Resonant spin tunneling in small antiferromagnetic particles

The paper reports a detailed experimental study on magnetic relaxation of n atural horse-spleen ferritin. ac susceptibility measurements performed on t hree samples of different concentration show that dipole-dipole interaction s between uncompensated moments play no significant role. Furthermore, the distribution of relaxation times in these samples has been obtained from a scaling of experimental chi " data, obtained at different frequencies. The average uncompensated magnetic moment per protein is compatible with a diso rdered arrangement of atomic spins throughout the core, rather than with su rface disorder. The observed field dependence of the blocking temperature s uggests that magnetic relaxation is faster at zero field than at intermedia te field values. This is confirmed by the fact that the magnetic viscosity peaks at zero field, too. Using the distribution of relaxation times obtain ed independently, we show that these results cannot be explained in terms o f classical relaxation theory. The most plausible explanation of these resu lts is the existence, near zero field, of resonant magnetic tunneling betwe en magnetic states of opposite orientation, which are thermally populated. [S0163-1829(99)08817-7].