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

Stable organic biradical crystals PNNNO, F2PNNNO, and PIMNO of the PNNNO family were synthesized. $PNNNO52-@48(N-tert-butyl-N-oxyamino)phenyl]-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl 3-oxide, F2PNNNO52-@28,68,-difluoro248-(N-tert-butyl-N-oxyamino)phenyl#4,4,5,5-tetramethyl-4, 5-dihydro-1H-imidazol-1-oxyl 3-oxide, PIMNO52-@482(N-tert-butyl-N-oxyamino)-phenyl#-4,4,5, 5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl.% PNNNO and PIMNO crystallize to form quasi-onedimensional lattices, but F2PNNNO 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 S51/2 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, F2PNNNO 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. @S0163-1829~99!00342-2#