Changes in Ba0.6K0.4BiO3 produced by fast neutron irradiation

Physica C 169 (1990)265-270 North-Holland

C H A N G E S I N Bae.sKoaBiO3 P R O D U C E D BY FAST N E U T R O N IRRADIATION J.D. T H O M P S O N a, J.R. COST b, G.H. KWEI a and ICC. O T T c • Physics Division; b Nuclear Materials Technology Division; c Exploratory Research and Development Center, Los Alamos National Laboratory, Los Alamos, N M 87545, USA

Received 5 June 1990

Fast (E> 0.1 MeV) neutron irradiation of Bao.6Ko.4BiO3suppresses its superconducting transition temperature Tc at a rate substantially less than in high-Tocuprates. Also unlike the cuprates, neutron irradiation has little effect on the magnetization critical current density. A neutron fluence of ~ 1.4× 10TM n/cm 2 qualitatively alters the temperature dependence of the normal state susceptibilityof Bao.6Ko.4BiO3.

1. Introduction The effects of neutron irradiation on the superconducting transition temperature Tc and magnetic hysteresis have been studied in both sintered cuprates [ 1 ] and single crystals [2,3] ofcuprates having very high-To. The general behavior found in these studies is a monotonic depression of T, and monotonic enhancement, for moderate fluences, of the hysteresis (critical current density J,), suggesting that neutron irradiation creates defects detrimental to superconductivity but favorable to flux pinning. However, the precise nature of these defects has not been determined. Comparable observations have been made on cubic A15-structure compounds [4 ] in which the defect structure produced by neutron irradiation has been determined to be predominantly anti-site disorder. Bao.6Ko.4BiO3 (BKBO) is a cubic, non-cuprate superconductor with T, ~ 30 K, a value intermediate to A 15"s and the cuprates. Magnetic hysteresis and flux creep experiments on polycrystalline BKBO [ 5 ] reveal a small (relative to the La- and Y-based cuprates) flux pinning energy consistent with a small d~ that decreases rapidly with applied magnetic field. Given this apparently small critical current density in BKBO and the beneficial effect of neutron irradiation on Jc in cuprates and A 15 compounds which have relatively larger J,'s, one might expect neutron irradiation to enhance J, substantially in BKBO. 0921-4534/90/$03.50 © ElsevierSciencePublishers B.V. ( North-Holland )

However, as will be discussed, contrary to this expectation, irradiation of BKBO with E > 0 . 1 MeV neutrons has little effect on the magnetic hysteresis (Jc), at least to fluences of ~ 1.4× 1018 n / c m 2.

2. Experimental Two different BKBO samples, of essentially identical weight but synthesized by different routes, were used in this study. Briefly, sample ~1 was prepared by grinding, pelletizing and heating overnight at 700 K stoichiometric amounts of BaNO3 and Bi203. This mixture then was added to an appropriate amount of KNO3, pelletized, heated and annealed following the procedures described by Hinks et al. [6,7 ]. Further details are given in ref. [ 5 ]. X-ray diffraction showed the sample to be cubic ( I m 3 m ) with a room temperature lattice constant of 4.2869 A. Rietveld refinement of neutron diffraction data gave an average stoichiometry of Bao.s2Ko.42BiO3.o [ 8 ]. Sample #2 was prepared from stoichiometric amounts of BaBiO3, KO2 and Bi203. The powder was allowed to react at 725°C in a gold boat for two hours before cooling under Ar to 170 ° C. Pure, dry oxygen was introduced into the reaction chamber and the temperature ramped to 412 °C in two hours, cooled in four hours to 200 ° C and then cooled to room temperature. After regrinding, the powder was heated rapidly to 950°C (above its melting point) for one

