Low-temperature specific heat ofBaCuO2andBaCuO2.14in magnetic fields to 7 T

Abstract

Listen

Translate article

The specific heats of samples of ${\mathrm{BaCuO}}_{2}$ and ${\mathrm{BaCuO}}_{2.14},$ on which magnetization and/or neutron-diffraction measurements had been made earlier, were measured for the temperature range $0.35<~T<~30\mathrm{K}$ in magnetic fields to 7 T. ${\mathrm{BaCuO}}_{2+x}$ has a complex structure with 90 formula units in a bcc unit cell; 6 Cu are ``lone spins,'' 48 are in 8 ${\mathrm{Cu}}_{6}{\mathrm{O}}_{12}$ ``ring clusters,'' and 36 are in 2 ${\mathrm{Cu}}_{18}{\mathrm{O}}_{24}$ ``sphere clusters.'' The ring and sphere clusters have ferromagnetically ordered ground states with spins $S=3$ and 9, respectively. Antiferromagnetic ordering of the ring clusters occurs with a N\'eel temperature ${T}_{N}(0)\ensuremath{\sim}15\mathrm{K}.$ The specific heat of ${\mathrm{BaCuO}}_{2}$ shows a cooperative ordering anomaly associated with the antiferromagnetic ordering of the ring clusters. Schottky-like anomalies, having maxima at \ensuremath{\sim}5 and \ensuremath{\sim}0.7 K, are identified with the ordering of the sphere clusters and the lone spins, respectively. Only Schottky-like anomalies are observed for the specific heat of ${\mathrm{BaCuO}}_{2.14}.$ It is suggested that the increase in the Cu oxidation state, due to the addition of 0.14 mol of O, increases Cu-O covalent bonding (spin compensation) and/or produces nonmagnetic ${\mathrm{Cu}}^{3+},$ which in addition to the known increase in the Cu-O bond lengths, disrupts the superexchange paths that lead to the antiferromagnetic ordering of the ring clusters in ${\mathrm{BaCuO}}_{2}.$ For ${\mathrm{BaCuO}}_{2}$ the magnetic entropy was 90% of that predicted for the ordering of the three Cu structures. On the other hand, the magnetic entropy for ${\mathrm{BaCuO}}_{2.14}$ was only 65% of that predicted, which suggests a relative large suppression of some magnetic entities due to the addition of 0.14 mol of O. Although ${\mathrm{BaCuO}}_{2+x}$ is an insulator, the specific heat has a T-proportional component that is magnetic field dependent and is presumably associated with the magnetic degrees of freedom.