Tunneling and EPR linewidths due to dislocations in Mn12 acetate

– We compute the width and shape of the EPR and tunneling resonances due to dislocations in Mn-12 acetate crystals. Uncorrelated dislocations produce the Gaussian shape of resonances while dislocations bound in pairs produce the Lorentzian shape. We stress that the uniaxial spin Hamiltonian together with crystal defects can explain the totality of experimental data on Mn-12. The discovery of resonant spin tunneling in Mn12 acetate [1, 2] has triggered an avalanch of theoretical and experimental works on molecular nanomagnets (see Ref. [3] for references). Despite of a significant progress made in understanding Mn12 and later discovered Fe8 spin-10 systems, a number of key questions remains unanswered. One of them is the width and shape of the tunneling resonance. Recently, we have suggested that quantum magnetic relaxation in molecular nanomagnets can be explained by dislocations in the crystal lattice [4, 3]. Four recent experimental works give evidence of the effect of defects on tunneling and EPR in single crystals of Mn12 and Fe8 [5, 6, 7, 8], in accordance with our suggestions. The analysis of these experiments requires computation of the width and shape of EPR and tunneling resonances due to dislocations, which is done in this Letter. We argue that dislocations at common concentrations provide the observed width of tunneling resonances and the observed width of the EPR in Mn12 and Fe8. Qualitatively, the importance of dislocations is clear from the fact that they give rise to long-ranged elastic strains which modulate crystal fields and thus create spatial dependence of the magnetic anisotropy. In spin tunneling and EPR experiments the resonant values of the magnetic field are determined by the anisotropy constants. For Mn12 crystals in the field parallel to the easy axis, with the Hamiltonian H = −DS2 z − HzSz + H′ (H′ being a small tunneling term), the resonant spin tunneling occurs at Hz = kD, k = 0,±1,±2, . . . ,±(2S − 1), (1) while the EPR between the levels m and m− 1 at frequency ω occurs at Hz = ω −D(2m− 1). (2) In our two recent works [4, 3] we computed H′ due to dislocations and neglected the effect of dislocations on the resonance condition. Meantime, the spatial dependence of the magnetic Typeset using EURO-TEX 2 EUROPHYSICS LETTERS