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J.D. Thompson et al. / Fast neutron irradiation of Bao.6Ko.,Bi03

hour, before slowly cooling to 725 ° C. Further details of this sample preparation are given in ref. [9 ]. Xray diffraction and electron microprobe analysis of this sample indicated significant grain-center to grainedge compositional gradients, with stoichiometric ranges from Bao.TKo.aBiO3 to Bao.55Ko.4sBiO3. Simultaneous neutron irradiation of both samples was performed at a port of the Los Alamos Omega West reactor which has a fast ( E > 0.1 MeV) neutron flux of 4 × 1012 n / ( c m 2 s). Thermal neutron flux was attenuated by a factor of 104 by cadmium shields. The samples were shuttled in and out of the reactor in an aluminum rabbit over which helium gas was flowed during the irradiation. The sample temperature was monitored during irradiation and did not exceed ~ 80 °C. The magnetic properties of these samples were measured with a Quantum Design superconducting quantum interference device (SQUID) magnetometer. Most measurements were performed in a mode where the sample was moved from 3 cm below to 3 cm above the center of the superconducting solenoid (6 cm scan). In this measurement mode, the sample is subjected to a magnetic field that varies from approximately 0.985Ho-to-Ho-to-O.985Ho, where Ho is the field at the center of the magnet. The effect of a non-uniform magnetic field on the hysteresis measurement will be discussed in the next section.

3. Results Figure 1 compares the zero field cooled (ZFC) and field cooled (FC) or Meissner response of both samples measured in an applied field Ho = 20 Pc. In calculating -4~X, external dimension of the samples were used to estimate a demagnetizing factor which was 0.15 and 0.10 for samples #l and ~2, respectively. The estimated uncertainty in the absolute value of -4~X, which arises primarily from uncertainty in calculating a demagnetizing factor, is + 5%. (In this regard, magnetization versus field measurements at 5 K show a departure from lioearity at 280 + 40 Oe for both unirradiated samples when these demagnetizing factors were used. This field value agrees well with the lower critical field Hc~ determined [ 10 ] on single crystals of Bao.6Ko.4BiO3.) Although the ZFC values are similar for both samples

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at low temperatures, the Meissner response is notably less for sample #2, suggesting a smaller superconducting volume fraction, enhanced flux pinning or both in $2. Superconducting transition temperatures, Tc=26.85 for sample #l and 27.68 K for sample #2, were determined by linearly extrapolating through data taken every 0.5 K from 3 K below Tc to the temperature at which -4~X=0. For consistency, after each irradiation the samples were remounted in the magnetometer in as-close-as-possible to the same orientation used to acquire the data of fig. 1 so that the demagnetizing factor changed little from run to run and T~ was determined as described above. The effec~ of neutron irradiation on T~ is shgwn in fig. 2(a). After an initial sharp drop in T~, both samples show an approximately linear decrease in T¢ with fluence at rates of - 4 . 2 K / l 0 ~9n / c m 2 and - 4 . 5 K / l019 n / c m 2 for sample #1 and #2, respectively. These

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Fig. 2. (a) Superconducting onset transition temperature T, of two BKBO samples vs. fluence of fast neutrons. (b) T~ normalized by its value T~a before sample irradiation as a function of neutron fluenc*. rates compare to -- 27 K / 1 0 t9 n / c m 2 for YBa2Cu307 [ 11 ], - 2 1 K / 1 0 t9 n / c m 2 for LaLssSro.,sCuO4 [ 12] and about - 2 . 5 K/1019 n / c m 2 for A15 c o m p o u n d s [ 13 ]. Figure 2 (b) shows T, normalized to its unirradiated value as a function o f fluence for both samples. The relative rate o f depression for fluences greater than ~ l X 1 0 ~7 n / c m 2 is approximately - 0 . 1 5 % / 1 0 1 9 n / c m 2, which is comparable to A15 materials and about two times smaller than found in YBa2Cu307 or La,.ssSro.~sCuO4. During irradiation, neither the ZFC nor FC values o f - 4 x Z changed within experimental uncertainty. In a simple critical state model [ 14 ], the magnetization critical current density at a fixed field is proportional to the difference in magnetization A M for field increasing and decreasing and inversely proportional to the dimension R around which the critical current density flows, i.e. J¢ocAM/R. Figure 3 shows typical results for M versus H on the two BKBO samples at 5 K after irradiation to 1.4× 10 ts

