The effect of growth temperature on the irreversibility line of MPMG YBCO bulk with Y2O3 layer

Cryogenics 85 (2017) 51–57

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Research paper

The effect of growth temperature on the irreversibility line of MPMG YBCO bulk with Y2O3 layer Sedat Kurnaz a, Bakiye Çakır a,b, Alev Aydıner a,⇑ a b

Physics Department, Karadeniz Technical University, Trabzon 61080, Turkey Vocational Health High School, Artvin Çoruh University, Artvin 08000, Turkey

a r t i c l e

i n f o

Article history: Received 12 December 2016 Received in revised form 29 May 2017 Accepted 31 May 2017 Available online 1 June 2017 Keywords: MPMG method Y2O3 layer Irreversibility temperature Irreversibility field Vortex glass Vortex liquid Giant flux creep

a b s t r a c t In this study, three kinds of YBCO samples which are named Y1040, Y1050 and Y1060 were fabricated by Melt–Powder–Melt–Growth (MPMG) method without a seed crystal. Samples seem to be single crystal. The compacted powders were located on a crucible with a buffer layer of Y2O3 to avoid liquid to spread on the furnace plate and also to support crystal growth. YBCO samples were investigated by magnetoresistivity (q–T) and magnetization (M–T) measurements in dc magnetic fields (parallel to c–axis) up to 5 T. Irreversibility fields (Hirr) and upper critical fields (Hc2) were obtained using 10% and 90% criteria of the normal state resistivity value from q–T curves. M–T measurements were carried out using the zero field cooling (ZFC) and field cooling (FC) processes to get irreversible temperature (Tirr). Fitting of the irreversibility line results to giant flux creep and vortex glass models were discussed. The results were found to be consistent with the results of the samples fabricated using a seed crystal. At the fabrication of MPMG YBCO, optimized temperature for crystal growth was determined to be around 1050–1060 °C. Ó 2017 Elsevier Ltd. All rights reserved.

1. Introduction YBCO superconductor was discovered in 1987 [1]. A great many of the efforts have been made to improve the superconducting, mechanical, structural and flux pinning properties of the superconductors to make them applicable in high temperature and magnetic field applications. YBCO bulk superconductors have considerable potential for high magnetic field engineering applications [2]. The Melt–Powder–Melt–Growth (MPMG) process is based on the reaction: Y2BaCuO5 (2 11) + L (3BaCuO2 + 2CuO) ? YBa2Cu3O7
x (1 2 3). As a result of this process it is possible to make the 2 1 1 remain in the final structure by changing the starting composition toward the 2 1 1 rich regions. When the distribution of 2 1 1 is not uniform, the final structure becomes inhomogeneous and results in weak-connectivity of the superconducting phase, leading to the lower critical current density (Jc). In order to promote the growth of the superconducting phase, the fine 2 1 1 particles must be dispersed uniformly in the liquid [3]. Since the 2 1 1 phase nucleates from Y2O3, it is possible to control the distribution of the 2 1 1 phase if the distribution of Y2O3 is controlled [4]. Pellerin et al. [5] reported that the Y–Ba–Cu–O liquid reacted with the Y2O3 ⇑ Corresponding author. E-mail address: [email protected] (A. Aydıner). http://dx.doi.org/10.1016/j.cryogenics.2017.05.010 0011-2275/Ó 2017 Elsevier Ltd. All rights reserved.

