T-jump spectroscopy under pulsed THz radiation: a universal way to measure T1 of magnetically concentrated single-molecule magnets

Spin-lattice and spin-spin interactions play a crucial role in various phenomena with an applied potential. In particular, any technological application of single-molecule magnets (SMMs) requires a low rate of spin-lattice relaxation (T₁). To determine T₁ of SMM, the magnetic susceptibility of a macroscopic powder sample is usually measured at different temperatures by alternating current SQUID magnetometry. Another direct method is pulsed electron paramagnetic resonance (EPR) spectroscopy that can be used only for diamagnetically diluted samples to suppress spin-spin interactions.

Temperature jump (T-jump) spectroscopy is a convenient alternative to SQUID magnetometry and pulsed EPR. It can use magnetically concentrated samples, but still obtain spectroscopic information, namely EPR spectrum and relaxation times (usually T₁). T-jump spectroscopy is a modification of time-resolved (TR) EPR spectroscopy. Traditional TR EPR spectroscopy uses pulsed laser radiation in the visible or near ultraviolet range for photochemical generation of nonequilibrium population states, while recording the kinetics of microwave absorption. The shape and sign of the signal reflect the difference in the EPR signals of the spin system before and after the external stimulus. In T-jump spectroscopy, the only effect of the laser pulse is heating of the sample. It gives rise to a change in the population of spin levels that manifests itself as a strong negative signal in the TR EPR spectrum. The use of far-infrared radiation (30-80 cm⁻¹) prevents photochemical reactions, but keeps the sample heated, thereby initiating spin dynamics as the population of spin levels equilibrates with the characteristic spin relaxation time.

Herein, we propose a numerical approach for modelling the spin dynamics under continuous MW irradiation and a T-jump induced by pulsed THz radiation and exemplify it with results obtained for cobalt(II) bis[tris(pyrazolyl)borate] with 100% cobalt(II) ion content. The numerical simulation is based on the solution of the Liouville – von Neumann equation in the Julia programming language. The developed model allowed us to determine the T₁ values for cobalt(II) ions in a wide temperature range at several magnetic field positions in the powder spectrum. T-jump experiments were carried out at the EPR spectroscopy endstation utilizing high power pulsed THz radiation of the Novosibirsk Free Electron Laser with a wavenumber of 77 cm⁻¹ and duration of 50 μs.

This work was supported by the Russian Science Foundation 23-73-00042.