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Fig. 3. (a) Magnetization Mnormalizcd per unit mass of sample as a function of applied magnetic field for which BKBO sample ~1, which has been neutron irradiated to a fluenc¢ of 1.4X 10ns n/em 2. Data have not been corrected for demagnetizing effects. See text for a discussion of the difference between 3 and 6 cm. (b) Magnetization normalized per unit mass of sample vs. applied magnetic field for BKBO sample ~2, which has been neutron irradiated to fluence of 1.4× 10nsn/crn2. Data have not been corrected for demagnetizing effects. n / c m 2. These data have not been corrected for demagnetizing effects. At the longer 6 cm scan lengths used to measure M, M versus H is nearly reversible for fields greater than ~ 15 kOe for sample ~¢1 and ~ 30 kOe for sample ~2. Around zero field, hysteresis in M is clearly larger for sample ~2, consistent with the notion that its smaller Meissner fraction is due to relatively larger flu x pinning than in sample ~¢1. (Enhanced pinning might arise frqr~ t~c known intragranular compositional gradient in this sample. ) This assumes, however, that the dimension R around which Jc flows is comparable in both samples. If the relevant current path is the external sam-

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J.D. Thompson et al. / Fast neutron irradiation of Bao.sKo.,BiOs

pie dimension perpendicular to the applied field, then this argument holds. However, this may not be the case in field cooling or at fields sufficiently large to break intergranular connections. In this case, the relevant dimension for calculating Jc is the average particle size. Analysis of scanning electron micrographs shows an average particle site of 19 + 8 p m and 6 + 3 pm for sample #l and #2, respectively. Thus, if these are the relevant current-path dimensions, sample #2 indeed does have a larger J~ than sample ~l. For the shorter 3 cm-scan length, in which the sample moves in a more homogeneous field, the qualitative difference between samples #l and #2 holds, although the hysteresis is considerably larger at high fields for both samples. This difference between the 6 cm- and 3 cm-scan measurements arises because the sample executes a minor hysteresis loop at each He as it is moved from a low field region to a higher field region and then back. For a 6 cm-scan, the sample experiences a field change of 1.4%; but at a 3 era-scan, the field change is only 0.05%. Therefore, the shorter scan more accurately reflects the intrinsic hysteresis. We note, though, that for increasing fields, data for both samples for both 3 cm- and 6 era-scan lengths coincide and it is only for decreasing fields that a difference between scan lengths is observed. (This is understandable in terms of the minor hysteresis loop made during measurements at the longer scan length. ) Because in a simple critical state model, the difference in magnetization AM for increasing and decreasing magnetic fields should be symmetrical about the equilibrium magnetization, a difference in flux pinning (J~) should be reflected equally in the field increasing 6 cm-scan data and 3 cm-scan data. Therefore, 6 era-scan results give qualitative changes in J~ with irradiation, although the magnitude of Jc will be underestimated. The effect of neutron irradiation on the hysteresis in M versus H curves is summarized in fig. 4 where AM, normalized per unit mass of sample, is plotted on a logarithmic scale, for various applied fields He, as a function of fluence. Contrary to what was found in an earlier study [ 1 ] on YBa2Cu307, measured also in the 6 cm-scan mode, in which AM increased by a factor of three or more with fluences to this level, we find no, or at most little, increase in Jc with neutron irradiation of BKBO. Accompanying the change in T~ with neutron flu-

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perature dependence of Z is unknown but possibly could arise from site disorder among K and Ba ions or to the creation of vacancies in BKBO. Further work is required to clarify this behavior. Whatever the radiation damage structure that is responsible for the depression of Tc and change in temperature dependence of Z, it apparently does not affect flux pinning.

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on cubic A15 compounds. In these later materials, superconductivity is believed to arise from strong electron-phonon coupling. There is mounting evidence [ 16-18 ] that this may be true for BKBO as well. Thus, moderate smearing of an already low density-of-states in BKBO by neutron irradiation may not be expected to suppress Tc as rapidly as in the cuprates in which the pairing mechanism is not clearly electron-phonon coupling alone. The insensitivity of AM to fast neutron irradiation is remarkable, and at present, we have no clear understanding of why this should be so. For flux lines to be pinned effectively by defects, the radial extent of the defect should be comparable to the normal-core radius of the flux line, which is the superconducting coherence length ~. From measurements of the upper critical field [15 ], a coherence length ~ = 6 0 - 8 0 / k has been deduced for BKBO. A crude estimate for the number of atoms displaced by a fluence of 1 . 4 X 10 Is n / c m 2 indicates an average damaged volume of ~ (25 atoms) 3 or roughly a 200/k diameter which is 2-3 times ~. Thus, if the damaged area has a substantially depressed superconducting order parameter relative to undamaged areas, at some fluence somewhat less than our maximum, optical pinning should have occurred. However, this is not reflected in AM versus fluence at any field value. Perhaps the pinning potential of polycrystalline BKBO is so weak that additional defects do not affect significantly the flux pinning. Clearly, additional work is called for to understand this unusual behavior.