substrate. Furthermore, it was observed that the Y2O3 buffer layer on crucible also prevents the liquid to spread on the furnace plate during the crystal growth of bulk YBCO [6]. Vortex dynamics in superconductors give information about the motion of vortices as influenced by different factors (interactions, defects, etc.). It is related to important quantities as the irreversibility line (IL) and the depinning critical currents. Many models have been proposed to describe the magnetic behavior of HTS (High Temperature Superconductor). Some of them assume that the IL is a phase transition granular inhomogeneous superconductors. The other models try to explain it as a conventional flux creep phenomenon [7]. Yeshurun and Malezomoff [8] suggested that strong, anisotropic magnetic relaxation of the field-cooled and zero-field-cooled magnetization along the principal axes of an YBCO single crystal and interpret it with a thermally activated flux creep model. A simple scaling argument shows that high thermal activation causes magnetic irreversibility. In a clean system, the vortex–lattice phase can melt due to thermal fluctuations. Pinning of vortices due to impurities or the other defects destroys the long–range correlations of the vortex lattice, probably by replacing it with a new vortex–glass phase that has spin–glasslike off–diagonal long–range order and is truly superconducting, in contrast to conventional theories of ‘‘flux creep” [9,10].

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In our previous works, the effect of the Y2O3 layer on the melt growth process was investigated and the effects on the magnetic properties were described [2]. In this study, we present the effect of growth temperature on irreversibility line of MPMG YBCO bulk material with Y2O3 layer using magnetoresistivity and magnetization measurements as a function of temperature. Additionally, we present the giant flux creep and vortex glass models results of the three samples fabricated without a seed crystal. These are consistent with the results of the samples fabricated using a seed crystal.

T (°C) Tmax

2°C/min

T1 T2 T

1°C/min 7°C/min

90 min

2. Experimental procedure Appropriate amounts of Y2O3, BaCO3 and CuO powders in stoichiometric ratio of 1:2:3 were mixed by a mortar machine for 1 h to get a homogeneous mixture. The mixture was placed in an alumina crucible after the milling process. Then, it was put into muffle furnace for calcination process. The powders were held at 900 °C for 30 h in an air atmosphere. After the first 15 h, the crucible was removed from the furnace and cooled in air to the room temperature. An intermediate grinding was performed for 15 min to increase the homogeneity of the bulk–like sample and is placed into the furnace. At the end of the calcination process, the black powders were ground again for 1 h to obtain a more homogeneous mixture. The fine powders were then placed into a platinum crucible and put into the muffle furnace at room temperature. The furnace temperature was increased to 1450 °C and held at that temperature for 5 min to get the molten material. The molten material was then poured onto a copper plate and afterward sandwiched with another copper plate to protect the phase condition during the melting process. Thin platelets obtained were ground again for 1 h to achieve fine powder. The precursor powder weighed 4 g and was pressed into a pellet 13 mm in diameter under 300 MPa pressure and held for 1 min under pressure. In the experiments three growth temperatures (1040, 1050 and 1060 °C) were chosen and three samples were prepared for these temperatures, in order to optimize the growth temperature. The samples prepared were named as Y1040, Y1050 and Y1060, in relation with the growth temperatures selected. The samples were located on crucible with a 1 mm thick buffer layer of Y2O3 (Fig. 1). Schematic diagram of thermal process was given in Fig. 2. As can be seen on Fig. 2, Tmax temperatures were 1040, 1050 and 1060 °C, T temperatures were 890, 900 and 910 °C, T1 temperatures were 990, 1000 and 1010 °C, T2 temperatures were 940, 950 and 960 °C, respectively, for the Y1040, Y1050 and Y1060 samples. Finally, the grown samples were annealed at 500 °C for 24 h in the flowing oxygen and then cooled to room temperature at a rate of 1 °C/min in oxygen [2].

Fig. 1. Y123 compound and Y2O3 layer on the Al2O3 plate.

1°C/h

2.5°C/min

t (min.)

Fig. 2. Schematic diagram of thermal process for YBCO samples.

The superconducting transition temperature was determined by a standard four–point method at temperatures between 40 and 100 K with a heating rate of 4 K/min using a Quantum Design physical properties measurement system (PPMS). Temperature measurements were made under various constant magnetic fields parallel to the pressing direction such as 0, 1, 2, 3, 4, and 5 T in the zero-field cooling (ZFC) regime. Zero-field cooling (ZFC) and the field cooling (FC) magnetization measurements under 0.01, 1, 2, 3, 4 and 5 T magnetic fields were performed using a vibrating sample magnetometer (VSM) made by the Quantum Design PPMS system. Magnetic field measurements were conducted by the magnetic field parallel to the pressing direction at temperatures between 40 and 100 K with a heating rate of 4 K/min.

