Determination of zero-field splitting in magnetically diluted cobalt(II)-based single molecule magnets using circular dichroism Fourier-transform IR spectroscopy

Single-molecule magnets (SMMs) with total electron spin S = 3/2 are promising platforms for optical magnetization control. Their study in such a context is relevant in both magnetically concentrated and diluted samples and assumes the excitation of magnetic dipole transitions by resonant radiation. Such experiments require precise determination of the energy splitting between Kramers doublets (EZFS), since this value determines the resonant radiation frequency. The direct spectroscopic method available for such measurements is frequency-domain Fourier transform terahertz electron paramagnetic resonance (FD-FT THz-EPR) spectroscopy. It usually requires a setup that includes an FTIR-spectrometer and a superconducting magnet to create a magnetic field in the sample area. In this work, we propose a method for the rapid determination of EZFS of SMMs without using a superconducting magnet. The method is based on circular dichroism of an SMM placed in a static external magnetic field. It was implemented using a standard FTIR spectrometer with a sample holder modified by inserting a permanent magnet. Circular polarization was created using a grid polarizer and a quarter-wave plate. Model Co(II)-based SMMs with a varying degree of magnetic dilution were investigated by the proposed method. The results were confirmed by FD-FT THz-EPR. The obtained EZFS values of 43.4-43.6 cm-1 demonstrate a slight downward trend with a decrease in the paramagnetic center content. Such preliminary measurements of EZFS in magnetically diluted SMMs pave the way for the resonant excitation of magnetic dipole transitions using narrowband sources and even for coherent control of magnetic quantum states.