Magnetization relaxation in the single molecule magnet Ni$_4$ under continuous microwave irradiation

TL;DR

This study investigates the spin relaxation mechanisms in the Ni4 single molecule magnet under continuous microwave irradiation, revealing phonon bottleneck effects at low temperatures and an Orbach process at higher temperatures.

Contribution

It provides detailed experimental data and theoretical analysis of spin relaxation in Ni4, highlighting the dominant relaxation pathways under steady microwave irradiation.

Findings

Relaxation rate strongly depends on temperature below 1.5 K

Phonon bottleneck dominates relaxation at low temperatures

Orbach mechanism involving excited spin levels at higher temperatures

Abstract

Spin relaxation between the two lowest-lying spin-states has been studied in the S=4 single molecule magnet Ni$_4$ 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.