Magnetic properties of low-dimensional quantum spin systems made of stableorganic biradicals PNNNO, F2PNNNO, and PIMNO

Stable organic biradical crystals PNNNO, F2PNNNO, and PIMNO of the PNNNO fa mily were synthesized. {PNNNO = 2-[4'-(N-tert-butyl-N-axyamino)phenyl]-4,4, 5,5 1H-imidazol-1-oxyl 3-oxide, F2PNNNO = 2-[2',6',-difluoro- 4'-(N-tert-bu tyl-N-oxyamino)phenyl]- 4,4,5,5-tetramethyl-4, 5-dihydro-1H-imidazol-1-oxyl 3-oxide, PIMNO = 2-[4'-(N- tert-butyl-N-oxyamino)-phenyl]-4,4,5,5-tetramet hyl-4,5-dihydro-1H- imidazol-1-oxyl.} PNNNO and PIMNO crystallize to form q uasi-one dimensional lattices, but F2PNNNO to form a quasi-two-dimensionaI lattice. The temperature dependences of the susceptibility and the high-fie ld magnetization process up to 34 T were measured down to 0.5 K. The result s 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 spin pairs (dimers) in which t he two spins are coupled ferromagnetically. At low temperatures, an antifer romagnetic ordering occurs in these crystals, which is confirmed by the the rmodynamic discussion through specific heat measurements. On the other hand , F2PNNO is thought to be a two-dimensional Heisenberg system, in which the spin pairs are connected by two types of antiferromagnetic interactions. T he ground state is singlet. The high-field magnetization process shows a tw o-step saturation with a plateau of the half value of the saturation magnet ization. [S0163-1829(99)00342-2].