Effect of paramagnetic Mn ion on the ...

JOURNAL OF APPLIED PHYSICS 112, 073920 (2012)

Effect of paramagnetic Mn ion on the superconductivity of Cu0.5Tl0.5Ba2Ca22xMnxCu3O102y A. A. Khurram,1,a) Qurat-ul-Ain,2 and Nawazish A. Khan2 1

Experimental Physics Labs, National Centre for Physics (NCP), Quaid-i-Azam University, Islamabad 45320, Pakistan 2 Materials Science Laboratory, Department of Physics, Quaid-i-Azam University, Islamabad 45320, Pakistan

(Received 12 August 2012; accepted 5 September 2012; published online 10 October 2012) The Mn-doped Cu0.5Tl0.5Ba2(Ca2xMnx)Cu3O10y(x ¼ 0, 0.15, 0.25, 0.35, 0.5, 0.75 1.0, and 1.25) superconductor samples were prepared at atmospheric pressure using solid state reaction method. The objectives of Mn doping at the Ca site were to improve the inter plane coupling and to see the effect of paramagnetic Mn atom on copper spins in the CuO2 planes and in turn on superconductivity in the light of charge stripe theory. The Mn doped samples have orthorhombic structure and the cell parameters were decreased after the increase of Mn concentration. The onset critical temperature and zero resistance critical temperature have been decreased, which were C 2012 American Institute of Physics. further decreased after oxygen annealing. V [http://dx.doi.org/10.1063/1.4757405]

I. INTRODUCTION

One of the general features of high temperature superconductors is the anisotropy parameter [c ¼ qc/qab ¼ nab/nc; qc and nc are resistivity and coherence length along c-axis, qab and nab are resistivity and coherence length along ab plane] resulting from the layered structure of these materials. The technological critical temperature [Z] and irreversibility field [Hirr] which play very important role in the use of high Tc superconductors in various applications are strongly related to the anisotropy parameter as Z ¼ Tc/c and Hirr ¼ Hc2/c.1 Among high Tc superconductors, the Cu0.5Tl0.5Ba2Ca2Cu3O10y has shown lowest anisotropy.2,3 In previous studies, it was found that a decreases in c has increased the critical temperature [Tc(0)], magnitude of diamagnetism, and the critical current density Jc(H).4 The low isotropic superconductors are simpler because of the symmetric wave function (s-wave type) assigned with the carriers. It is, therefore, advisable to have a superconductor with low anisotropy because the shape of the Fermi surface in such superconductors would be less intricated.5,6 The Cu0.5Tl0.5Ba2Ca2Cu3O10y superconductor has Cu0.5Tl0.5 Ba2O4y charge reservoir layer and three asymmetric CuO2 planes which are separated by Ca atoms.7,8 The outer CuO2 planes (OP) connected with Cu0.5Tl0.5Ba2O4y charge reservoir layer via apical oxygen atoms are identical whereas the third inner CuO2 plane (IP) is dissimilar from former two due to its different neighbouring environment. The OP planes due to their immediate connectivity with the charge reservoir layer are over-doped whereas the IP plane is under doped.9 In the present studies, we have substituted Mn at Ca site; the ionic radius of Mn is much smaller than that of Ca atom and it has multiple oxidation states as compared to the calcium atoms which have a fixed oxidation state (þ2). a)

Author to whom correspondence should be addressed. Electronic mail: [email protected]. Tel.: 92-51-289605. Fax: 92-51-2896084.

