Dynamic correlated Cu(2) magnetic moments in superconducting ${\text{YBa}}_2{\left({\text{Cu}}_{0.96}{\text{Co}}_{0.04}\right)}_3{\text{O}}_y\left(y\sim 7\right)$

We have examined the magnetic properties of superconducting ${\text{YBa}}_2{\left({\text{Cu}}_{0.96}{\text{Co}}_{0.04}\right)}_3{\text{O}}_y\left(y\sim 7,{\text{T}}_{sc}=65\text{K}\right)$ using elastic neutron scattering and muon spin relaxation $\left(\mu \text{SR}\right)$ on single-crystal samples. The elastic-neutron-scattering measurements evidence magnetic reflections, which correspond to a commensurate antiferromagnetic Cu(2) magnetic structure with an associated Néel temperature $T_{\text{N}}\sim 400\text{K}$. This magnetically correlated state is not evidenced by the $\mu \text{SR}$ measurements. We suggest that this apparent anomaly arises because the magnetically correlated state is dynamic in nature. It fluctuates with rates that are low enough for it to appear static on the time scale of the elastic-neutron-scattering measurements, whereas on the time scale of the $\mu \text{SR}$ measurements, at least down to $\sim 50\text{K}$, it fluctuates too fast to be detected. The different results confirm the conclusions reached from work on equivalent polycrystalline compounds: the evidenced fluctuating, correlated Cu(2) moments coexist at an atomic level with superconductivity.

Figure 1

From elastic-neutron-scattering measurements on single-crystal superconducting ${\text{YBa}}_2{\left({\text{Cu}}_{0.96}{\text{Co}}_{0.04}\right)}_3{\text{O}}_7\left(\text{y}\sim 7\right)$ mixed with ${\text{Y}}_2{\text{BaCuO}}_5$: temperature dependence of the scattering intensity at $\mathbf{Q}=\left(0.5,0.5,-2\right)$, which displays a re-entrant behavior below $\sim 90\text{K}$ and at $\mathbf{Q}=\left(0.5,0.5,-1.5\right)$, where the intensity grows below $\sim 90\text{K}$. The Néel temperature, $T_{\text{N}}\sim 400\text{K}$, is obtained by extrapolation as described in the text.

Figure 2

(Color online) $\mu \text{SR}$ spectra for single crystal ${\text{YBa}}_2{\left({\text{Cu}}_{0.96}{\text{Co}}_{0.04}\right)}_3{\text{O}}_y\left(\text{y}\sim 7\right)$ mixed with of ${\text{Y}}_2{\text{BaCuO}}_5$ at temperatures above the magnetic ordering temperature of ${\text{Y}}_2{\text{BaCuO}}_5$. Despite the fact that neutron-scattering measurements on this sample show the Cu(2) in ${\text{YBa}}_2{\left({\text{Cu}}_{0.96}{\text{Co}}_{0.04}\right)}_3{\text{O}}_7$ give rise to magnetic reflections, which correspond to a commensurate antiferromagnetic structure with an associated Néel temperature above room temperature, the presence of this correlated state does not markedly influence the depolarization (there is no reduction in the initial asymmetry and there are no oscillations).