Superconducting and magnetic properties of Y1Ba2Cu4O8±δ

Physica C I68 ( 1990) 40-46 North-Holland

SUPERCONDUCTING

AND MAGNETIC

PROPERTIES

OF Y1Ba2Cu408 i 6

G. TRISCONE, T. GRAF, A. JUNOD, D. SANCHEZ, 0. BRUNNER, J. MULLER

D. CATTANI and

Dkpartement de Physique de la Matiere Condenske, tJmversitPde GenPve. CH-121 I GenPve. 4. Swlrzerland Received

12 March

1990

We have carried out extensive characterization and measurements on a high purity Y IBa2Cu408+d ( 124) sample. Metallographic investigations, X-ray diffraction and microprobe analysis have been performed and reveal a single phase with traces of CuO. Resistivity measurements give a resistivity ratio R (300 K) /R( 100 K) of -_ 6.1, and a critical temperature r,, of 82.5 K. The AC susceptibility shows a sharp transition at about the same temperature. The Meissner effect, with a 55% fraction (at 4.2 K with H= 20.3 Oe) confirms the bulk nature of the superconductivity. The specific heat difference C( 0, T) - C( H, T) has been estimated from magnetization measurements in the vicinity of r, for&f= 8 T and compared to the direct calorimetric measurement. Both methods indicate that the specific heat jump amounts to approximately I5 mJ/K’mol. Fluctuation contributions are found up to several degrees above T,. Inter- and intra-grain critical currents were determined at 4.2 K by AC inductive measurements and by measuring the irreversibility of the magnetization curve at a constant temperature (Bean’s method). The results shows that there is no significant difference between the critical currents of the 124 phase and those of the 123 phase (Y ,Ba#Lt,O,).

1. Introduction The superconducting phase Y, Ba2Cu408 k 6 ( 124 ) discovered by electron microscopy in Y ,BazCuJO, (123) bulksamples [l] andinthinlilms [2] in 1988 is a very interesting and promising compound from many points of view. The 124 structure (space group Ammm) is closely related to the 123 structure, but with one additional Cu-0 chain in the unit cell [ 3 1. Each unit cell contains two Cu-0 chains and this leads to a much longer c-lattice parameter of 27.24 A and a smaller orthorhombicity (0.8% in the 124 phase, 1.8% in the 123 phase (0,) ) [ 41. Furthermore, the well known second order tetragonal-orthorhombic phase transition of the 123 structure at high temperature is suppressed in the 124 phase. The 124 compound is orthorhombic at all temperatures and no presence of the characteristic macroscopics twins observed in the orthorhombic 123 phase (created at the tetragonalorthorhombic transition) can be detected. While in the 123 structure the oxygen concentration is strongly sensitive to temperature and atmosphere change [ 5 1, this is not the case for the 124 compound charac0921-4534/90/$03.50 ( North-Holland )

0 Elsevier Science Publishers

B.V.

terized by a very stable oxygen content as a function of temperature [ 61. The critical temperature of the 124 phase is known to be about 80 K and its resistivity much smaller than the 123 phase [ 71. Recent work [8] has indicated that the addition of Ca in the 124 phase raises T, to 90 K. In this paper we present resistivity, AC susceptibility, inter- and intra-grain critical current, and Meissner effect measurements on a particularly well characterized single phase 124 sample. Additionally. we present an alternative way to estimate the specific heat anomaly at T, using magnetization measurements and we compare these results to the direct calorimetric measurement.

2. Sample preparation High purity Y,03 (99.999%, Fluka AG), BaCO, (99.99%) and CuO (99.95%, Alfa Johnson Mathey) powders were calcined in flowing oxygen (P( O2 ) = I atm) between 870°C and 920°C for a week and then reground. This procedure was applied twice. The

G. Triscone et al. /Superconducting

properties ofY,Ba2Cu,0Bkd

41

product is a two-phase black powder containing YBa2Cu30, and CuO (determined by X-ray analysis) as one would expect from the phase relations at P(0,) = 1 atm for a nominal composition Y : Ba: Cu = 1: 2 : 4 [ 9 1. The prereacted powder was cold pressed, fired under 85 atm O2 at 1000°C and reground several times. Finally the powders were cold pressed again under about 6.5 kbar to pellets with convenient dimensions for the following experiments and tired once more under high pressure.

