Cu(2) nuclear resonance evidence for a magnetic phase in aged 60-K superconductorsRBa2Cu3

A. V. Dooglav

¹

,

H. Alloul

²

,

O. N. Bakharev

³

et al.

Abstract

It is widely believed that the long-range antiferromagnetic order in the RBa2Cu3O6+x compounds (R=Y and rare earths except of Ce, Pr, Tb) is totally suppressed for the oxygen index x ≥ 0.4 (AFM insulator-metal transition). We present the results of the copper NQR/NMR studies of aged RBa2Cu3O6+x (R=Tm,Y) samples showing that a magnetic order can still be present at oxygen contents x up to at least 0.7 and at temperatures as high as 77K. I. INTRODUCTIONAn enormous amount of papers, both experimental and theoretical, are devoted to the NMR and NQR studies of copper in superconducting YBa 2 Cu 3 O 6+x layered cuprates. It is considered well known that in non-superconducting materials (x < 0.4) nuclei of copper located in the CuO 2 planes experience the influence of a strong internal magnetic field H int ≈ 80 kOe which is perpendicular to the c− axis, and that in superconductors (x > 0.4) such field is absent and 63,65 Cu(2) nuclei with the spin I = 3/2 give typical NQR spectra at frequencies of ν Q ≈ 25 ÷ 32 MHz, i.e., in the same frequency range as that for NQR spectra of two-fold coordinated "chain" copper 63,65 Cu(1) 1 . While studying NMR of thulium in oxygen-deficient TmBa 2 Cu 3 O 6+x compounds 2 we have discovered the wide and subtle absorption line ( Fig.1) which looked like copper NMR line but could not have been attributed to the mentioned Cu(2) and Cu(1) centers with ν Q ≈ 25 ÷ 32 MHz and H int = 0. We consequently undertook experiments with TmBa 2 Cu 3 O 6+x samples (x = 0.51, 0.6, 0.7) in zero external field at T=4.2K, which allowed us to observe a Cu(2) ZFNMR spectrum completely different from those described in the literature. The results of these experiments are presented in this paper. The analysis of Cu(2) spectra of both types corresponding to H int = 0 (type I, ν ZF N MR = 55 − 135 MHz) and H int = 0 (type II, ν Q ≈ 25 ÷ 32 MHz) shows that each of them belongs to two varieties of Cu(2) centers, A and B, possessing different NQR frequencies and that A I − and B I −centers differ much in the value of H int . Similar results are obtained for one YBa 2 Cu 3 O 6.66 sample 3 . II. EXPERIMENTALThe TmBa 2 Cu 3 O 6+x samples used in these experiments were c-axis-oriented powders mixed with paraffin (hereafter denoted as Tm6+x). They were previously used for Tm NMR studies 2 . All of them were stored at room temperature for almost 6 years after preparation. Critical temperatures (T onset c =53K, 61K and 64K for x = 0.51, 0.6 and 0.7, respectively) were obtained from measurements of the diamagnetic susceptibility at a frequency of 1 kHz. Home-built spin-echo NQR spectrometers were used for the Cu(2) NQR and ZFNMR measurements. Examples of the copper NQR spectra in Tm6.6 are shown in Fig.2. Comparing the non-saturated (NS) and saturated (S) spectra (Figs.2a,b) taken at 4.2K with pulse sequence repetition rates of 1 and 200 Hz, respectively, one can distinguish two contributions with different spin-lattice relaxation times T 1 (Fig.2c) and separate them (Figs.2d,e) by using a simple subtraction procedure. Tw...

Antiferromagnetism in hydrated 123 compounds

“…Some of these compounds are antiferromagnets having either low Néel temperature (15-30 K) or ZFNMR spectra very different from the one we observed in Ref. [1]. More recent studies (see Ref.…”

“…Some of these compounds are antiferromagnets having either low Néel temperature (15-30 K) or ZFNMR spectra very different from the one we observed in Ref. [1]. More recent studies (see Ref.[3] and references therein) using high-resolution electron microscopy and X-Rays have shown that the 123 superconductors react with water vapour via a topotactic mechanism.…”

“…Thus, further attempts were undertaken at somewhat higher temperatures. Finally we managed to get a small amount of magnetic phase exhibiting the characteristic ZFNMR spectra [1]. However, the mass of the samples during the ambient air annealing had increased, and controlled annealing of dry powder sealed in an ampoule showed neither the mass changes nor the appearance of the magnetic phase.…”

“…Thus, further attempts were undertaken at somewhat higher temperatures. Finally we managed to get a small amount of magnetic phase exhibiting the characteristic ZFNMR spectra [1]. However, the mass of the samples during the ambient air annealing had increased, and controlled annealing of dry powder sealed in an ampoule showed neither the mass changes nor the appearance of the magnetic phase.The most natural explanation of the mass increase is the occurrence of a chemical reaction of YBCO with components of the air, in particular, with water vapour, since the significant reactivity of 123 with water is well established.…”

Antiferromagnetic properties of a water-vapor-insertedYBa2Cu3O6.5compound studied by NMR, NQR, andDooglav1, Egorov2, Mukhamedshin3 et al. 2004Phys. Rev. BSelf Cite17120View full textAdd to dashboardCite

“…We had undertaken systematic Cu NMR experiments in zero field (ZFNMR) on this compound in which we had found sizable NMR signals in an internal field, indicating the existence of an antiferromagnetic (AF) order. 3 The spectroscopic features of these signals were totally different from those observed in the AF parent compound YBa 2 Cu 3 O 6 and had to be associated with a different magnetic phase. However, these signals were found to occur mostly in samples aged for a long time in air and appeared to result from a reaction with air moisture.…”