Experimental evidence of the dependence of spin tun - nelling on the concentration of dislocations in Mn 12 crys - tals

– We present experimental results on resonant spin tunnelling in a single crystal of Mn12-2Cl benzoate with different concentration of dislocations. The time evolution of the magnetisation follows the stretched exponential, M (t) ∝ exp[−(t/τ) β ] over a few time decades. The values of τ and β deduced from experiment have been used to determine the concentration of dislocations before and after the cooling-annealing process, using the algorithm recently suggested by Garanin and Chudnovsky. The crystals of molecular clusters are soft and fragile. They must contain a large number of dislocations which rearrange atoms in their immediate vicinity. Dislocations are formed by the frozen in lattice disorder when crystallization takes place. In the case of molecular clusters, the dislocations can also be introduced by performing thermal cycles which produce dilations and compressions of the lattice. Many techniques are now available for the study of dislocations. The two most important techniques are X-ray diffraction and transmission electron microscopy. a new theoretical approach [12] has been suggested which may fully explain the mechanism of thermally assisted resonant tunnelling in crystals of Mn 12 molecular clusters. Chudnovsky and Garanin have theoretically shown [12, 13] that spin tunnelling in Mn 12 clusters must be dominated by dislocations. Some of the theoretical suggestions of Chudnovsky and Garanin have been recently confirmed in experiments [14, 15, 16]. In this paper we report results of our study of the effect of crystal defects on the tunnelling process. For this purpose, we have compared the results of quantum magnetic relaxation experiments on a single crystal of Mn 12-2Cl benzoate (Mn 12 Cl) before and after suffering thermal shocks. At low temperature the intramolecular exchange interactions render an effective spin S=10 for this molecular cluster. The Mn 12 Cl crystallizes in an orthorhombic structure, a = 2.275