Abstract
The stability of the (x≥2) phase has been mapped over a wide range of temperature (300–1100 °C) and oxygen pressure (– bar). At ambient pressure and temperature, is found to be in a metastable state: long annealing at 450 °C tends to decompose the phase into and . Having obtained the phase stability domain in the [T,p(O] plane we were able to prepare single-phase samples of with different oxygen contents suitable for precise intrinsic thermodynamical measurements. We show that the behavior of the low-temperature specific heat (1.1≤T≤32 K) and its dependence on the magnetic field (0≤B≤14 T) may be understood by taking into account a many-level magnetic system directly related to the and structural blocks of . Depending on the oxygen concentration, competition between antiferromagnetic (AF) ordering and the many-level system is observed. With increasing oxygen content, the Néel temperature decreases whereas amplitude of the many-level system increases. The zero-field AF transition belongs to the three-dimensional isotropic Heisenberg universality class.
- Received 8 May 1995
DOI:https://doi.org/10.1103/PhysRevB.52.12833
©1995 American Physical Society