3. X-ray, metallographic investigations

and microprobe

The crystal structure was studied at ambient temperature by X-ray diffraction using a Guinier camera with Cu K, radiation. Silicon was added to the powdered samples as an internal standard (a= 5.4301 A). The lattice parameters were determined from least-squares fits of the diffractograms with more than 25 lines indexed (10” <40< 140”) with the space group Ammm (No. 65), as determined by Marsh et al. [ 3 1. We find the lattice parameters listed in table I and a resulting orthorhombicity 2 (b-a) / (a + b) of 0.76%. This is considerably lower than the anisotropy of 1.6% observed in oxygen deficient samples of the 123 phase having a T, of 80 K [ 51. Microstructural investigations were performed after polishing the sample using diamond paste. The surface was studied in a light microscope and in a highresolution scanning electron microscope coupled with Table I Parameters

c 2(b-a)l(a+b) d

T, (xAC) AT, (x AC, IO-90%) AT, (Resistivity lo-90%) AT, (Meissner 1O-90%)

AT, FC Meissner fraction x0 (constant term) ferromagnetic moments R(300K)/R(lOOK)

(a) nor-

energy dispersive X-ray analysis (EDAX). Occasionally clusters of CuO were found. These CuO clusters ( < 1% volume) are not visible on the X-ray diffractograms which showed the pure single phase 124. Figure 1 is a micrograph taken with normal (a) and polarized light (b) of the sample (ICO-3). The picture shows only the 124 phase. The sample density is about 93 to 94% of the theoretical value (6.11 g/cm3).

of sample ICO-3.

a b

Fig. 1. Optical micrograph of the sample ICO-3 under mal and (b) polarized light.

3.8397(7) 3.8691(5) 27.230(3) 0.76 5.74 * 0.04 81.42f0.5 1.54 0.96 6.8 16 55% 3.03 x lo-’ ~0.2 wt.ppm 6.1

IAl IAl [Al

4. Resistivity

%

[s/cm31 [Kl [Kl [Kl [Kl [mJ/K’mol] (H=20.3 [Oe]

[emu/s1 Fe equiv.

)

We used a standard four-point AC technique to measure to resistance as a function of temperature. The measuring current density was 1.7 x 10w3 A/cm2 and the frequency was set at 222 Hz. Four gold contacts were sputtered onto the sample to improve the contact resistance. Figure 2 shows the normalized resistance versus temperature and the inset is a blow-up of the transition. The behaviour is clearly metallic with a re-

(Y. Trmvne et al. /Superconducting

42

5 0.6 QG g 0.4 0.2 0.0

1

-L_i-_L-_-

100

150

Temperature

200

250

300

(K)

Fig. 2. Electrical resistivity vs. temperature. Full line: fit (see text). Inset shows a blow-up of the superconducting transition.

sistivity ratio, defined as R( 300 K) /R( 100 K), of 6.1 (twice the value of the 123 phase [ 10 ] ). A sharp superconducting transition is observed with a T,, (zero resistance) at 82.5 K, and a transition width (onset-7,,) of 1 K. The linearity of the R(T) curve of the 92 K-superconductor YBazCulO, ( 123) has been considered as an indication in favour of nonconventional scattering mechanisms. Indeed, for typical samples of 123. the resistivity curve extrapolated linearly below T, hits the origin. Clearly such an extrapolation for YBazCudOs+,? ( 124) would be unphysical, since it would lead to negative resistance values below 67 K (sample ICO-3 ). A more classical interpretation is in order. Figure 2 shows a fit of the Bloch-Griineisen type. The exact solution of the linearized Boltzmann equation for the case of an Einstein phonon distribution [ 1 1 1, a residual resistivity R. and a “parallel resistor” term R, are the ingredients included in the fit of the data from 100 to 300 K:

