Dynamic phenomena in superconducting oxides measured by ESR.

Dynamic electron spin resonance (ESR) measurements compare the paramagnetic and antiferromagnetic {AF) properties of superconducting oxides in the range 4 K to room temperature, at 8 MHz and 9.36 GHz. Two are derivatives of YBa2Cu307. I: Nd(Nd005Ba095)QCu307, T,0=72 K and II: Y0 zCaaasr2[Cu2(T10 &P10,)]07, T,&&=108 K and two are cases where AF ordering dominates the weak superconductivity: III: NbOi & 1.25 T,0 10 K and IU: La2Ni0400, 70 K & T,0~40 K. At temperatures 298~ T~64 K, the ESR absorption by I indicates orthorhombic symmetry. The peaks at g, =2.06, gb =2. 13, and g, =2.24 are identified with the presence of 5% Nd +( I9&2) in the Ba layer because the characteristic Cu + impurity hyperfine structure is absent and the ESR signal disappears several degrees below T, . Near T, the ESR absorption is reduced by two orders of magnitude. Proximity effects give rise to interference fringes with period ~f(T) independent of the field 8 and the rate of sweep dB, /dt. ESR is observed below T, because flux penetrates the superconductor. The temperature dependence of ~f leads to an activation energy for the flux motion E, {I )/R ~ 16 K and E, (III)/R = 3 K = T, /4. In the superconducting state a coherent flux expulsion response to a change in B, from 500 mT to zero is observed in times ~, =8 to 10 s. The inverse rate of noise spikes due to flux expulsion, when the samples are cooled through T, in a magnetic field, varies from ~„„„= 3.5 s for III to 21 s for IV. The microwave absorption spectra identify three temperature regimes: {i) For 3.5 K & T & T =- T & T„superconducting behavior was confirmed by the energy loss near zero magnetic field and the kinetics of high-field noise due to flux expulsion. Near g =2.00 ESR absorption is observed for all materials. A broad absorption near 50 to 100 mT at 9.36 GHz has been attributed to AF resonance. (ii) T =- T* ~ T ~ T, identifies the range where flux motion gives rise to interference fringes in the ESR absorption. (iii) ESR and AF resonance are observed immediately after warming above T, .