Low-temperature structure and ESR study of quasi-one-dimensional ferromagnet γ-p-NPNN

The low-temperature structure analysis and low-temperature ESR were performed in the γ-phase of p-NPNN, which is considered to be as a quasi-one-dimensional ferromagnet above the antiferromagnetic ordering temperature (0.65 K without a static magnetic field). No structural phase transition was observed below 30 K, but we found the large thermal shrinkage between the intra-chain p-NPNN molecules along c-axis. This result suggests that the one-dimensionality of the ferromagnetic chain is effectively enhanced at lower temperature, since the geometry through NO-NO₂ contact gives rise to the strong ferromagnetic interaction. Below 0.6 K, we succeeded in observing the antiferromagnetic resonance (AFMR) using the low frequency (∼300 MHz) and low-temperature (0.5 K) ESR. The frequency-field relation is well reproduced by the two-sublattice model with orthorhombic anisotropy.