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There were two objectives of substituting Ca by Mn; one was to increase the inter plane coupling and to lower the anisotropy parameter. Since the Fermi wave vector KF ¼ [3 p2(N/V)]1/3 depends on the volume of the unit cell, therefore, the decrease in volume of the unit cell increases KF; higher the KF longer will be the c-axis coherence length nc ¼ ÉKF/2 mD.3,9–11 The smaller sized atoms, somehow, develop a better inter-plane coordination and superconducting properties are expected to be enhanced. In our previous studies, we have successfully substituted Ca by Mg atom and observed enhanced superconducting properties in terms of increased critical current temperature and critical current density.12,13 Another objective was to see the influence of paramagnetic Mn atom on the superconductivity. Mn can exist in multiple oxidation states (þ2, þ3, þ4, þ5, þ6, etc.) which could modify the spins of immediate copper atoms.14–16 The doping effect of Mn on the superconductivity of Cu0.5Tl0.5Ba2Ca2Cu3O10y has not been reported so far although Mn doping causes a series of interesting phenomena in NdCaCuO, LaSrCuO, and YBaCuO.17–22 For example, Mn doping in NdCaCuO causes an orthorhombicto-tetragonal structure transition, and significantly suppresses Tc(0) and the superconducting volume fraction. The effective magnetic moment per unit cell was increased with increasing Mn content, resulting in an antiferromagnetic correlation at high doping levels and a complete destruction of superconductivity.17 Mn doping in LaSrCuO does not fit the Abrikosov and Gork’ov theory which predicts that the superconductivity transition temperature (Tc) should decrease in the presence of magnetic impurity.19 Mn doping in LaSrCuO also induces a spin-compensated state on some length scale which decreases the Meissner volume, along with a coexistence of superconductivity and ferromagnetism.15,18–20 The critical temperature of YBaCuO has been decreased drastically after Mn doping. XRD measurements showed that the structure of the YB2Cu4O8 phase was also largely affected even at low manganese-substitution

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levels;21,22 the Y-124 phase was completely disappeared after Mn doping. II. EXPERIMENTAL

The Cu0.5Tl0.5Ba2(Ca2xMnx)Cu3O10y(x ¼ 0, 0.15, 0.25, 0.35, 0.5, 0.75, 1.0, and 1.25) samples were prepared by the solid state reaction method in two stages. At the first stage, Cu0.5Ba2(Ca2xMnx)Cu3O10y(x ¼ 0, 0.15, 0.25, 0.35, 0.5, 0.75, 1.0, and 1.25) precursor material was prepared by thoroughly mixing Ba(NO3)2, Ca(NO3)2, MnO2, and Cu(CN) in a quartz mortar and pestle in appropriate ratios. The mixed material was fired twice at 860 C in a quartz boat for 24 h and furnace cooled to room temperature. At the second stage, the precursor material was ground for about an hour and mixed with Tl2O3 to give Cu0.5Tl0.5Ba2(Ca2xMnx)Cu3O10y(x ¼ 0, 0.15, 0.25, 0.35, 0.5, 0.75, 1.0, and 1.25) as final reactants composition. The pellets of thallium mixed material were prepared under 3.8 tons/cm2 pressure. The pellets were then enclosed in a gold capsule and heat treated for about 7 m at 860 C followed by quenching to room temperature. The superconducting properties of samples were characterized by resistivity and ac-susceptibility measurements. The structure of the material was determined by x-ray diffraction scan from Rigaku D/Max IIIC, using a CuKa source of wavelength ˚ and the cell parameters were determined by a cell 1.54056 A refinement computer program. The proton induced x-ray emission (PIXE) spectroscopy was used to confirm the incorporation of Mn in the final compound. The proton beam of energy 2.5 MeV was obtained from 5 MV tandem accelerator at NCP. The phonon modes related to the vibrations of various oxygen atoms in Cu0.5Tl0.5Ba2(Ca2xMnx)Cu3O10y unit cell were studied using Nicolet 5700 Fourier transform infrared spectrometer (FTIR) in 400–700 cm1 wave number range. The annealing of the samples in flowing oxygen atmosphere was carried out in a tubular furnace at 500 C for six hours.

FIG. 1. XRD pattern of Cu0.5Tl0.5Ba2(Ca2xMnx)Cu3O10y(x ¼ 0.25, 0.50, and 0.75) superconductors.

