Phase stability and low-temperature specific heat up to 14 T of BaCuOx as a function of oxygen stoichiometry.

Abstract

The stability of the ${\mathrm{BaCuO}}_{\mathit{x}}$ (x\ensuremath{\ge}2) phase has been mapped over a wide range of temperature (300--1100 \ifmmode^\circ\else\textdegree\fi{}C) and oxygen pressure (${10}^{\mathrm{\ensuremath{-}}5}$--${10}^{3}$ bar). At ambient pressure and temperature, ${\mathrm{BaCuO}}_{\mathit{x}}$ is found to be in a metastable state: long annealing at 450 \ifmmode^\circ\else\textdegree\fi{}C tends to decompose the phase into ${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{5}$ and ${\mathrm{BaO}}_{2}$. Having obtained the phase stability domain in the [T,p(O${)}_{2}$] plane we were able to prepare single-phase samples of ${\mathrm{BaCuO}}_{\mathit{x}}$ with different oxygen contents suitable for precise intrinsic thermodynamical measurements. We show that the behavior of the low-temperature specific heat (1.1\ensuremath{\le}T\ensuremath{\le}32 K) and its dependence on the magnetic field (0\ensuremath{\le}B\ensuremath{\le}14 T) may be understood by taking into account a many-level magnetic system directly related to the ${\mathrm{Cu}}_{6}$${\mathrm{O}}_{12}$ and ${\mathrm{Cu}}_{18}$${\mathrm{O}}_{24}$ structural blocks of ${\mathrm{BaCuO}}_{\mathit{x}}$. Depending on the oxygen concentration, competition between antiferromagnetic (AF) ordering and the many-level system is observed. With increasing oxygen content, the N\'eel temperature decreases whereas amplitude of the many-level system increases. The zero-field AF transition belongs to the three-dimensional isotropic Heisenberg universality class.