Low Temperature Phase Diagram in $\gamma-p$-NPNN

We performed the magnetic torque measurements of $\gamma $-phase of \textit{para}-Nitrophenyl Nitronyl Nitroxide (being $p$-NPNN), which is considered to be as a quasi-one-dimensional ferromagnet, in the vicinity of $T_{N}$ (=0.65K) using a piezo-resistive micro-cantilever. Typical sample dimension is about $0.25\times 0.10\times 0.10$mm$^{3}$. At 0.4K, a spin-flop transition ($H_{SF})$ and an antiferromagnetic-paramagnetic transition ($H_ {C})$ are clearly observed in the magnetic field of about 470 Gauss and 2100 Gauss, respectively. The spin-easy axis is almost parallel to the direction to phenyl ring from the ONCNO fragments. $H-T$ phase diagram is determined properly. These results are consistent with our recent low frequency ($\sim $ 300 MHz) and low-temperature (0.4 K) ESR. We will discuss the $H-T$ phase diagram of $\gamma -p$-NPNN in comparison with the one- dimensional Heisenberg ferromagnetic model.