3. Irreversibility line Irreversibility line (IL) appears the mixed state of high temperature oxides superconductors with a large anisotropy and short coherence length [11]. Magnetic fluxes in the type–II superconductors are pinned in the region of lattice defects and impurities. These regions act as the pinning centers and prevent vortex movements. The behavior of pinning centers is affected by defects, doping, thermal fluctuations, Lorentz force, magnetic field and temperature. IL plays a guiding role to understand the behavior of pinning centers. IL, also, gives information about the behavior of vortex and magnetization. The material exhibits an ideal magnetization with the reversible magnetization behavior, in the absence of pinned flux. This case is related with the fact that the magnetic fluxes penetrate the material in the applied magnetic field between Hc1 and Hc2. However, some flux will remain in the material due to the nature of the type–II superconductors. Remaining flux (called trapped flux) is less than the flux that entered the material when the external field applied. The presence of the trapped flux causes the superconductor to behave like a magnet [12]. IL consists of the irreversibility field (Hirr) and irreversibility temperature (Tirr). Between Tirr < T < Tc,onset and Hirr < H < Hc2 are defined as above the IL and reversible region where the effects of the pinning energy is negligible [13], the magnetization behavior shows reversible property. Because of trapped magnetic flux, the magnetization behavior between T < Tirr and H < Hirr is defined as below the IL and irreversible region which shows irreversible behavior. Below the IL, Lorentz force and thermal effect on mobilizing vortex movement is dominated by pinning force. When vortex pinning increases, the energy loss resulting from the vortex motion is reduced and critical current density is increased. At the same time, permanent magnetization increases. Above the IL, the vortex moves freely when the pinning force is dominated by Lorentz force

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4.1. Magnetoresistivity measurements The resistivity–temperature curves of Y1040, Y1050 and Y1060 samples are given on Fig. 3. The superconducting transition curves measured for all samples at 0 T magnetic fields were extremely sharp. Sharpness of the transition is proportional to the degree of interconnectivity of grains. Besides, the superconducting transition temperatures went down lower temperature with increasing magnetic field as listed in Table 1. The transition temperatures of Y1040 and Y1060 were over 94 K at 0 T magnetic field (Table 1). One of the reasons to this situation is thought to be the extremely fast heating rate (4 K/min). The width of superconducting transition temperature in 5 T magnetic field appears to be nearly 9 K in Y1050 sample. It means that all the samples are resistant to the magnetic field. Because the weak links of the grain boundaries firstly returned to the normal state by the applied magnetic field. Then, the magnetic field entering into the grain disrupted the superconductivity. Additionally, the magnetic dispersion of Y1060 was very little as shown in Fig. 3(c). When the polarized optical micrograph is analyzed [2], it can be seen that the superconducting grains of Y1040 were

1.2

(a) Y1040

ρ(Τ) /ρ(100 Κ)

1.0

0.6 0.4

0.0 1.2

(b) Y1050 1.0 0.8 0.6

0T 1T 2T 3T 4T 5T

0.4 0.2 0.0 1.2

ð1Þ

Tirr(0) is the irreversibility temperature at zero applied field. Tirr(H) is the irreversibility field at a different applied field. H0 and n are the fitting parameters and are calculated according to the equation y ¼ axb . According to the H0 and n, new Hirr values are obtained. Looking at these values, a fitting curve is drawn using Eq. (1). The n parameter varies from one high–Tc cuprate superconductor family to another. It takes values between 1.5 and 5.5 and is related to the degree of anisotropy of the system [19] for samples fabricated using a seed crystal. In the case of YBCO (for n  1.5), IL is dominated by the giant flux creep and vortex glass models [8,9].

