TUNNELING IN MESOSCOPIC MAGNETIC MOLECULES

We present a theoretical study of magnetic relaxation of big molecules , Mn12O12, at low temperature, when the relaxation time tau is tempera ture independent. If the magnetic field h(z) is not too low, tunneling can only take place if energy is exchanged with phonons. Then, 1/tau similar to h(z)(3) for weak h(z). A steeper increase is expected for h igher h(z). The effect of a transverse field h, is small. The maximum of tau(h(z)) observed experimentally at h(z) = 0.2 T might be due to t unneling without phonons in low field, with a relaxation rate reduced by hyperfine interactions.