Temperature jump spectroscopy under pulsed THz radiation: a way to measure T1 of magnetically concentrated substances

Spin-lattice and spin-spin interactions play an important role in a number of physical phenomena with applied potential, i.e. magnetic relaxation in single-molecule magnets (SMMs), entanglement of a quantum system with its environment, decoherence of magnetic qubits. In particular, for the technological application of SMMs, the low rate of spin-lattice relaxation (T₁) is one of the critical factors that requires a fundamental understanding. To determine T₁ of SMM, the magnetic susceptibility of a macroscopic sample is usually measured by alternating current SQUID magnetometry. Another direct method is pulsed EPR spectroscopy, which requires diamagnetically diluted samples to suppress spin-spin interactions.

Temperature jump spectroscopy can be used as an alternative to SQUID magnetometry and pulse EPR. This is a type of time-resolved (TR) EPR spectroscopy that records the change in microwave (MW) absorption caused by pulse heating of a sample. The sign and shape of the resulting signal reflect the difference in the EPR signals of the spin system before and after the external stimulus. Traditional TR EPR spectroscopy uses laser radiation in the visible range to photochemically generate nonequilibrium-populated triplet states. The use of far infrared radiation (30-60 cm⁻¹) does not cause photochemical reactions, but heat the sample, initiating spin dynamics.

Herein, we propose a numerical approach for modeling 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. Numerical modeling is based on solving the Liouville – von Neumann equation in the Julia programming language. The T₁ values were determined over a wide temperature range at several magnetic field positions of the powder spectrum. T-jump experiments were carried out at the EPR spectroscopy endstation utilizing pulsed THz radiation of the Novosibirsk Free Electron Laser.