0.8

0T 1T 2T 3T 4T 5T

0.2

1.0

ρ(Τ) /ρ(100 Κ)

Hirr

 n T irr ðHÞ ¼ H0 1
T irr ð0Þ

4. Results and discussion

ρ(Τ) /ρ(100 Κ)

and thermal effect. Finite resistance values with the movement of the vortex appear. Giant flux creep (gfc) model was proposed by Yeshurun and Malozemoff [8], the proposed model was based on flux creep model by Anderson and Kim [14]. To understand the complex pinning behavior, nonlinear magnetic behavior near Hc2 and the effect of thermal activation in the vortex lattice, depending on the magnetic field and temperature was proposed. The gfc phenomenon was observed in HTS, particularly in the yttrium-based materials, because of the relatively low pinning energies and the higher critical temperatures. The gfc apparently affects the flux dynamics, which relates to both flux pinning and thermal activation and influences the properties of HTS [15]. According the gfc model, the vortex begins to interact forming triangular vortex lattice leading to dissipation and thermal fluctuations in the matrix. The thermal fluctuations enhance further due to larger anisotropy and crystal imperfections such as local defects, dislocations, twins and stacking faults. The Lorentz forces induce flux motion by energy dissipation [16]. If the Lorentz force is greater than the pinning force, it can move freely above the IL. If T > Tc,onset or H > Hc2, superconductor will no longer show superconducting properties because of the extreme mobility of vortex and the interaction with each other. Fisher [9] suggested the vortex glass or the vortex solid model, taking the advantage of the IL and considering lattice defects in the high Tc oxide superconductor. The vortex glass model is used to the signify the destruction of long range translational order in the Abrikosov lattice due to the flux pinning by the underlying disorder in the superconducting material [17]. The mixed state of type–II superconductors, if superconductor has no impurities and lattice defects, vortices form properly sequenced like Abrikosov lattice. If the superconductor has impurities and lattice regions, these regions will attract vortices. Since these regions are in place in a random structure, they cause the formation of crystals by disrupting the regular vortex lattice. It will cause the vortex glass lattice. At low temperature, the vortex has minimal energy. Besides, the repulsive interaction between the vortices and the pinning centers causes to ignore the thermal fluctuations. Vortices gain heat energy and start to vibrate around their equilibrium position by the increase of temperature. This makes the thermal fluctuations become dominant. When temperature reaches a sufficient level (T = Tirr), vortex glass lattice is disordered will melt. As a result, the vortex liquid lattice occurs. IL gives information about the phase of vortices. Below IL, the vortices are found in the vortex glass phase. Above IL, the vortices are found in vortex liquid phases. The melting of the vortex lattice is a real phase transition. This phase transition temperature (Tirr), linear and nonlinear resistance value will occur. Irreversibility line is drawn using the Eq. (1) [18].

0.8 0.6

(c) Y1060 0T 1T 2T 3T 4T 5T

0.4 0.2 0.0 75

80

85

90

95

100

T (K) Fig. 3. The temperature dependence of normalized resistance for the sample (a) Y1040, (b) Y1050 and (c) Y1060 at different applied field.

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S. Kurnaz et al. / Cryogenics 85 (2017) 51–57

Table 1 The values of critical transition temperature for the sample Y1040, Y1050 and Y1060 under different magnetic fields. 0T

1T

2T

3T

4T

5T

Y1040

Tc,onset (K) Tc,offset (K) DT (K)

94.31 92.65 1.66

94.27 90.48 3.79

93.91 89.02 4.89

93.83 87.48 6.35

93.79 86.51 7.28

93.73 86.50 7.23

Y1050

Tc,onset (K) Tc,offset (K) DT (K)

93.45 92.92 0.53

93.04 89.74 3.30

91.62 87.50 4.12

91.41 85.87 5.54

91.08 83.62 7.46

90.70 82.78 7.92

Y1060

Tc,onset (K) Tc,offset (K) DT (K)

