) 11.666(2) 11.665(2) 11.668(2) 11.668(2) 11.683(2) 11.673(2) 11.629(2) 11.619(2)

173.17(7) 172.99(7) 172.14(7) 172.29(7) 172.41(7) 173.32(7) 172.87(7) 170.67(7) 170.48(7)

0.0082 0.0082 0.0079 0.0077 0.0080 0.0088 0.0079 0.0056 0.0062

91.85 76.60 – 73.50 71.85 75.92 – 72.60 72.30

(a)

(c)

(b)

(d)

Fig. 4. SEM photographs of samples with content of Mg y = 0.02 and of Ca (a) x = 0.05, (b) x = 0.10, (c) x = 0.15, (d) x = 0.20. The white arrows indicate grains having rectangular shape.

content of Mg (Fig. 6c; y = 0.02) does not change significantly the Tc but the width of the transition is lowered by about 50% without return to the situation with no Mg. The change of charge carrier density (n) due to doping with Mg [18,26], the induction of mobile holes by substitution with Ca [27,28] coupled or no with various structural changes [29], overall oxygen content and the disorder in CuO2 planes [10,30] are various mechanisms responsible for variation of Tc. The Tc and the temperature of irreversibility Tirr were extracted from these measurements. The Tc is usually measured at the onset of diamagnetism, thus when MFC or MZFC becomes negative. This is true when the applied DC field is not low. We used a field of 10 Oe which is sometimes overcome by the remnant field of the superconducting magnet of the PPMS. This effect happens because a high field M(H) cycle has been made, after the M(T) ZFC and FC measurements, on each sample. The effect of the remnant field on the ZFC or FC measurements is a non zero value,

positive or negative, of M(T) before the transition. To avoid this, we took Tc as the point where the slope of the curve begins to change compared to the part of the normal state of the sample. Fig. 7 illustrates how Tirr and Tc are determined for samples containing 0.01 Mg and 0.05 and 0.2 Ca. Fig. 7a shows the curves of DM(T) near the transition. DM(T) has been calculated by interpolating linearly MFC(T) and MZFC(T) between 70 K and 100 K and making the difference. Tirr has been chosen at the point where DM(T) leaves the zero base line corresponding to the part of the curves where MFC(T) and MZFC(T) overlap [31]. Fig. 7b shows the FC and ZFC v(T) near Tc. Not all the measured data have been reported in this figure for more clarity. Here the baseline is also the part where the ZFC and FC curves overlap. The point where these curves leave the baseline, where Tc is chosen, is materialised by the arrows. The same figure gives an example of the susceptibility (magnetisation) positive (sample with 0.2 Ca) or negative

40

S. Attaf et al. / Physica C 477 (2012) 36–42

Fig. 5. EDS spectrum of samples with content of Mg y = 0.02 and of Ca (a) x = 0.05, (b) x = 0.2.

Table 2 Results of EDS analysis in atomic % of samples with (a) Mg = 0.02, Ca = 0.05, (b) Mg = 0.02, Ca = 0.2. Elements

O

Mg

Ca

Cu

Y

Ba

Sample (a) Sample (b)

53.98 53.94

0.89 0.80

0.37 1.44

22.27 23.12

6.66 5.40

15.83 15.30

(sample with 0.05 Ca) before the transition from the normal to the superconducting state. Tc, Tirr and DM (10 K), measured for all the samples in the way explained above, have been reported in Fig. 8. Fig. 8a shows that, with Mg, Tc is reduced much more but, except for sample with 0.2 Ca, there is an increase of about 1 K when the content of Mg is higher. The decrease of Tc with x, the ratio DTc/ Dx, is more important with Mg than without Mg. The effect of Mg is also to increase the difference between Tc and Tirr, but the variation of the rate of Mg has no effect on Tirr itself. The variation of DM (10 K) when the content x of Ca increases is not the same for all the values of y: without Mg, Ca increases DM (10 K) but the variation of this last one is small with x; with 0.01 Mg, DM (10 K) goes through a minimum corresponding to x = 0.15; with 0.02 Mg, DM (10 K) goes through a maximum corresponding to the same value of x. There is apparently a change of regime of the trapped flux. The isovalent substitution of Mg on the Cu site is supposed to do not change the density of charge carrier in the CuO2 planes. The situation is different if the substitution is made on the Cu site of the chains where the valence of copper is +1. With content of Mg near the limit of solubility, this event is possible. For the samples containing 0.02 Mg, the maximum of DM (10 K) may

