Magnetic field versus temperature phase diagram of the spin-1/2 alternating-bond chain compound F ₅PNN

We have measured the specific heat of the S - 1/2 alternating-bond Heisenberg antiferromagnetic chain compound pentafluorophenyl nitronyl nitroxide in magnetic fields using a single crystal and powder. A sharp peak due to field-induced magnetic ordering (FIMO) is observed in both samples. The H-T phase boundary of the FIMO of the single crystal is symmetric with respect to the central field of the gapless field region HC1< H whereas it is distorted for the powder whose ordering temperatures are lower. We discuss possibility that an effective pressure caused by mixing the powder with grease, which is reported for various organic compounds, plays an important role for the distorted phase boundary. An analysis employing calculations based on the finite temperature density matrix renormalization group suggests that the pressure-induced frustration enhances incommensurate spin correlation leading to the distorted phase boundary for the powder.