94.93 93.68 1.27

94.90 92.68 2.22

94.84 91.98 2.87

94.80 91.58 3.22

94.77 91.24 3.53

94.41 90.90 3.78

6

Reversible region 4

ð3Þ

Reversible region

4

n = 1,46 ± 0,06 μ0H //c

3 2

Normal state

Vortex liquid

Vortex glass

1 0

(c)

Reversible region

5 4

Vortex glass 3 2

μ0H //c n = 1,60 ± 0,11 Hirr - Y1060 Hc2 - Y1060 fit - Hirr - Y1060 fit - Hc2 - Y1060

1 0 85

Normal state

id

Fig. 5 shows magnetization versus temperature curves of three YBCO samples while 0.01, 1, 2, 3, 4 and 5 T is applied parallel to the pressing direction. Looking at the XRD pattern of Y1040, Y1050 and Y1060 samples [2], it seems to texturing and c-axis orientation of grown crystals. According to ZFC curves, the samples become diamagnetic after separation from the abscissas. If 2 1 1 particles are homogenously dispersed in the samples, they serve as the pinning centers. Low magnetization value is seen in these pinning centers because of more trapped flux.

Hirr - Y1050 Hc2 - Y1050 fit - Hirr - Y1050 fit - Hc2 - Y1050

iqu xl rt e

4.2. Magnetization measurements

(b) 5

Vo

here, qn indicates the normal-state resistance of the samples at Tc,onset. Finding Hirr and T (Tirr) from Eq. (2) drawn in fieldtemperature plane and both H0 and n parameters were extracted from fitting data, using the Eq. (1). Fig. 4 shows the vortex glass, the irreversibility line, the reversible region (or vortex liquid) and the normal state from the magnetoresistivity measured for the Y1040, Y1050 and Y1060 samples and temperatures for the Hirr and Hc2. A wider reversible region is not suitable for superconducting applications. Because of the vortex mobility, the resistance and the transition from superconducting state to normal state occurs. In Fig. 4(a), Y1040 has the widest reversible region. Y1060 has the narrowest reversible region and the IL with the increasing magnetic field. When the all samples in Fig. 4 are considered, at only Y1060 sample the vortex glass transition shifted to higher temperature. When n parameters are evaluated for all samples produced using 1 mm thick buffer layer of Y2O3 (Table 2), it is seen that Y1040 [21], Y1050 [8] and Y1060 [22] samples exhibit behavior of gfc and vortex glass models. As a result of the measurements, it can be said that the optimized crystal growth temperature is 1060 °C and 2 1 1 particles of Y1060 seem to become more homogeneous and dominant.

Hirr - Y1040 Hc2 - Y1040 fit - Hirr - Y1040 fit - Hc2 - Y1040

0

μ0H (T)

qðHc2 ; TÞ ¼ 0:9qn

μ0H //c n = 1,32 ± 0,69

1

μ0H (T)

ð2Þ

Normal state

Vortex liquid

Vortex glass 3 2

4.1.1. Irreversibility line from magnetoresistivity measurements The Hirr and Hc2 are estimated from the resistivity versus the applied magnetic field curves. As is well known from the literature [20], the Hirr and Hc2 are defined at various magnetic fields as the fields where the temperature-dependent resistance is

qðHirr ; TÞ ¼ 0:1qn

(a)

5

μ0H (T)

completely surrounded by the non-superconducting second phase. The non-superconducting second phase resulted in the weak links between the grains, because the sample was not crystallized thoroughly or the temperature was not enough to grow crystals in a certain value. Y1050 and Y1060 samples had a very small amount of secondary non-superconducting phases which were located in the grain boundaries although the boundaries were still good in shape. Despite this small negation, the optimized temperature for crystal growth is 1060 °C.

86

87

88

89

90

91

92

93

94

95

96

T (K) Fig. 4. Hirr, Hc2, irreversibility line, reversible region, vortex glass, vortex liquid and normal state from magnetoresistivity measurements for the sample (a) Y1040, (b) Y1050 and (c) Y1060.