be correlated to the minimum of the orthorhombic distortion but nothing may be said for the other parameters. Fig. 9 shows the critical current density Jc(H) deduced from M(H) measurements, not reported here, by use of Bean’s model. Taking into account the average grain size (calculated from more than 100 statistical measurements made on SEM photographs), the used formula is Jc = 30Mirr/R where Mirr represents the difference between the upper and the lower parts of the cycle taken for the positive values of H [32] and R the average radius of the grains (see Table 3). We consider only fields above 20 kOe, where the demagnetising field and other shape and surface effects may be ignored. Fig. 9a shows that, without Mg, Jc decreases with the content x of Ca. The samples doped with Ca exhibit a Jc lower than the undoped sample one. The same decrease versus x is observed with the introduction of 0.01 Mg (Fig. 9b) but Jc has higher values and is even higher than the undoped sample one for x = 0.05. This effect of low level doping has been shown for Ca alone (x = 0.025; 0.04), not considered by our work, in enhancement of the intragranular Jc [33,34]. In the same figure the Jc(H) of the sample with 0.2 Ca exhibits a plateau like shape at fields higher than 4 T. The applied field being oriented parallel to the a–b planes, to minimise demagnetizing effects, this variation of Jc(H) suggests that the vortices are locked between the planes and consequently that the sample has a good degree of texture. The same sample exhibits ZFC and FC v(T) with transition shape different from the other ones (see Fig. 6b). With 0.02 Mg (Fig. 9c), the behaviour changes and the values of Jc, going versus x through a minimum, are all higher than those of the undoped sample one. This minimum has been also observed in the DM (10 K) versus x variation of the same samples (see Fig. 8b). The situation is not the same in the two measurements.

41

S. Attaf et al. / Physica C 477 (2012) 36–42

0 0.0

20

40

60

80

0.0

100

(a)

90

-0.5

Tc,Tirr (K)

FC

y=0

0.1

0.2

(a)

80

0.0

(b)

χ (SI)

FC -0.5

1.0

3

ZFC

ΔM10K(emu/cm )

70

Pure x = 0.10 x = 0.20

-1.0

(b) y 0 0.01 0.02

0.5

y = 0.01 Pure x = 0.05 x = 0.15 x = 0.20

-1.0 ZFC 0.0

0.0

0.1

0.2

x Fig. 8. Effect of the content x of Ca and y of Mg on (a) Tc (filled symbols) and Tirr (unfilled symbols) and (b) DM measured at 10 K.

(c) FC

12 -0.5

y=0

4.2 K 10

y = 0.02 Pure x = 0.05 x = 0.15 x = 0.20

-1.0 ZFC 0

20

40

60

80

Pure x = 0.10 x = 0.20

8 6 4

100

T (K) 2

(a)

Fig. 6. ZFC and FC DC susceptibility measurements with an applied field of 10 Oe for the various content x of Ca and of Mg: (a) y = 0, (b) y = 0.01, (c) y = 0.02.

