We discovered that an organic radical crystal, p-nitrophenyl nitronyl nitroxide (p-NPNN), is a quasi-1D ferromagnet induced by unpaired electrons on radicals. The magnetization and the susceptibility of the triclinic γ-phase crystal are very well represented by the Bethe-ansatz solution of the S=1/2 1D Heisenberg ferromagnet with nearest-neighbor coupling J≃4.3 K. Low-temperature specific heat and ac susceptibility measurements revealed that the crystal undergoes a ferromagnetic transition at 0.65 K. The interchain coupling ${\mathit{J}}^1$ was estimated to be about 0.1 K.

• Received 25 April 1991

DOI:https://doi.org/10.1103/PhysRevLett.67.746