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 YBa2Cu3O7: I: Nd(Nd0.05Ba0.95)2Cu3O7, Tc0=72 K and II: Y0.2Ca0.8Sr2[Cu2(Tl0.5Pb0.5)]O7, Tc0=108 K and two are cases where AF ordering dominates the weak superconductivity: III: NbO1.1, 1.25Tc010 K and IV: La2NiO4.00, 70 K Tc040 K. At temperatures 298T64 K, the ESR absorption by I indicates orthorhombic symmetry. The peaks at gc=2.06, gb=2.13, and ga=2.24 are identified with the presence of 5% Nd3+(4I9/2) in the Ba layer because the characteristic Cu2+ impurity hyperfine structure is absent and the ESR signal disappears several degrees below Tc. Near Tc 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 B and the rate of sweep dBz/dt. ESR is observed below Tc because flux penetrates the superconductor. The temperature dependence of f leads to an activation energy for the flux motion Ea(I)/R16 K and Ea (III)/R3 K Tc/4. In the superconducting state a coherent flux expulsion response to a change in Bz from 500 mT to zero is observed in times r=8 to 10 s. The inverse rate of noise spikes due to flux expulsion, when the samples are cooled through Tc in a magnetic field, varies from noise=3.5 s for III to 21 s for IV. The microwave absorption spectra identify three temperature regimes: (i) For 3.5 K <T<TmT*<Tc, 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) TmT*TTc 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 Tc. © 1994 The American Physical Society.