Spin relaxation between the two lowest-lying spin-states has been studied in the S=4 single molecule magnet Ni4 under steady state conditions of low amplitude and continuous microwave irradiation. The relaxation rate was determined as a function of temperature at two frequencies, 10 and 27.8 GHz, by simultaneously measuring the magnetization and the absorbed microwave power. A strong temperature dependence is observed below 1.5 K, which is not consistent with a direct single-spin-phonon relaxation process. The data instead suggest that the spin relaxation is dominated by a phonon bottleneck at low temperatures and occurs by an Orbach mechanism involving excited spin-levels at higher temperatures. Experimental results are compared with detailed calculations of the relaxation rate using the universal density matrix equation.
@article{arxiv.0804.3444,
title = {Magnetization relaxation in the single molecule magnet Ni$_4$ under continuous microwave irradiation},
author = {G. de Loubens and D. A. Garanin and C. C. Beedle and D. N. Hendrickson and A. D. Kent},
journal= {arXiv preprint arXiv:0804.3444},
year = {2008}
}