Pure x = 0.05 x = 0.15 x = 0.20

2

JC(10 A/cm )

y = 0.01

4.2 K

15

-3

(a)

x 0.05 0.20 2

0.5

1

10

6

ΔM(10 emu/cm

3

)

1.0

5

0

(b) y = 0.02

4.2 K 0.000

(b)

20

Pure x = 0.05 x = 0.15 x = 0.20

χ (SI)

15 FC

ZFC 2 FC

-0.002 70

10

1 ZFC

5 80

90

T (K) Fig. 7. Plot, for the samples containing 0.01 Mg and x = 0.05 and 0.2 of Ca, (a) of the difference DM = MFC  MZFC, and (b) details showing how varies the FC and ZFC v(T) near Tc. The arrows indicate the point where the temperature of irreversibility Tirr and the Tc are determined. The numbers 1 and 2 beside the arrows are for x = 0.05 and 0.20 respectively.

With a low applied field, ZFC M(T) is a result of intragrains and intergrains superconducting currents. M(H), and subsequently Jc(H), measured under a high applied field is a result of intragrains superconducting currents only. The similar behaviour versus x, for the same content of Mg, suggests that the intragrains currents are

0

(c) 0

30

H (kOe)

60

90

Fig. 9. Jc(H) at 4.2 K for the various content of x of Ca and Mg: (a) y = 0, (b) y = 0.01, (c) y = 0.02.

majority suggesting also that the grains connectivity is better. This fact is supported by Jc higher than the undoped sample one. The behaviour of lowering Jc, in our free Mg samples, has been observed, in thin films, only when the content of Ca is about 20% [35]. The enhancement of Jc has been observed, at 60 K and 70 K, when the grain boundary, not the grain itself, is doped with Ca

42

S. Attaf et al. / Physica C 477 (2012) 36–42

Table 3 Average grains size of Y1xCaxBa2(Cu1yMgy)3O7d samples. Ca doping level

Mg doping level

Average grains size (m)

0.00 0.05 0.10 0.15 0.20 0.05 0.10 0.15 0.20

0.00 0.01 0.01 0.01 0.01 0.02 0.02 0.02 0.02

7.9 8.1 6.5 4.6 3.6 4.3 3.2 2.8 1.7

(3) (5) (3) (3) (2) (3) (2) (2) (1)

when there is Mg. The variation of the difference DM (10 K), between the values at 10 K of the ZFC and FC magnetisation, shows versus x a different behaviour when the content of Mg is higher. A correlation with the orthorhombic distortion is possible for 0.02 Mg but nothing may be said for the other contents. Substitution by Ca alone for x = 0.1; 0.2 reduces Jc. Substituting by Mg together with Ca seems to have a compensating effect which enhance Jc and leads to values higher than those of the undoped sample when the content of Mg is near the solubility limit. Together with the behaviour of DM (10 K) versus x for 0.02 Mg, these results suggest the possibility that Mg may occupy the Cu site of the chains. References

