Reversible and irreversible magnetization in the high-T_{c ...

PHYSICAL REVIEW B VOLUME 39, NUMBER 16 1 JUNE 1989Reversible and irreversible magnetization in the high-T, superconductor Tl2ca2Ba2Cu301pY. Wolfus and Y. YeshurunDepartment of Physics, Bar Ilan-University, Ramat Gan -52100, IsraelI. FeinerRacah Institute of Physics, The Hebrew University of Jerusalem, Jerusalem 91904, Israel(Received 25 January 1989)dc magnetic measurements on the high-temperature superconductor TlqCa28aiCu30io (T, =113K), are reported. The zero-field-cooled and field-cooled magnetization curves coincide above atemperature T;„which scales with the applied field as 0' . The magnetization curves in the irreversibleregion scales; the scaling field is a simple function of geometrical factors and criticalcurrents. The magnetization above T, is not linear with the field, suggesting magnetic correlationsin the Cu02 layers in the normal state.SuperconductingI. , INTRODUCTIONoxides such as Tl-Cu-Ba-Cu-0 exhibita variety of interesting features which depend on theircomposition. ' Electrical resistivity and magnetizationstudies of polycrystalline samples of T12CazBa2Cu30io(2:2:2:3)have already been published;3 however, to thebest of our knowledge, little work has been reported todate on the reversibility and irreversibility properties ofthese systems. From a technical standpoint, these propertiershave great significance in high-T, compounds. Thetemperature, T;„, which is defined as the temperaturewhich separates reversible and irreversible regions, ismagnetic-field dependent and scales with the applied fieldas 0". The exponent n varies considerably in high-T, systems.In the present paper we present a detailed study of thereversible and irreversible properties of the new 2:2:2:3system with T, =113K. %'e find in general that the qualitativemagnetic behavior of this system is very similar tothat found in other high-T, superconductors. The irreversibilityline is pushed to relatively low temperaturescompared to, for example, Y-Ba-Cu-O. An interestingand intriguing feature of the present data is that all magnetizationcurves scale below T;„. The Meissner fraction,that is, the field-cooled moment normalized by — I/4tr,depends strongly on the field applied during the experiment.Even in the low-field limit its value is relativelysmall compared to other high-T, systems, which meansthat most of the fiux is trapped during the cooling procedure.Above T, the magnetic moment is not linear withthe applied field; the nature of this behavior will be discussed.II. EXPERIMENTAL DETAILSThe sample of nominal composition T12Ca28a2Cu30)0(2:2:2:3 phase) was prepared by thoroughly mixing andgrinding appropriate amounts of T1203, Ba02, CaO, andCuO, pressed into a pellet and put into an alumina cruciblein an oven preheated to 890 C. There it was reactedfor 10 min in flowing oxygen and then furnace cooled toroom temperature. X-ray-powder diffractomemetry revealedthe presence of two phases, the majority phase(more than 90%, the 2:2:2:3 phase) and the minorityphase (less than 10%) T12CaBazCu20s (2:1:2:2phase).The lattice parameters for the 2:2:2:3 phase area 3.827(2) A and c -36.18(2) A in complete agreementwith Ref. 2. The dc susceptibility measurements on asolid ceramic piece were carried out in a commercialSHESQUID magnetometer in various fields 3.5 Oe (H (40kOe as a function of temperature in the range of 5-180 K.The magnetization was measured by two diff'erent procedures:(a) The sample was zero-field cooled (ZFC) to 5K, a field H was applied, and the magnetization of theshielding branch was measured as a function of temperature.(b) The sample was field cooled (FC) from above T,in a field H and the Meissner branch was measured.From magnetic measurements we have clear indicationsthat the state of our Tl material is metastable on a timescale of months at room temperature. It is also possiblethat low-temperature thermal cycling during the variousmeasurements affects the composition stability. The measurementsreported here have been performed on a freshsample (within one week of preparation).~ -o.oaC4-0.06OQ -0.1S-O.ia10I I I50 70TEMPERATURE (K)90 110FIG. 1. ZFC and FC magnetic moment at 3.5 Oe for the2:2:2:3phase.39 11 690 1989 The American Physical Society

