Probing Magnetic Excitations in Co^II Single-Molecule Magnets by Inelastic Neutron Scattering

Co(acac)₂(H₂O)₂ (1, acac = acetylacetonate), a transition metal complex ( $S=3/2$ ), displays field-induced slow magnetic relaxation as a single-molecule magnet. For 1 and its isotopologues Co(acac)₂(D₂O)₂ (1-d ₄ ) and Co(acac-d ₇)₂(D₂O)₂ (1-d ₁₈ ) in approximately $D_{4h}$ symmetry, zero-field splitting of the ground electronic state leads to two Kramers doublets (KDs): lower energy $M_S=\pm 1/2\left({\varphi }_{1,2}\right)$ and higher energy $M_S=\pm 3/2\left({\varphi }_{3,4}\right)$ states. This work employs inelastic neutron scattering (INS), a unique method to probe magnetic transitions, to probe different magnetic excitations in 1-d ₄ and 1-d ₁₈ . Direct-geometry, time-of-flight Disk-Chopper Spectrometer (DCS), with applied magnetic fields up to 10 T, has been used to study the intra-KD transition as a result of Zeeman splitting, $M_S=-1/2\left({\varphi }_1\right)\to M_S=+1/2\left({\varphi }_2\right)$ , in 1-d ₁₈ . This is a rare study of the $M_S=-1/2\to M_S=+1/2$ excitation in transition metal complexes by INS. Indirect-geometry INS spectrometer VISION has been used to probe the inter-KD, ZFS transition, $M_S=\pm 1/2\left({\varphi }_{1,2}\right)\to M_S=\pm 3/2\left({\varphi }_{3,4}\right)$ in both 1-d ₄ and 1-d ₁₈ , by variable-temperature (VT) properties of this excitation. The INS spectra measured on VISION also give phonon features of the complexes that are well described by periodic DFT phonon calculations.