We have succeeded in synthesizing two types of new organic radical crystals 3-I-V [= 3-

We report magnetization, specific heat, and NMR measurements of 3-Br-4-F-V [= 3-(3-bromo-4fluorophenyl)-1,5-diphenylverdazyl], strong-rung S=1/2 Heisenberg spin ladder with ferromagnetic leg interactions. We explain the magnetic and thermodynamic properties based on the strong-rung regime. Furthermore, we find a field-induced successive phase transition in the specific heat and the nuclear spin-lattice relaxation rate 1/T 1 . 19 F-NMR spectra for higher and lower temperature phases indicate partial magnetic order and incommensurate long-range order, respectively, evidencing the presence of frustration due to weak interladder couplings.

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