Resonant magnetic quantum tunneling through thermally activated states

A theory of the magnetic relaxation of a large spin (S=10) having a large u niaxial magnetocrystalline anisotropy is outlined. The theory explains magn etic relaxation observed in Mn,, acetate. The joint action of local fields and a fourth order distortion of the magnetocrystalline anisotropy is neces sary to account for the tunneling that is observed, between the m = -4 and m = 4 unperturbed states. Even in resonance, tunneling takes place mainly i ncoherently in Mn-12 acetate. It proceeds through the lowest energy state d oublet which is not blocked by longitudinal local fields. The relaxation ra te Gamma has been calculated using a master equation. The model gives magne tization hysteresis loops and ac magnetic susceptibility curves which are i n quantitative agreement with experimental results. For temperatures below 0.5 K approximately, nonresonant tunneling from the ground state becomes th e dominant relaxation mechanism. (C) 1998 American Institute of Physics. [S 0021-8979(98)18611-5].