Multiple phase transitions in CuO observed with thermal expansion

A. Rebello, Z. C. M. Winter, S. Viall, and J. J. Neumeier

Phys. Rev. B 88, 094420 – Published 16 September 2013

Abstract

High-resolution thermal-expansion measurements of single-crystalline CuO (tenorite) are reported for the temperature range $5 < T < 350$ K. The data reveal three transitions ( $T_{N 1}$ $=$ 213 K, $T_{N 2}$ $=$ 229.2 K, and $T_{N 3}$ $=$ 229.8 K), which corroborate the recently proposed magnetic phase diagram [Villarreal, Quirion, Plumer, Poirier, Usui, and Kimura, Phys. Rev. Lett. 109, 167206 (2012)] revealing three distinct antiferromagnetic (AFM) phases. Analysis of the region surrounding $T_{N 2}$ and $T_{N 3}$ suggests that these phase transitions are continuous and yields estimates for the heat-capacity critical exponents of $α_{T_{N 2}}$ $=$ 0.033(2) and $α_{T_{N 3}}$ $=$ 0.040(9). Magnetic susceptibility measurements reveal spin-flop transitions at temperatures below $T_{N 1}$ , confirming that the $b$ axis is the easy AFM axis.