Influence of Al substitution in PrBa2Cu3O7−δ single crystals

Physica C 301 Ž1998. 141–146

Influence of Al substitution in PrBa 2 Cu 3 O 7yd single crystals S. Uma a , G. Rangarajan b, E. Gmelin a

a,)

Max-Planck-Institut fur Heisenbergstraße 1, D-70569 Stuttgart, Germany ¨ Festkorperforschung, ¨ b Department of Physics, Indian Institute of Technology, Madras 600 036, India Received 20 January 1998; accepted 14 March 1998

Abstract The role of Al substitution in single crystals of PrBa 2 Cu 3 O 7y d ŽPBCO. has been investigated using low temperature magnetic susceptibility and heat capacity measurements. The crystal composition and the amount of impurity have been determined using electron probe microanalysis ŽEPMA. and the lattice parameters have been determined using X-ray diffraction. Heat capacity measurements on single crystals of PrBa 2 Cu 3yy Al yO 7y d reveal a sharp peak at 16.6 K for y s 0.0 originating from an antiferromagnetic ordering of the Prion moments ŽTN . and broader transitions at f 10 K for y s 0.25 and f 5.3 K for y s 0.4. Subtle anomalies are also found at these temperatures in the magnetic susceptibilities. The present results along with the neutron diffraction studies demonstrate the deleterious role of Al on the magnetic properties in this sample. q 1998 Elsevier Science B.V. All rights reserved. PACS: 74.72.Bk; 75.30; 75.40.Cx Keywords: Y1y x Pr x Ba 2 Cu 3 O 7y d ; Magnetic susceptibility; Specific heat

1. Introduction Y1y x Pr x Ba 2 Cu 3 O 7y d ŽYPBCO. has engaged the attention of many research groups since substitution of Pr for Y depresses Tc and superconductivity disappears for x ) 0.55 w1x. PrBa 2 Cu 3 O 7y d ŽPBCO. finds potential applications since it lattice matches with YBa 2 Cu 3 O 7y d ŽYBCO. and is an insulator which enables the formation of trilayer Josephson junctions of the type YBCOrPBCOrYBCO. This has kindled several researchers to study the basic properties of this sample. A notable property is that the temperature of magnetic ordering of Pr in PBCO is almost two orders of magnitude larger than the )

Corresponding author.

value extrapolated from the ordering of the other rare earths in RBa 2 Cu 3 O 7y d ŽRBCO.. The unique properties of Pr in YPBCO offer a challenge to the scientific community to understand the physics of the interplay between superconductivity and magnetism in this system. Growth of large YPBCO single crystals of high quality is essential for understanding the relationship between the composition, crystal structure, magnetic and superconducting properties. It has been found that large dense YBCO type compounds can be grown with the use of alumina ŽAl 2 O 3 . crucibles which are among the cheapest crucibles. However, the high melting point of the sample causes the crucible material ŽAl. to react with the melt thereby introducing contamination from the crucible material

0921-4534r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 9 2 1 - 4 5 3 4 Ž 9 8 . 0 0 1 4 9 - X

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S. Uma et al.r Physica C 301 (1998) 141–146

in the single crystals w2,3x. For the preliminary measurements, small amounts of substitutional Al 3q can be tolerated without significantly compromising the property under investigation. However, for the ultimate goal of understanding the compound Žrelationship between the composition and various physical properties., it is essential to have contamination-free single crystals. We have therefore investigated the influence of Al substitutionrcontamination on the low temperature magnetic properties of oxygenated single crystals of PBCO. Al substitution in the crystals has been achieved by growing the crystals in alumina crucibles by the flux method w2,4x. Single crystals without Al substitution were grown in BaZrO 3 crucibles w3x. Electron probe microanalysis ŽEPMA. measurements have been performed to determine the amount of Al that enters the lattice. The low temperature magnetic susceptibility and heat capacity measurements have been performed on PrBa 2 Cu 3yy Al yO 7y d single crystals with y s 0.0, 0.25 and 0.4.

the Nernst step heating method. The sample contribution to the measured heat capacity is f 20% at 4.2 K and an error of 5% in heat capacity is estimated. Heat capacity measurements were also performed with magnetic fields up to 14 T on y s 0.4 single crystal using a home-made calorimeter in an Oxford Instruments cryostat assembly which includes a superconducting magnet and a lambda point refrigerator. The homogeneity of the magnetic field is better than 1 part in 1000 over a spherical volume of diameter 10 mm. Compact Cernox thin film resistance sensors ŽCX1050SD. having low magnetic field-induced errors, high sensitivity at low temperatures and good stability were used for temperature measurement and control. The in-field calibrations of the sample thermometer have also been performed in order to take into account the low magnetic field-induced errors w5x.

