Dynamic phenomena in superconducting oxides measured by ESR (1994)

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 ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$: I: Nd(${\mathrm{Nd}}_{0.05}$${\mathrm{Ba}}_{0.95}$${)}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$, ${\mathit{T}}_{\mathit{c}0}$=72 K and II: ${\mathrm{Y}}_{0.2}$${\mathrm{Ca}}_{0.8}$${\mathrm{Sr}}_{2}$[${\mathrm{Cu}}_{2}$(${\mathrm{Tl}}_{0.5}$${\mathrm{Pb}}_{0.5}$)]${\mathrm{O}}_{7}$, ${\mathit{T}}_{\mathit{c}0}$=108 K and two are cases where AF ordering dominates the weak superconductivity: III: ${\mathrm{NbO}}_{1.1}$, 1.25\ensuremath{\le}${\mathit{T}}_{\mathit{c}0}$\ensuremath{\le}10 K and IV: ${\mathrm{La}}_{2}$${\mathrm{NiO}}_{4.00}$, 70 K \ensuremath{\ge}${\mathit{T}}_{\mathit{c}0}$\ensuremath{\ge}40 K. At temperatures 298\ensuremath{\ge}T\ensuremath{\ge}64 K, the ESR absorption by I indicates orthorhombic symmetry. The peaks at ${\mathit{g}}_{\mathit{c}}$=2.06, ${\mathit{g}}_{\mathit{b}}$=2.13, and ${\mathit{g}}_{\mathit{a}}$=2.24 are identified with the presence of 5% ${\mathrm{Nd}}^{3+}$${(}^{4}$${\mathit{I}}_{9/2}$) in the Ba layer because the characteristic ${\mathrm{Cu}}^{2+}$ impurity hyperfine structure is absent and the ESR signal disappears several degrees below ${\mathit{T}}_{\mathit{c}}$. Near ${\mathit{T}}_{\mathit{c}}$ the ESR absorption is reduced by two orders of magnitude.Proximity effects give rise to interference fringes with period ${\mathrm{\ensuremath{\tau}}}_{\mathit{f}}$(T) independent of the field B and the rate of sweep ${\mathit{dB}}_{\mathit{z}}$/dt. ESR is observed below ${\mathit{T}}_{\mathit{c}}$ because flux penetrates the superconductor. The temperature dependence of ${\mathrm{\ensuremath{\tau}}}_{\mathit{f}}$ leads to an activation energy for the flux motion ${\mathit{E}}_{\mathit{a}}$(I)/R\ensuremath{\le}16 K and ${\mathit{E}}_{\mathit{a}}$ (III)/R\ensuremath{\simeq}3 K \ensuremath{\simeq}${\mathit{T}}_{\mathit{c}}$/4. In the superconducting state a coherent flux expulsion response to a change in ${\mathit{B}}_{\mathit{z}}$ from 500 mT to zero is observed in times ${\mathrm{\ensuremath{\tau}}}_{\mathit{r}}$=8 to 10 s. The inverse rate of noise spikes due to flux expulsion, when the samples are cooled through ${\mathit{T}}_{\mathit{c}}$ in a magnetic field, varies from ${\mathrm{\ensuremath{\tau}}}_{\mathrm{noise}}$=3.5 s for III to 21 s for IV. The microwave absorption spectra identify three temperature regimes: (i) For 3.5 K T${\mathit{T}}_{\mathit{m}}$\ensuremath{\simeq}${\mathit{T}}^{\mathrm{*}}$${\mathit{T}}_{\mathit{c}}$, 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) ${\mathit{T}}_{\mathit{m}}$\ensuremath{\simeq}${\mathit{T}}^{\mathrm{*}}$\ensuremath{\le}T\ensuremath{\le}${\mathit{T}}_{\mathit{c}}$ 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 ${\mathit{T}}_{\mathit{c}}$.