PHYSICAL REVIEW B VOLUME 36, NUMBER 7 1 SEPTEMBER 1987 Magnetism and critical fields in the high-T, superconductors YBa2Cu307 — S (x 0, 1): An ESR study Carlos Rettori, Dan Davidov, Igal Belaish, and Israel Feiner Racah Insititute of Physics, Hebrew University of Jerusalem, Jerusalem, Israel (Received 16 June 1987) We have used an ESR spectrometer to study magnetism and superconductivity in YBa2Cu307- S„(x 0, 1). All samples exhibited an ESR signal. All the superconducting sam- ples showed a low-field signal due to transition from the Meissner state to the mixed state. We demonstrated a reverse relation between magnetism (ESR intensity) and superconductivity. The temperature dependence of the critical field H, 1 is anomalous. Since the initial discovery of high-temperature super- conductivity in oxides ' extensive research has been carried out in attempt to understand the mechanism of supercon- ductivity and to find new superconductors. ' Recently, Feiner, Yeshurun, and Nowik have discovered that YBa2Cu30sS and YBa2Cu307 have the same supercon- ducting transition temperature, but the former exhibits a significantly larger Meissner effect (see inset of Fig. 1). This remarkable feature is not clearly understood at present. This Rapid Communication reports on magnetic and superconducting properties of the high-T, supercon- ductors YBa2Cu307-„S„(x 0, 1). Using an ESR spec- trometer we have obtained valuable information on (a) the properties of the unpaired spins which contribute to the ESR line, (b) the lower critical field H, t, and (c) the superconducting transition temperatures. This allows a better identification of the unpaired spins and their effect on superconductivity. We believe that our results clarify somewhat the origin of the large Meissner eftect for YBa2Cu306S. Measurements on four different samples [to be denoted by YBa2Cu307(I), YBa2Cu307(II), YBa2Cu307- (quenched), and YBa2Cu30sS] are reported here. The method of preparation was described previously. It is known that slow cooling of the melt down to room temper- ature stabilizes a superconducting orthorhombic phase while fast cooling and quenching leads to a nonsupercon- ducting state. Consequently, YBa2Cu307(I) and YBa2Cu307(II) were prepared by the slow-cooling pro- cedure using different batches of starting materials. Both compounds exhibit superconductivity at T, =- 89 K. YBa2Cu307 (quenched) was prepared by fast cooling and quenching [using the same batch as for YBa2Cu307(II)l and no evidence for superconductivity was observed down to T 8 K. YBa2CusOsS (in which an oxygen is replaced by a sulfur atom) was prepared by the slow-cooling pro- cedure. This sample is superconducting at T, =-89 K (Ref. 8) and exhibits over 80% Meissner effect, as com- pared to about 25% Meissner effect in other materials (see the results of Feiner, Yeshurun, and Nowik in the inset of Fig. 1). All the compounds exhibit a single phase. The superconducting material has an orthorhombic phase with lattice parameters of a 3.822 4, b 3.891 A, and c 11 67 A for YBa2Cus07 (I), and a 3 855 A. , b 3. 914 A, and c 11. 75 A for YBa2Cu30sS. The measurements (including transition temperatures, critical fields, and ESR spectra) were conducted using a standard E-line spectrometer at X band frequency and TEpi i cavity. The temperature was regulated and con- trolled by an "Air Product" helium flux system and mea- T=12. 7 K YBaz Cu&07 (Quenched) x& LLI LLI Cl CD CL 4J Cf& (0 LLJ Z Cl 0 40 w(K) I 500 1000 M AGNETIC FIELD (GAUSS ) 80 I 1500 FIG. 1. Derivatives of the "superconducting" signal vs mag- netic field. This signal is due to change in the diamagnetic sus- ceptibility upon transition from the Meissner state to the mixed state. The minimum yields H, &. Inset: magnetism M (in emu jg) vs temperature measured using a SQUID magnetometer and an external field of H 2 6 (Ref. 5). Note that the di- amagnetic susceptibility for YBa2Cu306S is significantly larger than that of YBa2Cu307(I) and roughly scales with the intensity of the superconducting signals. Inset was taken from Feiner, Yeshurun, and Nowik (Ref. 5). 4028