Pressure-induced ferro- to antiferromagnetic transition in a purely organic compound, β-phasepara-nitrophenyl nitronyl nitroxide

Abstract

A pressure-induced ferro- to antiferromagnetic transition in genuine organic compounds has been observed in the \ensuremath{\beta}-phase para-nitrophenyl nitronyl nitroxide crystal. The transition temperature ${T}_{\mathrm{C}}(p)$ in the ferromagnetic state decreases as ${T}_{\mathrm{C}}{(p)=T}_{\mathrm{C}}{(p}_{0})(1+ap)$ for pressures $pl~{p}_{c}=6.5\ifmmode\pm\else\textpm\fi{}0.5\mathrm{kbar}$ [${T}_{\mathrm{C}}{(p}_{0})=0.61\mathrm{K},$ $a=\ensuremath{-}0.048({\mathrm{kbar}}^{\mathrm{\ensuremath{-}}1})$]. For $pg{p}_{c},$ however, experiment demonstrates intrinsically the existence of an antiferromagnetic state as well as a pressure-induced enhancement of ${T}_{\mathrm{C}}(p)$ with $a=0.004({\mathrm{kbar}}^{\mathrm{\ensuremath{-}}1}).$ The results are discussed within the context of theories derived from the unrestricted Hartree-Fock approximation.