[36]. The reduced Jc may be due to chain disorder induced by Ca doping. The co doping with Mg seems to compensate the effect of Ca on the chain disorder. This compensation is also observed in the effects on the width of the transition showed by ZFC and FC M(T) measurements. Mg induces a reduction of the grain size. But Mg, substituting Cu, may also occupy a site on the chains changing the valence from +1 to +2. This variation of charge may explain the enhancement of Jc. The scenario could be that part of Mg atoms substitute on CuO2 plan sites, reducing Tc, and the other part substitute on the chains, enhancing Jc and reducing the width of the transition. The change introduced by Mg concerns also the bound with oxygen atoms in CuO2 planes. The hybridations of energy states in the Cu–O bound is reduced when Mg takes the place of Cu. This gives structural change which may enhance intrinsic pinning. The situation of co doping is more complicated than doping with one element. It has been shown that the Tc increase in Y1_xCaxBa2Cu2.8Zn0.2Oy is related to the direct Ca ions effect. The mechanism of the calcium influence on the Tc value is discussed with respect to the Ca-induced conduction band modification [19]. The calcium restores the suppression of Tc in YBa2Cu3Oy with oxygen deficit, as well as in pre-doped YBa2Cu3Oy with over stoichiometric oxygen content induced by nonisovalent impurities in different positions. In our case for low doping content of Mg (Y1_xCaxBa2Cu2.97Mg0.03Oy and Y1_xCaxBa2Cu2.94Mg0.06Oy) there is no improvement of Tc with increasing Ca doping level but Jc increases with Mg doping level from 1% to 2%. Note that it has been found that Mg substitution appears to be as detrimental to Jc in YBCO samples as reported by various authors [18,37–39]. This result suggests that Ca impurity at low doping level may lead to this variation of Jc. Our results suggest that the enhancement of Jc may have two different causes: the possibility of substitution of Mg on chain’s site and the effect of Ca. The enhancement of Jc may also have origin from nano sized defects, resulting from non reacted MgO particles, and ageing as pinning centre. More investigation is needed to elucidate the influence of the two dopants. 4. Conclusion The effects of double substitution of Y by Ca and Cu by Mg have been studied in polycrystalline YBa2Cu3O7d. EDS analysis shows an homogenous distribution of Ca in the sample while the distribution of Mg seems inhomogeneous. Ca alone reduces Tc and increases the width of the transition. A low rate of Mg, the Tc is reduced more and the width of the transition is much more increased. A higher rate of Mg results in a decrease of the width of the transition which returns comparable to those of the free Mg ones. The rate of decreasing Tc versus the x content of Ca is higher when Mg is present. The difference between Tc and Tirr increases