With R,=0.6465,

&=315.1 K, R,=8.165 and of the fit is equal to the experimental scatter (AR =0.0024 rms). All resistance values are normalized to the room-temperature resistance RxoO. Note that a simpler two-parameter fit with R,=0.5071 and 0,=279.7 K (Ro=O, R,=m) is still satisfactory (AR=O.O058 rms), and also features a small negative curvature at high temperature without resorting to the parallel resistor Ro=O.O1 115, the residual

propertres q/~i’,Ra,C’u,O,~ _ ,,

scheme. These fits tell us that: ( 1 ) an interpretation in terms of electron-phonon scattering is not in contradiction with experiment: (2) the apparent linearity of the R ( T) curve down to the origin observed for some superconducting oxides (but actually measured only above 7; ) can be reproduced by the present data by simply adding a higher residual resistivity; (3) the residual resistivity of YBa,Cu,O,L,b is particularly low. In order to document this last point we measured the absolute room-temperature resistivity by the DC van der Pauw method [ 121. The value obtained. p=355 p!&m, is less accurate than the relative data of fig. 2. The resistivity at 100 K. c 60 F&m. is similar to that of very homogeneous 123 thin films, and much smaller than that of typical 123 ceramics. The residual resistivity as deduced from the fit is 4 pRcm, a value that would be considered as low even for ordinary metallic compounds (e.g. Nb,Sn). It could be underestimated if superconducting fluctuations did contribute at temperatures much higher than expected.

5. AC susceptibility

and the Meissner

effect

A RF SQUID magnetometer [ 131 was used fat Meissner (field cooling), susceptibility and magnetization measurements. Figure 3 illustrates the sharpness of the field cooling transition (AT~C1”S”e’ ( lo-90%) =6.8 K). The constant applied field (20.3 Oe) was calibrated using a Pb standard. The Meissner flux expulsion ratio

-0.5

1

-0.7

m.

1 Temperature

(K)

Fig. 3. Field cooling susceptibility below T,. representing 55% of the perfect Meissner effect at 4.2 K (H=20.3 Oe). Inset shows the AC susceptibility at li,,,=O. I Oe.

G. Triscone et al. /Superconducting properties of Y,Ba2Cu408+6

f= 55% was evaluated using an effective sample volume given by m/d where m is the mass and d is the X-ray density (6.11 g/cm3). We took into account the geometric demagnetization factor D=O. 136. The observed Meissner fraction is rather higher for this value of the magnetic field. For comparison, a very high quality sample of 123 phase with a large specific heatjumpat T, (AC/T,=67 mJ/K’mol) hadf=65% in a magnetic field of 18 Oe [ 14 1. The inset of fig. 3 shows the AC susceptibility measured in a helium atmosphere at 73 Hz in a magnetic field of 0.1 Oe rms, and recorded at a rate of 15 K/h. The transition midpoint is 8 1.4? 0.5 K and the width (lo-90%) is 1.5 K.

6. Normal state susceptibility Figure 4 shows the normal-state susceptibility (full squares) measured between 80 and 250 K in a magnetic field of 20 kOe. No significant ferromagnetic impurities ( ~0.2 wt.ppm Fe equiv.) could be detected using Honda’s method [ 15 1. The value of the susceptibility is lower than that of the 123 phase ((4.2-5) x lo-’ emu/g at 100 K (0,) ) and increases markedly with temperature. With respect to the latter feature, one might ask if CuO impurities could be responsible for the positive dX/dT. CuO indeed exhibits a susceptibility [ 161 almost an order of magnitude larger than that of the 124 phase, as shown in the inset of fig. 4. Taking a 4.0

.,,.I

,,,I

, ,I,,,

,,,

,

, ,,, . ’ . . . n

G 3.5 ‘3 E 3,0;

fi

2

f?

5 x

~8 2.5

-

0

.

l

q

q

.

.

. .

