Field-induced incommensurate phase in the strong-rung spin ladder with   ferromagnetic legs
  We report magnetization, specific heat, and NMR measurements of 3-Br-4-F-V [=3-(3-bromo-4-fluorophenyl)-1,5-diphenylverdazyl], a 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/T1. 19F-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. 
H. Yamaguchi, H. Miyagai, M. Yoshida, M. Takigawa, K. Iwase, T. Ono,   N. Kase, K. Araki, S. Kittaka, T. Sakakibara, T. Shim...
cond-mat.str-el