ESR of YBa2Cu3O7 single crystals : on the nature of the magnetic centres

2014 Previous ESR results from YBa2Cu3O7 powders are contradictory. We present results on YBa2Cu3O7 single crystals, obtained through systematic intensity measurements, a comparison of X and Q-band spectra, a comparison with the spectra from other related Y/Ba/Cu oxides, and a study of the influence of grinding the crystals. The ESR of conduction electrons was not detected. Although anisotropic, the ESR spectrum is not produced by YBa2Cu3O7, but by oriented inclusions of Y2BaCuO5. At 300 K the rf field in the crystal undergoes a moderate, isotropic attenuation which disappears after grinding. The thermal variation of the ESR intensity is quite different before and after grinding. Before grinding it strongly decreases below Tc, which reflects the expulsion of the rf field. The absence of ESR from the Cu2+ of the YBa2Cu3O7 phase could be due to the formation of exchange-pairs with a non magnetic ground state. J. Phys. Francé 50 (1989) 2357-2374 1er SEPTEMBRE 1989,