Dislocation-induced spin tunneling inMn12acetate

Abstract

Comprehensive theory of quantum spin relaxation in Mn12 acetate crystals is developed, that takes into account imperfections of the crystal structure and is based upon the generalization of the LandauZener effect for incoherent tunneling from excited energy levels. It is shown that linear dislocations at plausible concentrations provide the transverse anisotropy which is the main source of tunneling in Mn12. Local rotations of the easy axis due to dislocations result in a transverse magnetic field generated by the field applied along the c-axis of the crystal, which explains the presence of odd tunneling resonances. Long-range deformations due to dislocations produce a broad distribution of tunnel splittings. The theory predicts that at subkelvin temperatures the relaxation curves for different tunneling resonances can be scaled onto a single master curve. The magnetic relaxation in the thermally activated regime follows the stretched-exponential law with the exponent depending on the field, temperature, and concentration of defects.

Electronic Transport in Single-Molecule Magnets on Metallic Surfaces

“…We adopt the model parameters from Refs. 6 and 10: D = 0.292K, E = 0.046K, C = −3.2 × 10 −5 K. The tunnel splitting ∆ M is calculated for H x = 0.1H z at the resonant field by employing the numerical diagonalization [10] or the perturbation method [15]. We obtain qualitatively the same results when H x has the fixed value at all resonant fields [16].…”

Resonant Tunneling in Truly Axial SymmetryMn12Single-Molecule Magnets: Sharp Crossover between Thermally Assisted and Pure Quantum Tunneling

“…It was also the first system to show thermally assisted tunneling of magnetization [2,3,4]. During the last several years, many more SMMs have been discovered and they are now among the most promising candidates for observing the limits between classical and quantum physics since they have a well defined structure, spin ground state and magnetic anisotropy [5,6,7,9].Nevertheless, Mn 12 -Ac is still the most widely studied SMM [10,11,12,13,14,15,16,17,18,19,20,21,22]. While a rough understanding of the quantum phenomena in Mn 12 -Ac was clear from the early studies, a detailed understanding has not yet emerged.…”

Tunneling and EPR linewidths due to dislocations in Mn acetate

“…In spin tunneling and EPR experiments the resonant values of the magnetic field are determined by the anisotropy constants. For Mn 12 crystals in the field parallel to the easy axis, with the Hamiltonian H = −DS 2 z − H z S z + H ′ (H ′ being a small tunneling term), the resonant spin tunneling occurs atwhile the EPR between the levels m and m−1 at frequency ω occurs atIn our two recent works [10,3] we computed H ′ due to dislocations and neglected the effect of dislocations on the resonance condition. Meantime, the spatial dependence of the magnetic anisotropy D due to dislocations causes resonances to spread over a certain field range.…”

“…[3] for references). Despite of a significant progress made in understanding Mn 12 and later discovered Fe 8 spin-10 systems, a number of key questions remains unanswered.…”

“…In our two recent works [10,3] we computed H ′ due to dislocations and neglected the effect of dislocations on the resonance condition. Meantime, the spatial dependence of the magnetic anisotropy D due to dislocations causes resonances to spread over a certain field range.…”