Ooco”:

l

43

safe upper limit of 1% CuO by volume, we find the maximum possible correction also shown in the main part of fig. 4 (open squares). It is evident that the susceptibility still increases with temperature in much the same way, i.e. dx/dT> 0 is an intrinsic property of the 124 compound. This behaviour suggests either antiferromagnetic ordering at high temperature or alternatively a dip in the density of states with strong positive curvature at the Fermi energy. The diamagnetic core susceptibilities of the 124 and the 123 phases are almost equal, 1.34 and 1.35x 10m5 emu/g-at, respectively. If we tentatively assume that the orbital Van Vleck-type contributions are also comparable for the two phases and roughly compensate the core diamagnetism, the observed differences in the extrapolated x0( T=O) can be attributed to a different (possibly exchange enhanced) Pauli susceptibility. Quantitatively, xA24amounts to 5.6 x 1O-5 emu/mol-Cu, whereas previous measurements [ 17-201 indicate values for xh23 between 9.3 and 11.1 x 10e5 emu/mol-Cu. Taking into account the slight uncertainty resulting from the CuO correction, one concludes that Xpauliof 124 is 40 to 50% lower than that of 123. This trend is expected to reflect itself in the coefficient y of the electronic specific heat. Considering the respective T, values and assuming that AC( T,) / yT, is similar for both phases, we anticipate that the mean field specific heat discontinuity at T, of 124 should be lowered by 50 to 60% compared to 123. The calorimetric measurements [ 211 at least qualitatively confirm this prediction.

7. Magnetization

measurements and specific heat

l

c Oi

;z

‘0

2.2

‘;

2.0

;

1.8

0

50

loo

150

200

250.

Exploiting the precise measurement of the reversible isothermal magnetization M(H) between H= 0 and H=H*, we can thermodynamically determine the specific heat difference C(H=O, T)-C(H=P, T) [22-241 through the relation:

lCO-3 295”’

ibd

iL$e~~~,~~~;~~~?-Z”

is0

[C(H*,

T)-C(0,

T)],T=,u,$~A4dH’. 0

Fig. 4. Susceptibility above T, vs. temperature (full squares). The open squares represent the susceptibility with the worst case correction for CuO impurities. Inset shows the susceptibility of CuO in the same temperature range.

Below T,, if H*> Hcz, C(IJy, T) is equal to the normal state ( z field independent) specific heat and the total specific heat anomaly can, in principle, be

44

G. Trmone

et al. /Superconducting

determined. The validity of this thermodynamic relation is however not restricted to temperatures below T,. It holds if the M( H, T) curves are reversible. In fig. 5, we show the measurements of M(H) in the vicinity of T,. The data were taken at constant temperature after zero-field cooling in both increasing and decreasing fields. An onset of irreversibility below 5 kOe appears at the lowest isotherm (76 K). This measurement was not used for the specific heat determination. These results establish that the “reversibility window” extends to 5 K below T, for the 124 phase. Figure 6 shows the magnetic specific heat difference calculated with the above relation. Below 82 K, the determination is based on the magnetization curves. whereas above we used the measurement of the field independent susceptibility. .As can be seen in fig. 6 we find a maximum spe-

,-0.2 g-0.4

properfres

g -0.6

8.

-1.0 I co-3 _eL--.__.l

1

2

3

4

5

6

7

X

9

H,,, (x1040e) Fig. 5. Magnetlratlon

isotherm

of YBa,Cu,O,,,

Temperature

near 7;

(K)

Fig. 6. Specific heat difference between H= 0 and&P = 8 T. Inset: measured specific heat in H=O (dots) and predicted specific heat in&P=8 T (open circles).

+,)

cific heat difference C’(0)-C ‘(puoH*= 8 T ) of I 3 mJ / K’mol for the 124 phase. For the 124 phase we had previously obtained 24 mJ/K’mol using the same method [ 221. The above maximum exhibited by the 124 phase compares favourably with the anomaly obtained by direct calorimetric measurement. 16 mJ/ K’mol [21]. The inset of fig. 6 shows the calorimetric measurement (dots) and the difference between the calorimetric measurement and C( O)-c’(pOH* = 8 T) determined by the magnetization measurements. i.c. the predicted specific heat at 8 T. The data represent the normal state specific heat in the temperature interval where poHcz -C8 T. The residual anomaly is related to the slope of the (anisotropic) critical field. An interesting point is that the residual anomaly is more important in the 123 phase [22] than in the 124 phase. In other words, the slope of the critical field dHcz/dTI ,c must be higher in the 123 phase than in the 124 phase. Finally, we note that the present analysis of the magnetic data also predicts a small but sizcable variation of the specific heat versus H in the fluctuation regime just above 7;.

