Thermal expansion study of anisotropic magnetolattice coupling and antiferromagnetic transition in CuO

Transition metal oxides have been the subject of intense research over the past few decades since they form the basic building block of many materials showing exotic properties such as high temperature superconductivity, spin and charge ordering, magnetoresistance, multiferroicity etc. Recently, Kimura et al. demonstrated an intriguing coupling between electric and magnetic dipole ordering in CuO, which opened a new route for finding materials exhibiting induced multiferroic behavior.\footnote{T. Kimura et al., Nature Mater. \textbf{7}, 291 (2008).} Here we present results on anisotropic thermal expansion in single crystalline CuO in the temperature range $5 < T < 350$ K. Our results demonstrate anisotropic magnetolattice coupling in CuO around the two known antiferromagnetic phase transitions at $T_{N1}$= 230 K and $T_{N2}$= 213 K. We also discuss the pressure dependence of $T_{N1}$ and critical behavior in CuO using the scaling of heat capacity and thermal expansion data.