Resonant tunnelling of the magnetization in molecular crystal of (Mn_1-xCr_x)₁₂O₁₂(CH₃COO)₁₆(H₂O)₄\cdot2CH₃COOH\cdot4H₂O (x=0,0.03,0

The quantum tunnelling of magnetization (QTM) in single crystals of the single molecule magnet (Mn_1-xCr_x)12-Ac (x=0, 0.03, 0.04, 0.05) has been investigated. In comparison with its parent Mn12-Ac, a greater rate of magnetization relaxation and a lower effective potential-energy barrier have been observed in Cr-doping samples. This modulation of QTM due to the Cr-doping could be attributed to the small change of S_z due to the smaller spin of Cr itself and additional intrinsic but distributed transverse and longitudinal anisotropy raised by a subtle change of the local environment in the magnetic Mn12 core.