Magnetization relaxation in the single-molecule magnet Ni₄ under continuous microwave irradiation

¹  Department of Physics, New York University - 4 Washington Place, New York, NY 10003, USA
²  Department of Physics and Astronomy, City University of New York - 250 Bedford Park Boulevard West Bronx, NY 10468-1589, USA
³  Department of Chemistry and Biochemistry, University of California San Diego - La Jolla, CA 92093, USA

Corresponding author: gregoire.deloubens@cea.fr

Received: 22 April 2008
Accepted: 16 June 2008

Abstract

Spin relaxation between the two lowest-lying spin-states has been studied in the 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.