DYNAMIC PHENOMENA IN SUPERCONDUCTING OXIDES MEASURED BY ESR

Dynamic electron spin resonance (ESR) measurements compare the paramag netic and antiferromagnetic (AF) properties of superconducting oxides in the range 4 K to room temperature, at 8 MHz and 9.36 GHz. Two are d erivatives of YBa2CU3O7: I: Nd(Nd0.05Ba0.95)(2)CU3O7, T-c0=72 K and II : Y0.2Ca0.8Sr2[Cu-2(Tl0.5Pb0.5)]O-7, T-c0=108 K and two are cases wher e AF ordering dominates the weak superconductivity: III: NbO1.1, 1.25 less than or equal to T(c0)less than or equal to 10 K and IV: La2NiO4. 00, 70 K greater than or equal to T(c0)greater than or equal to 40 K. At temperatures 298 greater than or equal to T greater than or equal t o 64 K, the ESR absorption by I indicates orthorhombic symmetry. The p eaks at g(c)=2.06, g(b)=2.13, and g(a)=2.24 are identified with the pr esence of 5% Nd3+(I-4(9/2)) in the Ba layer because the characteristic CU2+ impurity hyperfine structure is absent and the ESR signal disapp ears several degrees below T-c. Near T-c the ESR absorption is reduced by two orders of magnitude. Proximity effects give rise to interferen ce fringes with period tau(f)(T) independent of the field B and the ra te of sweep dB(z)/dt. ESR is observed below T-c because flux penetrate s the superconductor. The temperature dependence of tau(f) leads to an activation energy for the flux motion E(a) (I)/R less than or equal t o 16 K and E(a) (III)/R congruent to 3 K similar or equal to T-c/4. In the superconducting state a coherent flux expulsion response to a cha nge in B-z from 500 mT to zero is observed in times tau(r)=8 to 10 s. The inverse rate of noise spikes due to flux expulsion, when the sampl es are cooled through T-c in a magnetic held, varies from tau(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<T-m congruent to T<T-c, superconducting behavior was confirmed by the energy loss near zero ma gnetic field and the kinetics of high-field noise due to flux expulsio n. Near g=2.00 ESR absorption is observed for all materials. A broad a bsorption near 50 to 100 mT at 9.36 GHz has been attributed to AF reso nance. (ii) T-m congruent to Tless than or equal to T less than or eq ual to T-c identifies the range where flux motion gives rise to interf erence fringes in the ESR absorption. (iii) ESR and AF resonance are o bserved immediately after warming above T-c.