2. Experimental details

The chemical compositions of the single crystals of PrBa 2 Cu 3yy Al yO 7y d , as well as YP-BCO crystals grown in Al 2 O 3 crucibles analyzed using EPMA are tabulated in Table 1. The starting composition and the measured composition are found to be different since the crystals are grown in alumina crucible. Inspection of Table 1 shows that Al content increases with increasing Pr content which is similar to the observation of Widder et al. w6x. During the structure refinement using the X-ray intensity data of Y1y x Pr x Ba 2 Cu 3yy Al yO 7y d single crystals, it was found that the Al 3q occupies the CuŽ1. site ŽCu–O chains. and the occupancy of Al in CuŽ2. site ŽCu–O planes. is almost zero w5x. This is also supported by results in the literature w7x. Hence, the percentage of Al occupying the CuŽ1. sites has been determined and is also tabulated in Table 1. It is found that up to 40% of Cu–O chains are disrupted by aluminium. Table 2 shows the lattice parameters of the twinned PrBa 2 Cu 3yy Al yO 7y d single crystals. It is noticed that a and b increase and c decreases with increase in Al content causing an overall increase in the volume. The Pr–OŽ2., Pr–OŽ3. bond lengths for y s 0.0 w8x and that of y s 0.25 w5,9x are found to be almost the same which shows that the Al contamina-

Composition analysis was performed using EPMA with a wavelength dispersive spectrometer from JEOL JSM-840A electron microscope mounted in a JXA series electron microprobe. Y2 O 3 , Pr6 O 11 , BaF2 , Cu and Al were used as standards for Y, Pr, Ba, Cu, Al, respectively, for a quantitative analysis. The sample was probed at various spots and the average value of the element concentration was taken to determine the chemical composition. The detection limit of the instrument for impurity trace is f 0.2 at.% and the error in the composition analysis is less than 0.4 at.%. A Siemens four circle diffractometer with Mo K a radiation was used to determine the lattice constants. DC magnetic susceptibility measurements were performed using a Quantum Design SQUID magnetometer with a magnetic field of H s 1 T applied parallel Ž xc ., perpendicular Ž x a b . to the c-axis and in the temperature range 2–300 K. The heat capacity of single crystals of PrBa 2 Cu 3yy Al yO 7y d with mass 11.8 mg Ž y s 0.0., 12.7 mg Ž y s 0.25., 41.8 mg Ž y s 0.4. was measured in the temperature range 2–50 K using a quasi-adiabatic calorimeter based on

3. Results and discussion

S. Uma et al.r Physica C 301 (1998) 141–146

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Table 1 The chemical composition of the YPBCO single crystals grown in alumina crucibles as determined from the EPMA measurements Starting composition

Measured composition

Al Ž%. in chains

PrBa 2 Cu 3 O 7y d PrBa 2 Cu 3 O 7y d Y0.3 Pr0.7 Ba 2 Cu 3 O 7y d Y0.5 Pr0.5 Ba 2 Cu 3 O 7y d Y0.6 Pr0.4 Ba 2 Cu 3 O 7y d Y0.7 Pr0.3 Ba 2 Cu 3 O 7y d Y0.8 Pr0.2 Ba 2 Cu 3 O 7y d Y0.85 Pr0.15 Ba 2 Cu 3 O 7y d

PrBa 2 Cu 2.72 Al 0.25 O 7y d PrBa 2 Cu 2.6 Al 0.4 O 7y d Y0.32 Pr0.68 Ba 2 Cu 2.79 Al 0.21O 7y d Y0.51 Pr0.49 Ba 2 Cu 2.73 Al 0.17 O 7y d Y0.6 Pr0.4 Ba 2 Cu 2.74 Al 0.16 O 7y d Y0.73 Pr0.27 Ba 2 Cu 2.84 Al 0.16 O 7y d Y0.8 Pr0.2 Ba 2 Cu 2.86 Al 0.14 O 7y d Y0.82 Pr0.18 Ba 2 Cu 2.89 Al 0.11O 7y d

