Frustration induced one-dimensionality in the isosceles triangular antiferromagnetic lattice of $delta$-(EDT-TTF-CONMe$_{2}$)$_{2}$AsF$_6$

The $1/4$-filled organic compound, $delta$-(EDT-TTF-CONMe$_{2}$)$_{2}$AsF$_6$ is a frustrated two-dimensional triangular magnetic system as shown by high-frequency (111.2 and 222.4 GHz) electron spin resonance (ESR) and structural data in the literature. The material gradually orders antiferromagnetically below 40~K but some magnetically disordered domains persist down to 4 K. We propose that in defect free regions frustration prevents true magnetic order down to at least 4 K in spite of the large first- and second-neighbor exchange interactions along chains and between chains, respectively. The antiferromagnetic (AFM) order gradually developing below 40 K nucleates around structural defects that locally cancel frustration. Two antiferromagnetic resonance modes mapped in the principal planes at 4~K are assigned to the very weakly interacting one-dimensional molecular chains in antiferromagnetic regions.