Spin Tunneling via Dislocations in ${\mathrm{Mn}}_{12}$ Acetate Crystals

We show that dislocations should be the main source of spin tunneling in ${\mathrm{Mn}}_{12}$ crystals. Long range strains caused by dislocations produce broad distribution of relaxation times that has been seen in many experiments. When the external magnetic field is applied along the $c$ axis of the crystal, local rotations of the magnetic anisotropy axis due to dislocations result in the effective local transverse magnetic field that unfreezes odd tunneling resonances. Scaling law is derived that provides universal description of spin tunneling for all resonances.