III. RESULTS AND DISCUSSION A. As-prepared samples

The x-ray diffraction scans of representative samples of Cu0.5Tl0.5Ba2(Ca2xMnx)Cu3O10y(x ¼ 0.25, 0.5, and 0.75) superconductor are shown in Fig. 1 with very small amount of unknown impurity phase and CuTl-1212 superconducting phase. The planar reflections are fitted very well to the orthorhombic structure following P4/mmm space group. The XRD analysis has shown a decrease in the length of cell parameters with the increased Mn-doping, which was expected. The decrease in the unit cell volume could increase the c-axis coherence length resulting in lower anisotropy. The PIXE spectra of Mn doped samples are shown in Fig. 2. It can be seen from these spectra that the relative height of Mn k x-ray line has been increased with the increase of Mn concentration, which is a sign of Mn incorporation in the material. The resistivity measurements of Cu0.5Tl0.5Ba2(Ca2xMnx)Cu3O10y (x ¼ 0, 0.15, 0.25, 0.35, 0.5, 0.75, and 1.0) samples are shown in Fig. 3. A metallic variation of resistivity from room temperature down to the onset of superconductivity was observed in all the samples. The room temperature resistivity in these

samples was varied from 0.075 to 0.155 X cm. The superconductivity onset temperature [Tc(onset)] and critical temperature as a function of Mn concentration are plotted in Figs. 4(a) and 4(b). The Tc(onset) as well as Tc(0) have been systematically suppressed with the increased Mn concentration. The AC-susceptibility measurements of Cu0.5Tl0.5Ba2 (Ca2xMnx)Cu3O10y(x ¼ 0, 0.15, 0.25, 0.35, 0.5, 0.75, and 1.0) samples are shown in Figs. 5(a) and 5(b), which show bulk superconductivity in these samples. The FTIR absorption measurements of as-prepared Cu0.5Tl0.5Ba2(Ca2xMnx)Cu3O10y(x ¼ 0, 0.15, 0.25, 0.35, 0.5, 0.75, and 1.0) samples are shown in Fig. 6(a). In (Cu0.5Tl0.5)Ba2Ca2Cu3O10y oxide superconductors, the vibrations related to apical oxygen, planar oxygen, and Od oxygen atoms of Cu0.5Tl0.5Ba2O4y charge reservoir layer are observed around 400–540 cm1, 540–600 cm1, and 660–700 cm1, respectively.23 In Mn free (Cu0.5Tl0.5)Ba2 Ca2Cu3O10y samples, the apical oxygen mode of the type Cu(1)-OA-Cu(2) and Tl-OA-Cu(2) were observed around 540 cm1 and 491 cm1. The CuO2 planar oxygen mode

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FIG. 2. The PIXE spectra of Cu0.5Tl0.5Ba2 (Ca2xMnx)Cu3O10y(x ¼ 0, 0.15, 0.25, 0.35, 0.50, 0.75, and 1.0) superconductors.

and Od mode were observed around 572 cm1 and 694 cm1, respectively. The intensity of apical oxygen mode of type Cu(1)-OA-Cu(2) and Od mode of the charge reservoir layer has been decreased whereas that of CuO2 planar mode of oxygen atoms was increased after Mndoping. The decrease in the intensity of Od mode of oxygen atoms of Cu0.5Tl0.5Ba2O4y charge reservoir layer shows that oxygen content has been decreased with the increased concentration of Mn in the final compound. As far as the frequency of the phonon modes is concerned, the CuO2 planar oxygen mode was systematically hardened with the increased Mn-doping, which may be related to the shrinkage of the a and b axes of the unit cell. However, the two apical oxygen modes of type Cu(1)-OA-Cu(2) and Tl-OACu(2) behaved differently, i.e., the Cu(1)-OA-Cu(2) mode was softened and Tl-OA-Cu(2) mode was hardened with the increased Mn-doping. The hardening of Tl-OA-Cu(2) apical oxygen mode indicates that the charge state of Mn is different from þ2 and is most likely more than þ2, which attracts the Tlþ1 (the charge state of thallium largely depends on the oxygen content in the charge reservoir layer. As the oxygen content decreases, thallium changes its state from þ3 to þ2 and þ1 (Refs. 13 and 24)) towards itself making the Tl-OA-Cu(2) bond shorter. Moreover, the higher