25 40 21 17 16 16 14 11

tion does not affect the Pr–O bond length Žsee Table 3.. The bond lengths which are expected to be affected by the Al contamination are the ones which involve CuŽ1. since Al occupies the CuŽ1. site. A shortening of CuŽ1. –OŽ1., CuŽ2. –OŽ2. and lengthening of CuŽ1. –OŽ4., CuŽ2. –OŽ1. due to Al contamination are observed. The marginal increase in the CuŽ2. –OŽ2. bond length may reflect a decrease in the number of carriers in the Cu–O 2 planes w10x. Ga and Zn substitution also does not affect the Pr–O bond length but causes a shortening of CuŽ1. –OŽ1. and lengthening of CuŽ2. –OŽ2. w11x. Figs. 1 and 2 show the magnetic susceptibility of y s 0.0, 0.4 crystals with magnetic field applied parallel Ž xc . and perpendicular Ž x a b . to the c-axis, respectively. The magnetic susceptibility for y s 0.25 has already been reported w9x. It is interesting to note that for y s 0.0, 0.25, xc ) x a b whereas for y s 0.4, xc - x a b . This result is more clearly seen in Fig. 3 where the magnetic anisotropy D x Ž xc y x a b . is plotted as a function of temperature. We infer that a contamination causes a change in the crystal field interaction at the Pr 3q site and therefore a change in the magnetic anisotropy. Previous susceptibility measurements on PBCO single crystals grown in Al 2 O 3 crucible have shown both xc ) x a b w12x, as well as xc - x a b w13x. However, the content of Al was not Table 2 Lattice parameters of PrBa 2 Cu 3yy Al yO 7y d crystals y

˚. a ŽA

˚. b ŽA

˚. c ŽA

˚ 3. V ŽA

0.0 0.25 0.40

3.885Ž5. 3.913Ž5. 3.919Ž3.

3.898Ž4. 3.916Ž3. 3.919Ž3.

11.749Ž10. 11.712Ž11. 11.717Ž16.

177.92 179.45 179.96

given in these studies. Aligned polycrystalline powder samples have shown xc ) x a b w14x. The magnetic ordering of Pr is observed only as a small kink or a change of slope in all the samples. In the y s 0.0 sample, we observe two transitions in the xc as well as x a b . The transitions occur due to Pr ordering at 16.6 K coupled with a CuŽ2. spin reorientation, and due to a change in the magnetic structure of Pr at 13.5 K as observed by the neutron diffraction measurements w15x. The mean susceptibility, given by xmn s Ž2 x a b q xc .r3 Žinset of Fig. 1. shows the two transitions. The x a b , xc of the y s 0.25, 0.4 samples do not reveal any transitions. However, D x shows subtle changes, which in comparison with the heat capacity measurements ŽFig. 4. and neutron diffraction measurements on Al-contaminated crystals w16x indicate the magnetic ordering of Pr at f 11 and f 5.3 K for y s 0.25 and 0.4, respectively. The neutron diffraction studies on the Al-contaminated and oxygenated PBCO crystal show a 3D ordering of Pr below 11 K with the magnetic moments tilted at an angle u f 608 from the c-axis w16x. An imperfect order and ferromagnetic coupling was observed along the c-axis. It should be remarked that even small Table 3 Bond lengths of PrBa 2 Cu 3yy Al yO 7y d Bond lengths

y s 0.0 w8x

y s 0.25 w5,9x

Pr–OŽ2. Pr–OŽ3. CuŽ1. –OŽ1. CuŽ1. –OŽ4. CuŽ2. –OŽ1. CuŽ2. –OŽ2. CuŽ2. –OŽ3.

2.435Ž2. 2.462Ž3. 1.849Ž3. 1.964Ž0. 2.254Ž4. 1.950Ž1. 1.983Ž4.

2.443Ž10. 2.464Ž11. 1.791Ž18. 1.972Ž2. 2.330Ž18. 1.976Ž2. 1.973Ž2.

