Principal magnetic susceptibilities and uniaxial stress experiments in CoO

Using a single crystal from which nonstoichiometries have been removed, the principal susceptibilities, χ_∥ and χ_⊥, of CoO have been determined in the temperature range of 4.2 to 350 K, bracketing T_N~=289 K. A stress ~= 210 bars along [001] was applied during measurements and in cooling through T_N (to remove T domains), and the initial χ was measured for H--> ∥ [001] and H-->⊥[001] at H-->=250 Oe. These measured χ's are then related to χ_∥ and χ_⊥ using the known magnetic structure of CoO, from which χ_∥ and χ_⊥ at various temperatures are determined. Then, using the relation χ_p=(χ_∥+2χ_⊥)3, the calculated χ_p (powder) is found to agree well with the measured χ_p at all temperatures, thus providing a consistency check on the procedures used in this work. For T-->0 K, the experimental χ_∥ (0)~=39.5×10^-6 cm³/g. This value is about 50% higher than the only available theoretical estimate by Tachiki [although his estimates of χ_⊥ (0) and χ(T_N) are in good agreement with our measured values]. Various sources for this discrepancy are examined, and it is suggested that further theoretical work is needed to explain this discrepancy as well as the observed temperature dependence of χ_∥ and χ_⊥. A comparison with the recent results of a similar study in MnO is also made.