Optical control of magnetization in Co(II)-based single-molecule magnets via resonant pulsed THz excitation

Single-molecule magnets (SMMs) are a class of magnetic materials that combine a number of unique characteristics compared to conventional magnets, such as low density, low electrical conductivity, and optical transparency. A key obstacle to their practical application is the absence of an effective and reliable route to control their magnetization. Such control can be achieved by directly exciting the ground spin state with photons. This strategy was demonstrated in the early 2000s for the well-known Fe₈ single-molecule magnet, using a microwave generator (~110 GHz) as a source of resonant photons. Since then, considerable progress in the chemical tuning of SMMs has substantially increased the energy gap between the ground and first excited states, which now extends into the terahertz (THz) and far-infrared ranges. This has hindered the study of direct excitation of the SMM spin system, owing to the scarcity of suitable radiation sources operating in these ranges.

One of the few suitable sources is the Novosibirsk Free Electron Laser (NovoFEL), which provides high-power pulsed THz radiation with tunable frequency. The EPR endstation at NovoFEL integrates X-band continuous wave, time-resolved, and pulsed EPR with THz irradiation of the samples under study. These capabilities, together with direct measurements of changes in the sample magnetization, were employed to investigate THz-induced processes. A series of SMMs based on Co(II) complexes with Schiff base ligands served as the model objects.

By tuning the wavelength of the THz radiation over the 225–245 μm range, we achieved direct excitation of the spin transitions, giving rise to a non-equilibrium population of the ground Kramers doublet. Depending on the excited transition, the SMM magnetization could be polarized in either direction, corresponding to (i) heating or (ii) cooling of the spin system in terms of spin temperature. To quantify these effects, we combined pulsed THz radiation with pulsed EPR spectroscopy using the pulse sequence [THz – T – π/2 – τ – π – τ – spin echo]. By varying the delay T, we estimated the spin-system temperature of a magnetically diluted single crystal of one of the SMMs studied.

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