Antiferromagnetic ground state in organic quasi-1D ferromagnet γ-phase para-nitrophenyl nitronyl nitroxide

We have studied the ground state of an organic quasi-one-dimensional (Q1D) ferromagnet, the γ-phase para-nitrophenyl nitronyl nitroxide (γ-p-NPNN), by low-frequency ESR and magnetic torque measurements, and have found that it is an antiferromagnetically ordered state with an orthorhombic anisotropy. Antiferromagnetic resonance (AFMR) modes are observed below 0.6 K. From the angle dependence of AFMR, the spin easy axis is found to point the [011] direction. The spin-flop (SF) field is estimated to be 520 Oe. From the analysis of the AFMR modes in terms of a mean-field approximation, we have estimated the interchain interactions and the anisotropy field parameters in the ground state. The magnetic phase diagram along the spin easy axis has been determined using a high sensitive microcantilever technique. On the field dependence along the easy axis, the torque response shows anomalies at 520 Oe and around 2100 Oe, corresponding to the spin-flop and the full-polarization of spins, respectively. The easy axis and anisotropy in the antiferromagnetic state are found to be well explained by our calculations of the dipolar energy with respect to the spin structure obtained in this work.