Tetraoxolene radical-bridged lanthanide SMM systems were prepared for the first time by reduction of the respective neutral compounds. Magnetic measurements reveal the profound influence of the radical center on magnetic behavior. Strong magnetic couplings are revealed in the radical species, which switch on SMM behavior under zero applied field for Dy^III and Tb^III compounds. HFEPR spectra unravel the contributions of the magnetic coupling and the magnetic anisotropy. For Gd^III this results in much more accurate magnetic coupling parameters with respect to bulk magnetic measurements.

Electron paramagnetic resonance (EPR) is a powerful technique to investigate the electronic and magnetic properties of a wide range of materials. We present the first combined terahertz (THz) field and frequency domain electron paramagnetic resonance (HFEPR/FDMR) spectrometer designed to investigate the electronic structure and magnetic properties of molecular systems, thin films and solid state materials in a very broad frequency range of 85–1100 GHz. In this paper, we show high resolution frequency-field (Zeeman) maps (170–380 GHz by 0–15 T) recorded on two single-molecule magnets, [Mn₂(saltmen)₂(ReO₄)₂] and [Mn₂(salpn)₂(H₂O)₂](ClO₄)₂, which give direct access to the field-dependence of the energy level diagram. Furthermore, supression of standing waves in the described system and the sensitivity in field and frequency domain operations is evaluated and discussed.

Static and dynamic magnetic properties of the tetracoordinate CoII complex [Co(CH3‐im)2Cl2], (1, CH3‐im = N‐methyl‐imidazole), studied using thorough analyses of magnetometry, and High‐Frequency and ‐Field EPR (HFEPR) measurements, are reported. The study was supported by ab initio complete active space self‐consistent field (CASSCF) calculations. It has been revealed that 1 possesses a large magnetic anisotropy with a large rhombicity (magnetometry: D = −13.5 cm−1, E/D = 0.33; HFEPR: D = −14.5(1) cm−1, E/D = 0.16(1)). These experimental results agree well with the theoretical calculations (D = −11.2 cm−1, E/D = 0.18). Furthermore, it has been revealed that 1 behaves as a field‐induced single‐ion magnet with a relatively large spin‐reversal barrier (Ueff = 33.5 K). The influence of the Cl–Co–Cl angle on magnetic anisotropy parameters was evaluated using the CASSCF calculations.

Nanoscale magnetic systems play a decisive role in areas ranging from biology to spintronics. Although, in principle, THz electron paramagnetic resonance (EPR) provides high‐resolution access to their properties, lack of sensitivity has precluded realizing this potential. To resolve this issue, the principle of plasmonic enhancement of electromagnetic fields that is used in electric dipole spectroscopies with great success is exploited, and a new type of resonators for the enhancement of THz magnetic fields in a microscopic volume is proposed. A resonator composed of an array of diabolo antennas with a back‐reflecting mirror is designed and fabricated. Simulations and THz EPR measurements demonstrate a 30‐fold signal increase for thin film samples. This enhancement factor increases to a theoretical value of 7500 for samples confined to the active region of the antennas. These findings open the door to the elucidation of fundamental processes in nanoscale samples, including junctions in spintronic devices or biological membranes.