Resonant photon absorption and hole burning in Cr7Ni antiferromagnetic rings

Magnetization measurements on a crystal of ${\mathrm{Cr}}_{7}\mathrm{Ni}$ antiferromagnetic rings are presented. Irradiation with microwaves at frequencies between 1 and $10\phantom{\rule{0.3em}{0ex}}\mathrm{GHz}$ leads to observation of very narrow Gaussian resonant photon absorption lines which are mainly broadened by hyperfine interactions. The relaxation of magnetization after a nanosecond RF pulse is dominated by the phonon bottleneck effect. A two-pulse hole burning technique allowed us to estimate the characteristic energy diffusion time (spectral diffusion).