  We have succeeded in synthesizing single crystals of the verdazyl radical
beta-2,6-Cl2-V [= beta-3-(2,6-dichlorophenyl)-1,5-diphenylverdazyl]. The ab
initio MO calculation indicated the formation of an S = 1/2 Heisenberg
antiferromagnetic chain with four-fold magnetic periodicity consisting of three
types of exchange interactions.We have successfully explained the magnetic and
thermodynamic properties based on the expected spin model by using the quantum
Monte Carlo method. Furthermore, we revealed that the alternating and unique
Ising ferromagnetic chains become effective in the specific field regions and
observed a cooperative phenomenon caused by the magnetic order and quantum
fluctuations. These results demonstrate that verdazyl radical could form
unconventional spin model with interesting quantum behavior and provide a new
way to study a variety of quantum spin systems.