Table 2 Power law exponent n, the parameters H0, and Tirr values from fitting of the Y1040, Y1050 and Y1060 to the flux creep power law generated from the data given on Fig. 4.

Y1040 Y1050 Y1060

H0 (T)

n

Tirr (K)

191.16 157.01 1720.35

1.32 ± 0.69 1.46 ± 0.06 1.60 ± 0.11

93.03 92.82 94.00

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S. Kurnaz et al. / Cryogenics 85 (2017) 51–57

20

2

ZFC 0

ZFC H= 1 T//c

0

H= 0.01 T//c

-20

-40

3

M (10 A/m)

M (103 A/m)

-2

-4

-60

-6 -80

Y1040 Y1050 Y1060

-8

Y1040 Y1050 Y1060

-100

-120

-10

0

ZFC H= 2 T//c

0

ZFC H=3 T//c

-10

-20

M (10 A/m)

-30

3

-40

3

M (10 A/m)

-20

-40

-60 -50

Y1040 Y1050 Y1060

-80

-70

-100

ZFC H=4 T//c

0

ZFC H= 5 T//c

0

-10

-10

-20

3

M (10 A/m)

-20 3 M (10 A/m)

Y1040 Y1050 Y1060

-60

-30

-30

-40

ZFC Y1040 Y1050 Y1060

-50

Y1040 Y1050 Y1060

-40

-50

-60 40

50

60

70

80

90

100

T (K)

40

50

60

70

80

90

100

T (K)

Fig. 5. The ZFC magnetization behaviors for the samples Y1040, Y1050 and Y1060 to 0.01T  H  5T.

Magnetization of Y1050 sample values in different applied field is greater than Y1040 and Y1060. The reason for this is thought to be more liquid phase or 2 1 1 particles in the part cut from the Y1050 sample and more liquid phase or 2 1 1 particles don’t act as pinning centers. Therefore, the diamagnetism feature of Y1050 is poor, whereas, Y1060 has the lowest magnetization value, but, the diamagnetism feature is strong. These behaviors are mainly caused by their granular nature, together with the effect of the secondary phases. Thus, the grain boundaries show weak connectivity

[23]. In addition, the difference in magnetization and magnetic resistance measurement results is that the measurements are made in different regions of the sample. 4.2.1. Irreversibility line from magnetization measurements The DM(T) are determined by subtracting the ZFC magnetization from FC magnetization [13].

DMðTÞ ¼ MFC ðTÞ
M ZFC ðTÞ

ð4Þ

56

S. Kurnaz et al. / Cryogenics 85 (2017) 51–57 Table 3 Power law exponent n, the parameters H0, and Tirr values from fitting of the Y1040, Y1050 and Y1060 to the flux creep power law generated from the data given on Fig. 7.

1.0

3 ΔM (10 A/m)

0.8 Y1040 Y1050 Y1060

0.6

Y1040 Y1050 Y1060

94.31 93.45 94.93

(a)

84

86

88

90

92

T (K) μ0H (T)

4

Fig. 6. An example of the DM(T) measured at 1 T magnetic field. The vertical arrow indicates the irreversibility temperature Tirr(H).

n = 1,46 ± 0,06 n = 1,56 ± 0,25

3 2

6

Hirr - Y1050 (ρ-T) fit - Hirr - Y1050 (ρ-T) Hirr - Y1050 (M-T) fit - Hirr - Y1050 (M-T)

(a)

Vortex liquid

Vortex glass

2

0

(b)

n = 0,96 ± 0,43 μ0H //c

4

Tirr, Y1040 Tc,onset fit - Tirr - Y1040 fit - Tc,onset - Y1040

1

n = 1,60 ± 0,11

n = 1,78 ± 0,09

5

μ0H (T)

3

te al sta N or m

4

1

Reversible region

5

μ0H (T)

Tirr (K)

