Low-temperature spin dynamics of ferromagnetic molecular ring {Cr₈Y₈}

The spin dynamics of {Cr₈Y₈}, a rare example of ferromagnetic molecular rings, has been studied by inelastic neutron scattering (INS) and heat capacity (HC) methods. Clear evidence of low-lying magnetic excitation has been found. Magnetic Schottky anomalies are observed in low-temperature (low-T) HC curves measured under various fields and can be well fitted with a multi-level Schottky term, giving important information on the energy gaps between spin levels. The INS results obtained on TOFTOF and NEAT time-of-flight spectrometers show INS peaks corresponding to the transitions within S = 12 ground states and between the S = 12 ground state and the S = 11 excited state. The single-J model with a unique exchange constant J = 0.151 meV can well reproduce the low-lying energy levels and their Zeeman splitting upon applied magnetic fields. This work shows that the single-J model can be a good approach for the low-T spin dynamics of {Cr₈Y₈} and may have general significance for other weak ferromagnetic molecular rings. Determination of the Cr³⁺-Cr³⁺ exchange constant in {Cr₈Y₈} will benefit the study on the complicated magnetic interactions in chromium lanthanide complexes. The zero-field splitting which is suggested by HC data still calls for EPR or high-resolution INS technique to verify.