Z-0.8 t:

o,f Y,Ba2C‘u.&

G. Triscone et al. /Superconducting Table 11 Parameters geometric h

properties of Y,Ba,Cu+O,,,

45

of sample ICI-9. form

cylinder 7.00 2.88 3.8418(5) 3.8714(S) 27.238(2) 0.77 5.68 79.20 2.5 49.6%

@ a b c 2(b-a)/(a+b) d T, (xAC) ATc (xAC, lo-90%) FC Meissner fraction

[mm1 [mm1 [Al $1 % [g/cm’1 lK1 tK1 (H=20.5

[Oe]) Fig. 8. Critical currents vs. magnetic field at 4.2 K determined with Bean’s critical state model. If the grain size rather than the sample diameter had been used in the calculation, the ordinate scale would have been shifted by two orders of magnitude. The inset shows the irreversible magnetisation vs. magnetic field at 4.2 K.

1011”““““““”

“‘j””

“““‘J”

“‘I

(.1-._~ 0

12

3

4 5 H.,, (x l@Oe>

Fig. 7. Intergrain (a) and intragrain K vs. magnetic field.

6

(b) critical

7

8

currents

9

at 4.2

there is no really significant difference between the two phases. Critical current densities were also estimated using the magnitude of the hysteresis of the DC magnetization at low temperature. The relation based on Bean’s critical state model [ 271 reads (MKSA): .L (H) = -3(M+

(H) -M-

(H)

)/@,

where j, is the critical current, M, (H) and M_ (H) the magnetization when the external magnetic field increases and decreases, respectively, and @ the diameter of the cylinder perpendicular to the field [28,29] (in CGS units: j,z -3O(M+-M_)/@ where [Jc] =A/cm’, [M] =emu/cm3 and [@I =cm). Such a measurement is shown in fig. 8. The inset is the magnetization curve at 4.2 K measured up to 80 kOe. No correction for a demagnetizing field needs to be made in this case.

We found that the magnetization is practically reversible above 40 kOe, implying that the critical current above 40 kOe is very weak. The hysteresis appears only below 40 kOe. If one compares the critical currents obtained in our case with the results of Zhou et al. [ 291 for the 123 phase, we observe definitely less hysteresis in the high field range. A striking feature (inset of fig. 8 ) is that above 40 kOe at low temperature, the 124 phase is characterized by the persistence of almost constant diamagnetism with vanishing critical current density.

9. Conclusion The present study has enabled us to characterize pure polycrystalline YBa2Cu40,,, and to pinpoint significant differences with respect to the related phase YBa2Cu30,. In particular, the 124 phase exhibits a higher normal state resistivity ratio, a considerably lower electronic density of states and a strongly positive derivative of the normal state susceptibility versus temperature. The critical current densities appear to be similar for both compounds although the 124 phase approaches more closely ideal type II behaviour above about 40 kOe at low temperature. The 124 phase also shows a “reversibility window” extending 5 K below T, for the magnetization isotherms. The thermodynamic analysis of

46

these data yields a satisfactory cific heat anomaly at 7,.

G. Trrsconeetal. /Superconductingpropertles

estimate

of the spe-

Acknowledgements The authors are grateful to Ph. Bouvier, F. Liniger and A. Naula for their technical assistance. This work was supported by the Fonds National Suisse de la Recherche Scientifique.

