We have carried out 63,65 Cu NMR spectra measurements in magnetic field up to about 45 T on single crystal of a multiferroic triangular antiferromagnet CuCrO2. The measurements were performed for magnetic fields aligned along the crystal c-axis. Field and temperature evolution of the spectral shape demonstrates a number of phase transitions. It was found that the 3D magnetic ordering takes place in the low field range (H 15 T). At higher fields magnetic structures form within individual triangular planes whereas the spin directions of the magnetic ions from neighboring planes are not correlated. It is established that the 2D-3D transition is hysteretic in field and temperature. Lineshape analysis reveals several possible magnetic structures existing within individual planes for different phases of CuCrO2. Within certain regions on the magnetic H-T phase diagram of CuCrO2 a 3D magnetic ordering with tensor order parameter is expected.
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...
We have carried out 63,65 Cu NMR spectra measurements in a magnetic field up to about 15.5 T on a single crystal of the multiferroic triangular lattice antiferromagnet CuCrO 2 . The measure ments were performed for perpendicular and parallel orientations of the magnetic field with respect to the c axis of the crystal, and the detailed angle dependence of the spectra on the magnetic field direction in the ab plane was studied. The shape of the spectra can be well described in the model of spiral spin structure proposed by recent neutron diffraction experiments. When the field is rotated perpendicular to the crystal c axis, we observed, directly for the first time, a remarkable reorientation of the spin plane simultaneous with rotation of the incommensurate wavevector, by quantitatively deducing the conver sion of the energetically less favorable domain to a more favorable one. At high enough fields parallel to the c axis, the data are consistent with either a field induced commensurate spiral magnetic structure or an incommensurate spiral magnetic structure with a disorder in the c direction, suggesting that high fields may have influence on interplanar ordering.
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