Incommensurate antiferromagnetism in copper(II) oxide: Specific-heat study in a magnetic field.

Abstract

A specific-heat study of the incommensurate-to-commensurate antiferromagnetic transition at a temperature ${\mathit{T}}_{\mathit{L}}$\ensuremath{\sim}213 K in a CuO single crystal has been carried out in magnetic fields ${\mathbf{B}}_{\mathit{a}}$ up to 6 T. ${\mathit{T}}_{\mathit{L}}$ is found to increase nonlinearly with increasing ${\mathit{B}}_{\mathit{a}}$ for ${\mathbf{B}}_{\mathit{a}}$\ensuremath{\parallel}a and c axes, whereas a linear decrease of ${\mathit{T}}_{\mathit{L}}$ is observed with increasing ${\mathit{B}}_{\mathit{a}}$ for ${\mathbf{B}}_{\mathit{a}}$\ensuremath{\parallel}b axis. This behavior of \ensuremath{\Delta}${\mathit{T}}_{\mathit{L}}$/\ensuremath{\Delta}${\mathit{B}}_{\mathit{a}}$ and that of the magnetization, above and below ${\mathit{T}}_{\mathit{L}}$, can be understood thermodynamically by a magnetic analogy to Clausius-Clapeyron equation. The origin of the incommensurate-antiferromagnetic phase is discussed in the framework of the ``interacting-component mechanism'' of Heine and McConnell.