Nonexponential dynamic scaling of the magnetization relaxation in Mn12 acetate

We present new results on the zero-field magnetic relaxation in a single crystal of Mn(12) acetate. We find three relaxation regimes. Above 2.8 K, the relaxation is exponential. Below this temperature, deviations are observed. These deviations are more pronounced when temperature decreases between 2.4 and 2.0 K. Below 2.0 K they lead to a square root relaxation. A scaling analysis is proposed, which shows the dipolar interactions can be treated as a time-dependent local mean field in the thermally activated tunneling regime. These evolutions of local mean field produce deviations from the exponential relaxation and can explain the square root regime at lower temperature.