Principal magnetic susceptibilities and uniaxial stress experiments in CoO

Abstract

Using a single crystal from which nonstoichiometries have been removed, the principal susceptibilities, ${\ensuremath{\chi}}_{\ensuremath{\parallel}}$ and ${\ensuremath{\chi}}_{\ensuremath{\perp}}$, of CoO have been determined in the temperature range of 4.2 to 350 K, bracketing ${T}_{N}\ensuremath{\simeq}289$ K. A stress \ensuremath{\simeq} 210 bars along [001] was applied during measurements and in cooling through ${T}_{N}$ (to remove $T$ domains), and the initial $\ensuremath{\chi}$ was measured for $\stackrel{\ensuremath{\rightarrow}}{\mathrm{H}}\ensuremath{\parallel}[001]$ and $\stackrel{\ensuremath{\rightarrow}}{\mathrm{H}}\ensuremath{\perp}[001]$ at $\stackrel{\ensuremath{\rightarrow}}{\mathrm{H}}=250$ Oe. These measured $\ensuremath{\chi}$'s are then related to ${\ensuremath{\chi}}_{\ensuremath{\parallel}}$ and ${\ensuremath{\chi}}_{\ensuremath{\perp}}$ using the known magnetic structure of CoO, from which ${\ensuremath{\chi}}_{\ensuremath{\parallel}}$ and ${\ensuremath{\chi}}_{\ensuremath{\perp}}$ at various temperatures are determined. Then, using the relation ${\ensuremath{\chi}}_{p}=\frac{({\ensuremath{\chi}}_{\ensuremath{\parallel}}+2{\ensuremath{\chi}}_{\ensuremath{\perp}})}{3}$, the calculated ${\ensuremath{\chi}}_{p}$ (powder) is found to agree well with the measured ${\ensuremath{\chi}}_{p}$ at all temperatures, thus providing a consistency check on the procedures used in this work. For $T\ensuremath{\rightarrow}0$ K, the experimental ${\ensuremath{\chi}}_{\ensuremath{\parallel}}(0)\ensuremath{\simeq}39.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$ ${\mathrm{cm}}^{3}$/g. This value is about 50% higher than the only available theoretical estimate by Tachiki [although his estimates of ${\ensuremath{\chi}}_{\ensuremath{\perp}}$ (0) and $\ensuremath{\chi}({T}_{N})$ 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 ${\ensuremath{\chi}}_{\ensuremath{\parallel}}$ and ${\ensuremath{\chi}}_{\ensuremath{\perp}}$. A comparison with the recent results of a similar study in MnO is also made.