FMR Study of the Field Dependence of the Ferromagnetic Transition in an Organic Magnet

Organic heterocyclic thia/selenazyl radicals have unique magnetic properties. First and foremost, in their crystalline form, they experience a transition to a ferromagnetic state at temperatures that are the highest for any material containing only non-metallic elements. Second, their low temperature uniaxial anisotropy field is the highest among purely organic ferromagnets [Winter et al., JACS {\bf 133}, 8126 (2011)]. To investigate the effect of a magnetic field on the transition in the mixed Se-S compound ($T_c = 12.5$~K) at zero field, we employ ferromagnetic resonance (FMR) absorption as a measure of the anisotropy field for a single crystal. We also focus on the temperature and field dependence of the FMR linewidth. Our main finding is that the application of a field significantly broadens the ferromagnetic transition, with a noticeable FMR signal observed to as high as $2T_c$ in fields of a few tesla. Meanwhile, the FMR linewidth is relatively insensitive to frequency/field, though it becomes narrower upon decreasing the temperature and saturates below $T_c$. We will discuss the broadening of the ferromagnetic transition within the framework of scaling theory.