Resonant magnetic tunnelling in Mn-12 2-Cl benzoate

We present a detailed experimental study of magnetic relaxation on it singl e crystal of Mn-12 2Cl-benzoate and compare with results obtained on a sing le crystal of Mn-12 acetate. Relaxation times obtained from a.c. susceptibi lity experiments for both samples show sharp minima at the same resonant fi elds. Although magnetic relaxation of Mn-12 2-Cl benzoate is faster at any field and temperature, activation energies in zero field are approximately equal. This is an indication that a small increase of the transverse fields magnitude (arising from dipole-dipole interactions) does not modify apprec iably tunnelling rates between spin states which are degenerate at zero fie ld. This result agrees with the predictions of a new theoretical model rece ntly developed by some of the authors.