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Fig. 1. DC magnetic susceptibility of PBCO with H applied parallel Ž xc : `. and perpendicular Ž x a b : q. to the c-axis. Inset: xmn showing two transitions.

contaminationsrdopants cause a change in the coupling along the c-axis w15–17x. The temperature dependences of the magnetic susceptibility parallel and perpendicular to the c-axis also indicate that the easy axis of magnetization may be at an angle from

Fig. 2. DC magnetic susceptibility of PrBa 2 Cu 2.6 Al 0.4 O 7y d with H applied parallel Ž xc : `. and perpendicular Ž x a b : q. to the c-axis. Inset: D x Ž xc y x a b . in warming and cooling showing a transition.

the c-axis below T N . At temperatures above TN , the easy axis of magnetization is decided by the crystal field effects while at temperatures below TN , the Pr–Pr exchange interaction overcomes the crystal field anisotropy and decides the easy axis of magnetization. The heat capacity measurement ŽFig. 4. clearly shows that measurements on polycrystalline samples w13,18–20x give more accurate results on the sample property than the Al-contaminated samples. The Pr ordering, as well as spin reorientation has been observed in the heat capacity measurement of the y s 0.0 sample and has been described elsewhere w15x. Al-contaminated PBCO single crystals show broader transitions with lowered transition temperature. Application of a magnetic field of 2 T parallel to the c-axis ŽFig. 5. on the y s 0.4 single crystal makes the transition broader and shifts it slightly to lower temperatures which is characteristic of an antiferromagnetic interaction. Schottky-like anomalies are observed for magnetic fields greater than 2 T. It is interesting to note that in the case of the y s 0.0 crystal, magnetic field as high as 12 T was necessary to shift T N below 5 K w15,21x. The entropy of all the three samples under investigation is f R ln2 at 17 K. A trend of saturation to f R ln3 is observed, which is expected for the low-lying quasi-triplet of Pr 3q.

Fig. 3. M agnetic anisotropy D x Ž x c y x a b . PrBa 2 Cu 3yy Al yO 7y d for y s 0.0 Že., 0.25 ŽI., 0.4 Ž^.

of

S. Uma et al.r Physica C 301 (1998) 141–146

Fig. 4. Specific heat of PrBa 2 Cu 3yy Al yO 7y d for y s 0.0 Ž`., 0.25 Ždashed line., 0.4 Ž=.. The solid lines show the data of Hilscher et al. w13x on polycrystalline samples.

Since the spin reorientations usually cause small transitions, it has been overlooked by several groups who have performed the magnetic susceptibility or heat capacity measurements. For example, two transitions have been observed previously in the c p measurements of polycrystalline samples of PBCO. However, no significance was attached to the transition around f 13.5 K w13,22x. Similarly, a recent report w23x on heat capacity measurements of PrBa 2 Cu 4 O 8 shows a transition at 17 K which is attributed to the Pr ordering and a weaker anomaly at 3–4 K which, according to the authors, is of unknown origin. We think that the transition at 3–4 K may be due to PrrCu spin reorientation. Neutron diffraction studies on single crystals of PrBa 2 Cu 4 O 8 would throw more light on this presumption. We like to emphasize that susceptibility, heat capacity and neutron diffraction measurements performed on a single batch of crystals give very valuable information compared to any of the techniques independently. Neutron diffraction has been used as a tool to understand the magnetic ordering of Pr and Cu in polycrystalline w24–27x and single crystalline w15–17x PBCO. Neutron diffraction studies on single crystals of PBCO with and without Al contamination w15–17x indicate a complex phase diagram of Pr and Cu

145

ordering. The CuŽ2. spins order at f 280 K and reorientation of the spins occurs at low temperatures Žf 17 K. which is coupled with the onset of magnetic order of Pr. This is further complicated by the fact that Al contamination causes CuŽ1. to develop ordered moment at low temperatures w7,28,29x. This phase is absent in pure single crystals which do not have aluminium. However, in Al-contaminated PrBa 2 Cu 3 O 7 , it has not been possible to clearly identify the existence of CuŽ1. moments using neutron diffraction measurements w16x. Hence, the observed transitions in the magnetic susceptibility and heat capacity could be caused due to the Pr, as well as the CuŽ2. moments. It has been reported that Ga substitution at the CuŽ1. site has a slightly larger effect on the T N than the Zn substitution at the CuŽ2. site w11,30x. From the present work, we find that Al substitution affects TN more strongly than Zn or Ga substitution w11x. It is quite strange that doping in the CuŽ1. site, which is farther away from the Pr ion, has a larger effect on the Pr magnetic ordering than doping in the CuŽ2. site, which is nearer. The reason may be that Al substitution causes a change in the CuŽ2. –O bond distances which affects the hybridization of Pr 4f electron; with the Cu–O conduction holes w31x which in turn decides the magnetic properties of praseodymium.

Fig. 5. Specific heat of PrBa 2 Cu 2.6 Al 0.4 O 7y d with a magnetic field of 0, 2, 5, 10 and 14 T applied parallel to the c-axis.

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