Low-field EPR studies of levels near the top of the barrier in Mn12-acetate reveal a new magnetization relaxation pathway

We show that X-band electron paramagnetic resonance (EPR) measurements using a dual-mode resonance cavity can directly probe the levels near the top of the magnetization reversal barrier in the single-molecule magnet (SMM) Mn12-acetate. The observed transitions are much sharper than those reported in high-field EPR studies. The observed temperature dependence of the line positions points to the presence of a spin-diffusional mode. The correlation time for such fluctuations is of the order of 6×10−8 s at 10 K, and follows an Arrhenius activation energy of 35-40 K. These results open a new avenue for understanding the mechanism of tunneling and spin-lattice relaxations in these SMMs.