39 REVERSIBLE AND IRREVERSIBLE MAGNETIZATION IN THE. . . 11 6930Q 1 JI4.5-0.2 —,-0.8COg -04-0.5-Q.e0X-07 — +X-0.8— O0-0.9—h0TEMP(K)v+ RQO $0+ 400 5Q8070OICD43.510H/HmaxFIG. 7. Scaling of several magnetic curves at diA'erent temperatures(see text).3 1101 I l130 150TEMPERATURE (K)1170FIG. 8. Temperature dependence of magnetic susceptibilitymeasured at various fields above T,. In the inset: The variationof the susceptibility with the applied field is shown.where r 10 cm is the average radius of a grain; thedensity is 3 g/cm . J, has an error of 20% due to the uncertaintyof r.Qualitatively, very similar curves to Fig. 6 are obtainedat various temperatures, allowing us to scale all the magnetizationcurves using a simple procedure. %'e scale thefield axis with H, „, the field for which M reaches itsmaximum value M,„; similarly, we scale the M axis withMm, „. The result of this scaling for several temperaturesis exhibited in Fig. 7. Perfect scaling is obtained for fieldsand temperatures in the irreversible regions. Clear deviationsfrom scaling are observed for fields and temperaturesin the reversible range above the irreversibility line of Fig.5. A complete description of this phenomenon will bepublished elsewhere.E. Nonlinear magnetization above T,The temperature dependence of the normal-state magneticsusceptibility (M/H) for various fields is shown inFig. 8. It is apparent from this figure that the magnetizationdoes not vary linearly with H. The field dependenceof the susceptibility at 170 K is given in the inset. Thenonlinear behavior exists up to 40 kOe, the highest fieldused. M/H depends strongly on temperature in contrastto that of single-phase Y-Ba-Cu-O. '6 Each of the curvesin Fig. 8 can be fitted by a Curie-Weiss law plus a constantterm, i.e., M/H(T) go+ C/(T — 6). For example,for H 1 and 2.5 kOe, go 2.3x10 and 4.7x10emu/mole, C 0.59 and 0.63 emu/mole, and 8 11 and— 16 K, respectively. In general, go depends strongly onthe applied field. It decreases by more than an order ofmagnitude over the magnetic-field regime of the presentstudy, while the Curie constant increases slightly withfield. Similar field effects were also observed in Y-Ba-Cu-O. s From the above values of the Curie constants wederive P,ff 1.26pg (Cu atom) which corresponds to afraction of 73% spin- 2 local moments of the Cu + ions(1.71ps). It means that the Curie-Weiss contribution tothe susceptibility is intrinsic to the (2:2:2:3)single phase,and does not arise from impurity phases, since such alarge concentration of an impurity phase would be easilyvisible in the x-ray-diffraction pattern.It would be edifying to discover the source of the nonlinearmagnetization behavior exhibited in Fig. 8. The existenceof ferromagnetic impurities is excluded for the followingreasons: (i) The nonlinear behavior continues atleast up to 40 kOe (not shown in Fig. 8) and to relativelyhigh temperatures -300 K. No existing ferromagneticco~pound with such a high Curie temperature which isnot saturated at low temperatures at 40 kOe. (ii) To thebest of our knowledge, there is no compound containingsome of all the elements that constitute the 2:2:2:3phasewhich is ferromagnetically ordered at about 300 K. We'may assume that as in La2Cu04, magnetic correlationsin the Cu02 layers persist in the compound well above T,;their nature is now being studied.ACKNO%"LEDGMENTSThis research is supported by the German-Israel Foundationfor Scientific Research and Development GrantNo. I-40-100.10/87.'S. S. Parkin, V. Y. Lee, E. M. Engler, A. I. Nazzal, T. C.Huang, G. Gorman, R. Savoy, and R. Beyers, Phys. Rev.Lett. 60, 2539 (1988),2S. S. Parkin, V. Y. Lee, A. I. Nazzal, R. Savoy, T. C. Huang,G. Gorman, and R. Beyers, Phys. Rev. 38, 6531 (1988).W. Reith, P. Muller, C. Allgeier, R. Hoben, J. Heise, J. S. Shilling,and K. Andres, Physica C 156, 319 (1988).4M. M. Fang, J. E. Ostenson, J. K. Finnemore, D. E. Farrell,and N. P. Bansal, Phys. Rev. B 39, 222 (1989).5K. A. Muller, M. Takashige, and J. G. Bednorz, Phys. Rev.