Acknowledgements Work at Los Alamos was performed under the auspices of the U.S. Department of Energy, Office of Basic Energy Sciences. We thank M.P. Maley for interesting discussions and acknowledge the advice and assistance from staff and supervisors at the Los Alamos Omega West reactor.

4. Conclusions References

The effect of fast neutron irradiation on Tc of Bao.6Ko.4BiO3 is notably less severe than in the Y- or La-based cuprates but is comparable to observations

[ 1 ] J.R. Cost, J.O. Willis, J.D. T h o m p s o n and D.E. Peterson, Phys. Rev. B37 (1988) ! 563.

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J.D. Thompson et al. / Fast neutron irradiation o f Bao.6Ko.~Bi03

[2] A, Umezawa, G.W. Crabtree, J.Z. Liu, H.W. Weber, W.K. Kwok, LH. Nunez, TJ. Moran and C.H. Sowers, Phys. Rev. B36 (1987) 7151. [3] R.B. Van Dover, E.M. Gyorgy, L.F. Schneemeyer, J.W. Mitchell, ICV. Rao, R. Puzniak and J.V. Waszczak, Nature 342 (1989) 55. [4] See, for example, A.R; Sweedler, C.L. Snead, Jr. and D.E. Cox, in: Treatise on Materials Science and Technology, eds. J. Luhman and D. Dew-Hughes (Academic Press, New York, 1979) p. 349. [5]M.E. McHenry, M.P. Maley, G.H. Kwei and J.D. Thompson, Phys. Rev. B39 (1989) 7339. [6 ] D.G. Hinks, D. Dabrowski, J.D. Jorgensen, A.W. Mitchell, I).R. Richards, S. Pei and D. Shi, Nature 333 (1988) 836. [7 ] D.G. Hinks, D.R. Richards, B. Dabrowski, A.W. Mitchell, J,D. Jorgensen and D.T. Marx, Physica C 156 (1988) 477. [ 8 ] G.H. Kwei, J,A. Goldstone, A.C. Lawson, J.D. Thompson and A. Williams, Phys. Rev. B39 (1989) 7378. [9] ICC. Ott, M.F. Hundley, G.H. Kwei, M.P. Maley, M.E. McHenry, E.J. Peterson, J.D. Thompson and J.O. Willis, Mat. Res. Soc. Syrup. Proc. 156 (1989) 369.

[10]G.S. Grader, A.F. Hebard and L.F. Schneemeyer, unpublished. [ 11 ] J.O. Willis, J.R. Cost, R.D. Brown, J.D. Thompson and D.E. Peterson, Mat. Rcs. SOc. Syrup. Proc. 99 (1988) 391. [ 12 ] S.T. Sekula, D.IC Christen, H.R. Kerchner, J.R. Thompson, L.A. Boatner and B.C. Sales, Jpn. J. Appl. Phys. 26 (1987) 1185. [13] A.R. Seedler, D.E. Cox and S. Moehlecke, J. Nucl. Mater. 72 (1978) 50. [ 14] C.P. Bean, Phys. Rev. Lett. 8 (1962) 250. [ 15] B. Batlogg, R.J. Cava, L.W. Rupp, A.M. Mujsce, J.J. Krajewski, J.P. Remeika, W.F. Peck, A.S. Cooper and G.P. Espinosa, Phys. Rev. Lett. 61 (1988) 1670. [ 16 ] D.A. Papaconstantopoulos, A. Pasturel, J.P. Julien and F. Cryot-Lackman, Phys. Rev. B40 (1989) 8844. [ 17 ] M.F. Hundley, J.D. Thompson and G.H. Kwei, Solid State Commun. 70 (1989) 1155. [ 18] D.G. Hinks, D.R. Richards, B. Dabrowski, D.T. Marx and A.W. Mitchell, Nature 335 (1988) 419.