Precautions when interpreting EPR and dc magnetization measurements of high-T, RBa2Cu309 — -phase superconducting materials

Many electron-paramagnetic-resonance (EPR) and magnetic susceptibility measurements for the high-T, superconductors of the form RBa2Cu309 — „(R = Y or a rare earth) have been reported. Excluding local moment resonances due solely to the R atoms, we show that all of the EPR measurements known to us are not intrinsic to the superconducting phase, but rather are due to low concentrations of spurious phases typically present in the samples. BaCu02 is found to be the main source of the low-temperature EPR signal and of the magnetic susceptibility which has been reported for YBa2Cu309 — „above T, .


I. INTRODUCTION
The static magnetic properties of the high-T, ()90 K) superconductors are clearly of great interest, and numerous measurements of the dc magnetic susceptibility of YBa2Cu309 "have been reported, with various authors attributing an average local moment of -(0.1-0.5)lttt to the copper atoms. ' The dynamic magnetic properties are also of great interest and could, in principle, be determined via observation of electron-paramagnetic-resonance (EPR) of the carriers, or other local moments. Were such data available, they might be extremely helpful in clarifying the nature of the superconducting mechanism. The primary purpose of this note is to demonstrate why great care must be taken in interpreting both the magnetic susceptibility and EPR experiments, and to explain why it is unlikely that any of the EPR measurements reported so far ' for the usual granular-compressed-powder (GCP) samples of the form RBa2Cu309 "(R=Y or a rare earth), can be unambiguously related to the high-T, superconducting phase. The primary problem is that significant quantities of spurious phases (most notably Ba-Cu02) are readily present when the samples are made via the GCP method, and some of these phases have sufficiently strong magnetic properties to dominate over that of the high-T, superconducting phase material. (Htt ) for the LT line were found to be quite similar for all the rare-earth or Y hosts. ' An example of such data is presented in Fig. 1  The major phases present in the samples made only with BaCO3 and CuO powders were determined from xray powder diffraction measurements to be BaCu02, BaCO3, and CuO. Neither pure BaCO3 nor CuO were observed to exhibit an appreciable EPR signal at any temperature, which strongly suggests that the EPR signal observed in these samples arises from the BaCu02 phase.
By varying the BaCO3 to CuO starting ratios, we have prepared many samples which contain various amounts (10-93% mass fraction) of the BaCu02 phase. For these samples, as well as for numerous RBa2Cu309 "samples, we performed a careful, quantitative x-ray powder diffraction analysis, in combination with a determination of the LT EPR intensity per unit mass of sample. From the x-ray data, the amplitude of the most intense x-ray peak of each detectable phase was measured, and the mass fraction of each phase subsequently determined using standard procedures. " The mass absorption coefficients of each phase were taken into account, and numerous standards consisting of various known amounts of BaCuOz, BaCOs, CuO, and/or EuBa2Cu309 "were prepared for calibration purposes. This allowed the mass fraction of each phase to be determined to an accuracy of 10% or better, if the corresponding x-ray peak exhibited adequate signal-to-noise ratio. When necessary, large time constants and small angular sweep rates were used to improve the signal-to-noise ratio. The EPR intensities [taken as the amplitude times (hH~~) ] were determined at 6 K. Although we have taken into account the cavity Q factor, and normalized the EPR intensity per unit mass of sample, variations in sample size, shape, and position in the cavity limit the accuracy of our intensity measurements to -20% at best.
For the samples which were made from BaCO3 and CuO only, we find that the intensity of the observed EPR line is proportional to the amount of BaCu02 present. The intensity is large enough that a BaCu02 mass fraction of~3% would be sufficient to account for the intensity of the LT line observed in most of our RBa2Cu309 samples. We also found that a majority of our RBa2-Cu309 -"samples (including those doped with 3d or 4f impurities) contained a detectable amount of BaCu02, and that the amount of BaCu02 present correlated well with the measured LT amplitude. The results are summarized in Fig. 2 for various RBa2Cu309 superconducting samples, including a few doped with 3d impurities. Data points are shown for samples in which an appreciable amount of BaCu02 could be detected' and should not be taken as indicative of the amount of BaCu02 typically present for a particular rare-earth host R. The solid line in the figure indicates the expected dependence of the LT signal intensity on BaCu02 content, as deduced from a linear fit to the data of the samples made from BaCO3 and CuO only. The value of the slope of this linear fit has a standard deviation of~30%, which is mainly due to errors in determining the LT intensity. The data points also have measurement errors of -30%. Considering the various measurement errors involved, the data points fit this line quite well. The results presented in Figs. 1 and 2 strongly indicate that BaCu02 is responsible for the LT EPR signal observed by ourselves' and other investigators in the RBa2Cu309 superconductors. ' Further evidence supporting this conclusion is that, for all superconducting samples in which no BaCu02 was detected by x-ray diffraction, the LT signal intensity was weak enough that the amount of BaCu02 needed to produce this intensity would not have been detectable by x-ray diffraction. Also, for a few samples, no LT line was observed at all, nor was any BaCu02 detected.
The HT signal, when resolved, has g"=2.27, g~2 . 12, and g, =2. 05, and is most likely due to Cu + in a noncubic site. We suggest that the HT signal also cannot be unambiguously associated with the RBa2Cu309phase because of the following. (a) There are no changes in the HT linewidth or field for resonance which we can associate with T, . (b) The HT signal amplitudes increase with time for samples left at room temperature, sometimes increasing by as much as two orders of magnitude over a period of two weeks, but without any noticeable changes in either the x-ray spectra or T, . (c) We can find similar HT signals and time dependence in samples made with only the BaCO3 and CuO powders. (d) We find pure Mass Fraction BaCu02 (%) FIG. 2. Intensity of the LT EPR signal (normalized per unit mass of sample) observed for various RBa2Cu309samples or EuBa2Cu309 -"samples doped with Zn or Ni as a function of the mass fraction of BaCu02 present in the sample. The Yb data point is plotted with both the LT intensity (680 units) and mass fraction of BaCu02 (24.6%) reduced by a factor of 5 to facilitate the presentation of the data. The solid line indicates the expected dependence of the EPR intensity on BaCu02 content, as described in the text. Both the line and the data points contain measurement errors of -30%.