electronegativity of Mn as compared to Ca atoms attracts Cu(2) more towards itself as compared to Cu(1) and stretching the Cu(1)-OA-Cu(2) bond making it to vibrate at lower frequency. These results show that Mn has successfully replaced Ca in the unit cell of (Cu0.5Tl0.5)Ba2Ca2Cu3O10y samples. Now, we will discuss the possible mechanisms

FIG. 3. Resistivity curves of Cu0.5Tl0.5Ba2(Ca2xMnx)Cu3O10y(x ¼ 0, 0.15, 0.25, 0.35, 0.50, 0.75, and 1.0) superconductors.

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B. Oxygen annealing

In order to confirm the presence of either of the two mechanisms of Tc suppression, we have annealed Cu0.5Tl0.5Ba2(Ca2xMnx)Cu3O10y samples in oxygen. The increased concentration of oxygen will overcome the deficiency of holes in CuO2 planes and would recover the decrease in Tc(onset) and Tc(0). The resistivity measurements of oxygen annealed Cu0.5Tl0.5Ba2(Ca2xMnx)Cu3O10y(x ¼ 0, 0.15, 0.25, 0.35, 0.5, 0.75, and 1.0) samples are shown in Fig. 7. These samples have shown metallic variation of resistivity from room temperature down to onset of superconductivity. The room temperature resistivity of these samples was not significantly influenced by the oxygen annealing and it was varied from 0.04 to 0.15 X cm. The Tc(onset) and Tc(0) as a function of Mn-doping for the oxygen annealed samples are shown in Figs. 4(a) and 4(b). Both Tc(onset) and Tc(0) have been further suppressed after annealing in oxygen. The ACsusceptibility measurements of oxygen annealed Cu0.5Tl0.5Ba2 (Ca2xMnx)Cu3O10y(x ¼ 0, 0.15, 0.25, 0.35, 0.5, 0.75, and 1.0) samples are also shown in Figs. 8(a) and 8(b). The phonon modes of Cu0.5Tl0.5Ba2(Ca2yMny) Cu3O10d(x ¼ 0, 0.15, 0.25, 0.35, 0.5, 0.75, and 1.0) oxygen annealed samples are shown in Fig. 6(b). The salient features of these absorption spectra are that Cu(1)-OA-Cu(2) apical

FIG. 4. (a) The onset critical temperature as a function of Mn-doping concentration. (b) The zero resistivity critical temperature as a function of Mn concentration.

behind the suppression of superconductivity in terms of decrease of Tc(onset) and Tc(0) with the increase of Mn content. There may be two reasons of suppression of superconductivity; one is the decrease of the carrier concentration in CuO2 planes due to the decrease in the oxygen content in the charge reservoir layer and the higher charge state of Mn (>þ2), which supply extra electrons to CuO2 planes as compared to the Ca atoms and reduces the hole concentration below the optimum level. According to stripe theory, holes induced in CuO2 planes segregate into periodically spaced stripes that separate anti ferromagnetic isolating domains of copper atoms. Therefore, the second reason may be the paramagnetic spins of Mn ions which could interact with the copper spins in the neighbouring CuO2 planes and produce disorder in the anti ferromagnetic spin structure of the copper atoms, and forces the rearrangement of charge stripes in a manner to suppress superconductivity. Such effects have been observed in the form of decreased critical temperature and magnitude of diamagnetism in Mn doped Cu0.5Tl0.5Ba2(Ca2xMnx)Cu3O10y samples. In the samples with higher Mn-doping, the room temperature resistivity was significantly increased and the samples with Mn doping of x 1.25 have been found to be totally insulating.

