Measurements of the tetragonal elongation in a CoO single crystal in the critical region above the Néel temperature $T_{\mathrm{N}}$ indicate that $\left|\frac{\Delta L}{L}\right|\propto {\left(\frac{T}{T_{\mathrm{N}}-1\mathrm{}}\right)}^x$, and below $T_{\mathrm{N}}$ that $\left|\frac{\Delta L}{L}\right|\propto {\left(\frac{1-T}{T_{\mathrm{N}}}\right)}^{x^{\prime }}$, where $x=0.67\mathrm{}\pm 0.10$ above $T_{\mathrm{N}}$ and $x^{\prime }=0.60\mathrm{}\pm 0.08$ below $T_{\mathrm{N}}$. We compare these critical exponents with two theoretical predictions of strain behavior in the critical region and find agreement in both instances.

• Received 22 March 1971

DOI:https://doi.org/10.1103/PhysRevLett.26.1483