Comparison of the ESR spectra in ceramic YBa₂Cu ₃O_7-y (1>y>0) and related phases

The authors report ESR measurements on YBa₂Cu₃O _7-y(0<y<1), the 123 phase and related phases of the Y-Ba-Cu oxide system-Y₂BaCuO₅ (the green phase), Y ₂Cu₂O₅ (the blue phase) and BaCuO₂ (the dark phase)-over a temperature range between 5 and 300 K. The ESR observed in the different phases are due to Cu ions. The temperature behaviour of the linewidths, Delta H, of the green and blue phases is in accordance with the magnetic phase transition. In the dark phase they observe a very large variation of the linewidth with temperature, d( Delta H)/dT=25 G K^-1. The intensity of the 123 spectra varied by more than two orders of magnitude for different samples, indicating that its origin is due to impurity phases. Its lineshape, down to 80 K, resembles that of the green phase. On further cooling (below 60 K) the linewidth passes through a maximum at T_m, where T_m depends on y. Its signal intensity, as determined from the peak-to-peak intensity and linewidth, shows an exponential decrease with temperature. The authors show that the LT ESR line in the 123 samples originates from the interplay of the green and the dark impurity phases, and they present evidence of coupling between spins of these inclusions which is mediated by the conduction electrons of the 123 matrix. They also offer a qualitative explanation for the increase of the signal intensity of the LT line, and speculate about the rapid linear temperature dependence of the linewidth of the dark-phase ESR line.