References [ I ] H.W. Zandbergen, R. Gronsky, K. Wang and G. Thomas, Nature 331 (1988) 596. [ 21K. Char, M. Lee. R.W. Barton, A.F. Marshall, I. Bozovic, R.H. Hammond, M.R. Beasley, T.H. Geballe, A. Kapitulnik and S.S. Laderman, Phys. Rev. B38 ( 1988) 834. [ 31 P. Marsh, R.M. Fleming, M.L. Mandich, A.L. DeSantolo, J. Kwo, M. Hong and J.L. Martinez-Miranda, Nature 141 (1988) 334. [ 41 K. Yvon and M. Francois, Z. Phys. B76 ( 1989) 4 13. [ 51T. Graf. G. Triscone and J. Muller, to be published in J. Less-Comm. Met. [6] D.E. Morris, N.G. Asmar, J.H. Nickel, R.L. Sid and J.Y.T. Wei, Physica C 159 ( 1989) 287. [ 7 ] P. Berghuis, P.H. Kes, B. Dam, G.M. Stollman and J. van Bentum, Physica C 167 ( 1990) 348. [ 81 T. Miyatake, S. Gotoh, N. Koshizuka and S. Tanaka. Nature 341 (1989) 41. [ 91 T. Graf. J.L. Jorda and J. Muller, J. Less-Comm. Met. 146 (1989) 49. [ IO] M. Ishikawa, Y. Nakazawa, T. Takabatake. A. Kishi, R. Kato and A. Maesono, Proc. M2S-HTSC. Physica C 153-l 55 (1988) 1089.

qf Y,Ba2Cu40,y +,,

[ I I ] H.-L. Engquist. Phys. Rev. 7 I ( 1980) 2067. [ I ?] L.J. van der Pauw, Philips Res. Rep. I3 ( 1958) I [ 13) M. Pelizzoneand 4. Treyvaud, App. Phys. 24 (I981 ) 375. [ 141 A. Junod. D. Eckert. G. Triscone. V.Y. Lee and J. Muller. PhysicaC 159 (1989) 215. [IS] K. Honda. Ann. Physik 32 (1910) 1027. [ 161 A. Junod. D. Eckert. G Triscone, J. Muller and W Reichardt. J. Phys. Condens. Matter I ( 1989) 802 I. [ 171 T. Kawagoe. T. Mizoguchi, K. Kanoda, T. Takahashi. M. Hasumi and S. Kagoshima. J. Phys. Sot. Jpn. 57 ( 1988) 2272. [ 181 C. Allgeter. R. Sieburger. H. Diederichs. H. Neumaier. W. Reith, P. Muller and J.S. Schilling. Proc. M*S-HTSC. PhysicaC 162-164 (1989) 741. [ 191 Y. Nakazawa and M. Ishikawa. Physica C I58 ( 1989) 38 I. [ 201 W.C. Let and D.C. Johnston. to be published m Phys. Rev. B. [ 21 ] A. Junod, D. Eckert, 7. Graf. E. Kaldis. J. Karpinski. 5. Rusiecki, D. Sanchez, G. Trisconc and J. Muller, Physica C 168 (1990) 47 (following article). [22] G. Trisconc. A. Junod and J. Mulier. Proc. M’SHTSC‘. PhysicaC 162-164 (1989) 470. [23] A. Bezinge. J.L. Jorda, A. Junod and and J. Muller. Solid State Commun. 64 ( 1989) 79. [ 241 KS. Athreya. O.B. Hyun, J.E. Ostenson, J.R. C‘lem and D.K. Finnemore, Phys. Rev. B38 ( I988 ) I 1846. [25] R.W. Rollins, H. Ktlpfer and W. Gey. J. Appl. Phys. 45 (1974) 5392. [26] H. Kilpfer. I. Apfelstedt. R. Flilkiger, C. Keller. R. MeierHirmer, B. Runtsch, A. Turowski, V. Wiech and T. Wolf. Cryogenics 28 ( 1988 ) 650. [ 271 C.P. Bean, Phys. Rev. Lett. 8 ( 1962) 250. [28] H.W. Weber, in: Studies of High Temperature Superconductors. ed. A.V. Narlikar (Nova Science, New York, 1989). [29] H. Zhou, C.L. Seaman, Y. Dalichaouch, B.W. Lee. K.N. Yang, R.R. Hake and M.B. Maple, Physica C 152 ( 1988) 321.