D. C. VIER etal.
Y28aCu05 (the so-called green phase) to have a very strong HT signal, such that the presence of & 1% of this phase would account for the HT signals observed in the YBa2Cu309 "superconducting samples. ' 15.0 III. dc MAGNETIC SUSCEPTIBILITY We have mentioned that measurements of the dc magnetic susceptibility X for YBa2Cu309 "have been reported, which, if interpreted as being associated with the Cu ions, would correspond to an average moment ranging from -0.1 to 0.5 pg per ion. ' Of course such measurements represent a sum of the contributions from all the sample constituents. Since we have found that there are always small quantities of spurious phases present in our GCP samples, and some of these give observable EPR signals, they must also contribute to the total dc magnetization. We have measured a sample containing a 93% mass fraction of BaCu02 (the remainder being BaCO3, which was determined to give a negligible contribution to L), and present a plot of 1/Z versus temperature (adjusted to 100% BaCu02) in Fig. 3. At high temperature we find an eA'ective magnetic moment of 1.72ps per Cu ion in agreement with other published values. ' ' As the temperature is lowered, the efT'ective moment increases, reaching a value of 3.16pq below 30 K. Thus, at high temperatures a mass fraction of BaCu02 ranging from 0.3% to 8% would explain the reported results. This is in agreement with similar qualitative conclusions reached by other groups 1 y2y5 IV. SUMMARY We have shown that the LT EPR line observed in GCP samples of the RBa2Cu309 superconductors is most likely due to the presence of BaCu02 and is not to be interpreted as intrinsic to the superconducting material.
We have also shown that the HT signal cannot be unambiguously attributed to the EBa2Cu309 "phase, a1though it is not clear what phase (or phases) gives rise to this signal. In addition, the dc magnetic susceptibility of BaCu02 can be large enough to dominate over any intrinsic paramagnetic contribution from RBa2Cu309 "(when R = Y or a nonmagnetic rare earth) in the normal state. RBa2Cu309-"oxide systems, and we caution that, even for future work utilizing single crystals, particular care must be taken to ensure that EPR and magnetic-