0.96 ± 0.43 1.56 ± 0.25 1.78 ± 0.09

5

0.0 82

3 2

0

(b)

1

5

Reversible region

4

μ0H (T)

n

29.83 270.36 274.15

0.4

0.2

Vortex liquid

Vortex glass

0 80

Normal state

Hirr - Y1060 (ρ-T) fit - Hirr - Y1060 (ρ-T) Hirr - Y1060 (M-T) fit - Hirr - Y1060 (M-T)

82

84

86

88

90

92

94

96

T (K) 3 2

n = 1,56 ± 0,25 μ0H //c

Fig. 8. Irreversibility line and n parameters from magnetoresistivity and magnetization measurements for the sample (a) Y1050 and (b) Y1060.

Tirr, Y1050 Tc,onset fit - Tirr - Y1050 fit - Tc,onset - Y1050

1 0

(c) 5

3

Vortex liquid

Vortex glass n = 1,78 ± 0,09 μ0H //c

2

Tirr, Y1060 Tc,onset fit - Tirr - Y1060 fit - Tc,onset - Y1060

1 0 78

Normal state

Reversible region

4

μ0H (T)

H0 (T)

80

82

84

86

88

90

92

94

96

T (K) Fig. 7. Hirr, Hc2, irreversibility line, reversible region, vortex glass, vortex liquid and normal state from magnetization measurements for the sample (a) Y1040, (b) Y1050 and (c) Y1060.

The Fig. 6 shows a representative result of DM(T) data to the all samples for 1 T. The Tirr(H) is the temperature value where the difference Eq. (4) deviates from the zero baseline (DM(T = Tirr) = 0).

Fig. 7 shows the irreversibility line of Y1040, Y1050 and Y1060 samples using Eq. (1). Tirr(0) is the critical temperature at zero applied field in Table 1. The Tirr(H) defines the limit below which the pinning effects become important [24]. Y1040 has the widest reversible region. For this reason, it is considered that the crystal growth temperature for the Y1040 sample is not sufficient. The vortex glass transition is jumped to a lower temperature for Y1040 sample. Also, Y1050 sample appears to be narrower. When n parameters are evaluated for all samples produced with 1 mm thick buffer layer of Y2O3 (Table 3), Y1050 sample exhibits behavior of gfc and vortex glass model [25]. Looking at the all samples, it is seen that the optimized crystal growth temperature is 1050 °C, according to the magnetization measurement. Fig. 8 shows the IL and n parameter from magnetoresistivity and magnetization measurements of Y1050 and Y1060 samples. In Fig. 8(a), the n parameter calculated from both measurements appears to be compatible with each other. So, Y1050 sample exhibit behavior of gfc and vortex glass model. In Fig. 8(b), only the n parameter calculated from magnetoresistivity measurements for Y1060 sample seems to be compatible with gfc and vortex glass model. The main cause of the different n parameters, it is thought to result from the measurement of sample taken from different places of produced bulk YBCO. This difference is thought to be induced by

S. Kurnaz et al. / Cryogenics 85 (2017) 51–57

the lattice defects, the grain boundaries, dislocations and dispersion of 2 1 1 particles. 5. Conclusion In the magnetoresistivity measurements; 1. Y1060 is the most durable sample against to the increasing magnetic field and the narrowest superconducting transition temperature. 2. According to n parameters, Y1040, Y1050 and Y1060 samples exhibit behavior of gfc and vortex glass model. In the magnetization measurements; 1. Y1060 has the lowest magnetization value and its diamagnetism feature is strong. 2. According to n parameters, only Y1060 sample exhibits behavior of gfc and vortex glass model. The results were found to be consistent with the results of the samples fabricated using a seed crystal. The optimized growth temperature was determined to be around 1050 and 1060 °C. Acknowledgments This study was supported by the Turkish Scientific and Research Council (TUBITAK) research grant (TBAG-107T751) and Karadeniz Technical University – Turkey research grant (BAP2008.111.001.8).

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