[1] M. Karppinen, H. Yamauchi Int, J. Inorg. Mater. 2 (2000) 589. [2] J.J. Capponi, C. Chaillont, A.W. Hewat, P. Lejay, M. Marezio, N. Nguyou, B. Raveau, J.L. Sonbeyroux, J. Tholence, R. Tounier, Europhys. Lett. 3 (1987) 1301. [3] H. Yamauchi, M. Karppinen, Supercond. Sci. Technol. 13 (2000) R33. [4] R. Beyers, T.M. Shaw, Solid State Phys. 42 (1989) 135. [5] P. Starowicz, J. Sokolowski, M. Balanda, A. Szytula, Physica C 363 (2001) 80. [6] B. John Parise, M. Eugene McCarron III, J. Solid State Chem. 83 (1989) 188. [7] R.S. Liu, J.R. Cooper, J.W. Loram, W. Zhou, W. Lo, P.P. Edwards, W.Y. Liang, L.S. Chen, Solid St. Commun. 76 (1990) 679. [8] C. Legros-Gledel, J.-F. Marucco, E. Vincent, D. Favrot, B. Poumellec, B. Touzeline, M. Gupta, H. Alloul, Physica C 175 (1991) 279. [9] R.G. Buckley, D.M. Pooke, J.L. Tallon, M.R. Presland, N.E. Flower, M.P. Staines, H.L. Johnson, M. Meylan, G.V.M Williams, M. Bowden, Physica C174 (1991) 383. [10] V.P.S. Awana, A.V. Narlikar, Phys. Rev. B 49 (1994) 6353. [11] X.S. Wu, S.S. Jiang, J. Lin, J.S. Liu, W.M. Chen, X. Jin, Physica C 309 (1998) 25. [12] J.L. Tallon, J.W. Loram, G.V.M. Williams, J.R. Cooper, I.R. Fisher, J.D. Johnson, M.P. Staines, C. Bernhard, Phys. Status Solids (b) 215 (1999) 531. [13] Giri Rajiv, V.P.S. Awana, H.K. Singh, R.S. Tiwari, O.N. Srivastava, Anurag Gupta, B.V. Kumaraswamy, H. Kishan, Physica C 419 (2005) 101. [14] C. Bernhard, J.L. Tallon, C. Bucci, R. De Renzi, G. Guidi, G.V.M. Williams, Ch. Niedermayer, Phys. Rev. Lett. 77 (1996) 2304. [15] A.J. Zaleski, J. Klamut, Phys. Rev. B 59 (1999) 14023. [16] P. Mendels, J. Bobroff, G. Collin, H. Alloul, M. Gabay, J.F. Marucco, N. Blanchard, B. Grenier, Europhys. Lett. 46 (1999) 678. [17] Wu. Zheng, Pei.-Herng. Hor, Supercond. Sci. Technol. 22 (2009) 105012. [18] L. Raffo, R. Caciuffo, D. Rinaldi, F. Licci, Supercond. Sci. Technol. 8 (1995) 409. [19] O.A. Martynova, D.V. Potapov, V.E. Gasumyants, E.V. Vladimirskaya, Physica C 471 (2011) 308–313. [20] F. Ben Azzouz, M. Zouaoui, K.D. Mani, M. Annabi, G. Van Tendeloo, M. Ben Salem, Physica C 442 (2006) 13. [21] Xiao. Gang, A. Bakhshai, M. Z. Cieplak, Z. Tesanovic and C. L. Chien, Phys. Rev. B 39 (1989) 315. [22] R.J. Cava, A.W. Hewat, E.A. Hewat, Batlogg B, M. Marezio, K. M. Rabe, J.J. Krajeweski, W. F. Jr. Peck and L. W. Jr. Rupp, Physica C 165 (1990) 419. [23] J.F. Bringley, T.M. Chen, B.A. Averill, K.M. Wong, S.J. Poon, Phys. Rev. B 38 (1988) 2432. [24] Z. Hiroi, M. Takano, Y. Takeda, R. Kanno, Y. Bando, Jpn. J. Appl. Phys. 27 (1988) L580. [25] Jing.-He.Qiu. Ke-XiXu, Li.-yi. Shi, Supercond. Sci. Technol. 19 (2006) 178. [26] Anand. Vyas, C. C. Lam and L. J. Shen, Physica C 341–348 (2000) 935. [27] J.L. Tallon, C. Bernhard, H. Shaked, R.L. Hitterman, J.D. Jorgensen, Phys. Rev. B 51 (1995) 12911. [28] Y. Tokura, J.B. Torrance, T.C. Huang, Al. Nazzal, Phys. Rev. B 38 (1988) 7156. [29] V.P.S. Awana, S.K. Malik, W.B. Yelon, A. Claudio Cardoso, O.F. de Lima, A. Gupta, A. Sedky, S.B. Samanta, A.V. Narlikar, Physica C 338 (2000) 197. [30] V.P.S. Awana, S.K. Malik, W.B. Yelon, Physica C 262 (1996) 272. [31] V.N. Vieira, I.C. Riegel, J. Schaf, Phys. Rev. B 76 (2007) 024518. [32] S.J. Senoussi, Phys. III France 2 (1992) 1041. [33] H. Huhtinen, V.P.S. Awana, A. Gupta, H. Kishan, R. Laiho, A.V. Narlikar, Supercond. Sci. Technol. 20 (2007) S159. [34] A. Zahariev, E. Nazarova, K. Nenkov, T. Mydlarz, V. Kovachev, AIP Conf. Proc. 1203 (2009) 367. [35] S.H. Naquib, A. Semwal, Physica C 425 (2005) 14. [36] Y. Zhao, C.H. Cheng, Physica C 386 (2003) 286. [37] J. Figueras, A.E. Carrillo, T. Puig, X. Obradors, J. Low Temp. Phys. 117 (1999) 873. [38] J. Figueras, T. Puig, A.E. Carrillo, X. Obradors, Supercond. Sci. Technol. 13 (2000) 1067–1073. [39] X. Obradors, T. Puig, E. Mendoza, J. Plain, J. Figueras, X. Granados, A.E. Carrillo, E. Varesi, F. Sandiumenge, P. Tixador, Supercond. Sci. Technol. 13 (2000) 879.