Tunneling and inversion symmetry in single-molecule magnets: The case of the Mn-12 wheel molecule
Abbreviated Journal Title
Phys. Rev. B
ANISOTROPIC SUPEREXCHANGE INTERACTION; QUANTUM RELAXATION; WEAK; FERROMAGNETISM; DOMAIN-WALL; LOOP STRUCTURE; NUCLEAR SPINS; MAGNETIZATION; PARTICLES; INTERFERENCE; NANOMAGNETS; Physics, Condensed Matter
We present a detailed study of the influence of various interactions on the spin quantum tunneling in a Mn-12 wheel molecule. The effects of single-ion and exchange (spin-orbit) anisotropy are first considered, followed by an analysis of the roles played by secondary influences, e.g., disorder, dipolar and hyperfine fields, and magnetoelastic interactions. Special attention is paid to the role of the antisymmetric Dzyaloshinski-Moriya (DM) interaction. This is done within the framework of a 12-spin microscopic model, and also using simplified dimer and tetramer approximations in which the electronic spins are grouped in two or four blocks, respectively. If the molecule is inversion symmetric, the DM interaction between the dimer halves must be zero. In an inversion symmetric tetramer, two independent DM vectors are allowed but no new tunneling transitions are generated by the DM interaction. In an effort to explain the experiments, we consider a breaking of the molecular inversion symmetry, and we explore this in detail using both models, focusing on the asymmetric disposition and rounding of Berry phase minima associated with quantum interference between states of opposite parity. A remarkable behavior exists for the "Berry phase zeroes" as a function of the directions of the internal DM vectors and the external transverse field. However, in both dimer and tetramer models, a rather drastic breaking of the molecular inversion symmetry is required to explain the experiments using DM interactions. Such a symmetry breaking cannot be attributed to sample disorder, and there is no evidence for it in experiments. In the absence of DM interactions, we suggest a number of other ways to explain the experiments. These results are of general interest for the quantum dynamics of tunneling spins, and lead to some interesting experimental predictions.
Physical Review B
"Tunneling and inversion symmetry in single-molecule magnets: The case of the Mn-12 wheel molecule" (2010). Faculty Bibliography 2010s. 104.