Polarization of the ground Kramers doublet in single-molecule magnets based on Co(II) by pumping the spin system with resonant pulsed THz radiation

Single-molecule magnets (SMMs) exhibit unique magnetic properties but face challenges in practical use due to difficulties in controlling their magnetization. In this work, pulsed, frequency-tunable THz radiation from the Novosibirsk Free Electron Laser (NovoFEL) was combined with electron paramagnetic resonance (EPR) techniques to study THz-induced processes in Co(II)-based SMMs. By tuning the wavelength, direct excitation of magnetic dipole transitions was achieved, leading to non-equilibrium populations of the ground spin state.

One of the obstacles to the practical application of SMMs is the lack of an effective and reliable approach to control magnetization. This can be achieved by directly exciting the ground spin state with photons. Significant progress in the chemical tuning of SMMs has led to a substantial increase in the energy gap between the ground and first excited states, which now reaches the terahertz (THz) and far-infrared ranges, hampering the study of direct excitation due to the lack of suitable radiation sources. One of the few suitable sources is the NovoFEL, which provides pulsed THz radiation with high power and tunable frequency. The EPR endstation at NovoFEL combines X-band continuous wave, time-resolved, and pulsed EPR with THz irradiation of the samples under study. A series of SMMs based on Co(II) complexes with Schiff base ligands were used as objects.

By changing the wavelength of THz radiation in the range of 225–245 μm, we demonstrated direct excitation of the spin transitions, which led to a non-equilibrium population of the ground state. Depending on the excited transition, the magnetization of the SMM was polarized in both directions, namely toward (i) heating or (ii) cooling of the spin system in terms of spin temperature. To quantify the observed effects, we combined pulsed THz radiation with pulsed EPR spectroscopy and applied a pulse sequence [THz – T – π/2 – τ – π – τ – spin echo]. By varying the delay T, we estimated the temperature of the spin system of a magnetically diluted single crystal of one of the SMMs under study.

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