Resonant photon absorption and hole burning in Cr7Ni antiferromagnetic   rings
  Presented are magnetization measurements on a crystal of Cr7Ni antiferromagnetic rings. Irradiation with microwaves at frequencies between 1 and 10 GHz leads to observation of very narrow resonant photon absorption lines which are mainly broadened by hyperfin interactions. A two-pulse hole burning technique allowed us to estimate the characteristic energy diffusion time. 
W. Wernsdorfer, D. Mailly, G. A. Timco, and R. E. P. Winpenny