High field magnetic resonant properties of beta'-(ET)2SF5CF2SO3
  A systematic electron spin resonance (ESR) investigation of the low temperature regime for the (ET)2SF5CF2SO3 system was performed in the frequency range of ~200-700 GHz, using backward wave oscillator sources, and at fields up to 25 T. Newly acquired access to the high frequency and fields shows experimental ESR results in agreement with the nuclear magnetic resonance (NMR) investigation, revealing evidence that the transition seen at 20 K is not of conventional spin-Peierls order. A significant change of the spin resonance spectrum in beta'-(ET)2SF5CF2SO3 at low temperatures, indicates a transition into a three-dimensional-antiferromagnetic (3D AFM) phase. 
I.B. Rutel, S.A. Zvyagin, J.S. Brooks, J. Krzystek, P. Kuhns, A.P.   Reyes, B.H. Ward, J.A. Schlueter, R.W. Winter, and G.L....