Low temperature spin state of a model compound for organic ferrimagnets as studied by single crystal cw-ESR spectroscopy

We have studied a model compound for organic ferrimagnets by single crystal cw-ESR. The system under study is composed of two kinds of nitronylnitroxide molecules with the ground states of S=1/2 and S=1. These molecules are stacked in an alternating chain in the crystal. The cw-ESR signal of the compound split into two lines below 10 K, which were reproduced by the superposition of two Lorentzian lines. The X-ray measurements at 9.6 K revealed that the crystal structure remained unchanged at low temperatures, indicating that the origin of the ESR line splitting is attributable to some change in the spin state: Two distinguishable spin species appear in the crystal to give the line splitting. The magnetic interaction fields from the two species are elucidated on the basis of the angular dependence of the ESR linewidths: One has isotropic (three-dimensional) interaction field, while the other has one-dimensional field which is axially symmetric along the alternating chain. The appearance of the two kinds of spin species demonstrates that the model system violates the classical picture of ferrimagnetic states (antiparallel alignment of adjacent spins with different spin quantum numbers). This presumably results from the internal magnetic degree of freedom within the 5=1 molecule, i.e., finite intramolecular ferromagnetic interaction.