Journal
PHYSICAL REVIEW B
Volume 79, Issue 21, Pages -Publisher
AMER PHYSICAL SOC
DOI: 10.1103/PhysRevB.79.214404
Keywords
magnetic anisotropy; magnetic particles; magnetic relaxation; magnetic structure; manganese compounds; molecular magnetism; organic compounds; particle size; X-ray absorption spectra
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We show that reducing the crystal size of [Mn12O12(O2C6H5)(16)(H2O)(4)] single-molecule magnets from 11.5 to 0.4 mu m strongly affects the molecular magnetic anisotropy and magnetic-relaxation rates. The effective activation energy for the spin reversal of the standard clusters decreases by 13% with decreasing size, whereas it remains approximately constant for the fast relaxing species. The pre-exponential factor tau(0) increases with decreasing crystal size for both. The observed decrease in the effective energy barrier for the slow relaxing species seems to be associated with the existence of a distribution of second-order transverse anisotropy terms, centered on E=0, which broadens as the crystal size decreases. By contrast, the expected changes in the axial anisotropy parameter D with decreasing crystal size are too small to account for the change in U. The different effects that the reduction in crystal size has on the fast and slow relaxing species are discussed.
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