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Precise measurements have been made of the specific heats of the antiferromagnetic solid solutions pMnO-qCoO and of the pure oxides MnO and CoO in the temperature range 20 to 700K. From these measurements, the Neel temperatures have been deduced and, through the application of the BPW effective field model, the exchange integrals have been estimated: in spite of the change which occurs in the lattice parameter, these integrals appear to remain effectively constant over the range of compositions. A hybrid method based on a combination of the corresponding states method with the effective phonon spectrum has been found to give consistent results when the lattice contribution is separated for MnO and CoO. There is good agreement between the experimental and theoretical entropy for most of the range of compositions. However, there appears to be some anomalous behaviour for the magnetic energy where there is small substitution of one oxide in the other.

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