FIG. 5. (a) and (b) AC-susceptibility measurements of as-prepared Cu0.5Tl0.5 Ba2(Ca2xMnx)Cu3O10y(x ¼ 0, 0.15, 0.25, 0.35, 0.5, 0.75, and 1.0) superconductors.

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FIG. 6. Infrared absorption spectra of Cu0.5 Tl0.5Ba2 (Ca2xMnx)Cu3O10y(y ¼ 0, 0.15, 0.25, 0.35, 0.50, 0.75, and 1) superconductors (a) asprepared (b) oxygen annealed.

oxygen and CuO2 planar oxygen mode modes have been softened in comparison with as-prepared samples. The intensity of Od mode of oxygen atoms of Cu0.5Tl0.5Ba2O4d charge reservoir layer observed around 690 cm1 has been increased after annealing in oxygen. The increased intensity of Od mode of oxygen atoms is a manifestation of enhanced oxygen content. The results of oxygen annealing of Cu0.5Tl0.5Ba2(Ca2yMny)Cu3O10d(x ¼ 0, 0.15, 0.25, 0.35, 0.5, 0.75, and 1.0) show that even the oxygen annealing, which is a source of supply of holes to the CuO2 planes, could not recover the decrease in Tc(0). It could be inferred from the preceding results that the effect of paramagnetic spins of Mn atom on the anti ferromagnetic domains is so strong that the oxygen annealing could not increase the critical temperature, which shows that any disorder in the spin density wave (SDW) (anti ferromagnetic aligned spins of copper atoms in CuO2 planes) induced by magnetic impurities is very crucial. The disorder in the SDW could possibly

FIG. 7. Resistivity curves of Cu0.5Tl0.5Ba2 (Ca2xMnx)Cu3O10y(x ¼ 0, 0.15, 0.25, 0.35, 0.50, 0.75, and 1.0) superconductors.

FIG. 8. (a) and (b) AC-susceptibility measurements of oxygen annealed Cu0.5Tl0.5Ba2(Ca2xMnx)Cu3O10y(x ¼ 0, 0.15, 0.25, 0.35, 0.5, 0.75, and 1.0) superconductors.

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forces the charge stripes to rearrange themselves resulting in a suppression of superconductivity. Moreover, in our recent paper, we have also shown that the flux pinning characteristics are also suppressed after Mn substitution at Ca site.25 IV. CONCLUSION

In conclusion, we have successfully synthesized Mndoped Cu0.5Tl0.5Ba2(Ca2yMny)Cu3O10d(x ¼ 0, 0.15, 0.25, 0.35, 0.5, 0.75, and 1.0) samples and studied their superconducting properties. The Mn doping has suppressed superconductivity of these samples. The main object of substitution of Mn at the Ca site was to see the influence of the paramagnetic spins of Mn atoms on the charge/spin state of neighbouring Cu atoms in the CuO2 planes. These studies were important for understanding basic mechanism of high temperature superconductivity in the context of charge stripe model. The paramagnetic spins of Mn atom disturb the anti ferromagnetic alignment of copper atoms adjacent to the charge stripes, which possibly forces the creation of disorder in the charge stripe structure and Tc(0) is suppressed. After oxygen annealing, there was further suppression of superconductivity in terms of decreased Tc(0), which shows that the effect of paramagnetic spins of Mn atom on the anti ferromagnetic domains is so strong that the oxygen annealing could not increase the critical temperature. The disorder in the SDW induced by magnetic impurities could possibly forces the charge stripes to rearrange themselves resulting in a suppression of superconductivity. Moreover, the stronger inter plane coupling resulting from the decrease in the lattice parameters could not improve the superconducting properties too due to the dominant influence of the paramagnetic spins of Mn atom at Ca site. 1 2

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