Stable organic biradical crystals PNNNO, ${\mathrm{F}}_2\mathrm{PNNNO},$ and PIMNO of the PNNNO family were synthesized. $\{\mathrm{PNNNO}=2-[4^{\prime }-$ $(N-\mathit{tert}-\mathrm{butyl}-N-\mathrm{oxyamino})\mathrm{phenyl}]-4,4,5,5-\mathrm{tetramethyl}-4,5-\mathrm{dihydro}-1H-\mathrm{imidazol}-1-\mathrm{oxyl}$ $3-\mathrm{oxide},$ ${\mathrm{F}}_2\mathrm{PNNNO}=2-[2^{\prime }{,6\mathrm{}}^{\prime },-\mathrm{difluoro}-4^{\prime }-(N-\mathit{tert}-\mathrm{butyl}-N-\mathrm{oxyamino}\left)\mathrm{phenyl}\right]-\mathrm{}4,4,5,5-\mathrm{tetramethyl}-4,$ $5-\mathrm{dihydro}-1H-\mathrm{imidazol}-1-\mathrm{oxyl}$ $3-\mathrm{oxide},$ $\mathrm{PIMNO}=2-\left[4^{\prime }-\right(N-\mathit{tert}-\mathrm{butyl}-N-\mathrm{oxyamino})-\mathrm{phenyl}]-4,4,5,5-\mathrm{tetramethyl}-4,5-\mathrm{dihydro}-1H-\mathrm{imidazol}-1-\mathrm{oxyl}.\}$ PNNNO and PIMNO crystallize to form quasi-one-dimensional lattices, but ${\mathrm{F}}_2\mathrm{PNNNO}$ to form a quasi-two-dimensional lattice. The temperature dependences of the susceptibility and the high-field magnetization process up to 34 T were measured down to 0.5 K. The results are analyzed by comparing with the theoretical calculations based on the crystal structures. PNNNO and PIMNO are considered to be antiferromagnetic Heisenberg spin chains consisting of $S=1/2\mathrm{}$ spin pairs (dimers) in which the two spins are coupled ferromagnetically. At low temperatures, an antiferromagnetic ordering occurs in these crystals, which is confirmed by the thermodynamic discussion through specific heat measurements. On the other hand, ${\mathrm{F}}_2\mathrm{PNNNO}$ is thought to be a two-dimensional Heisenberg system, in which the spin pairs are connected by two types of antiferromagnetic interactions. The ground state is singlet. The high-field magnetization process shows a two-step saturation with a plateau of the half value of the saturation magnetization.

• Received 10 May 1999

DOI:https://doi.org/10.1103/PhysRevB.60.12924