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Register a new userA Microscopic and Spectroscopic View of Quantum Tunneling of Magnetization
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This chapter takes a microscopic view of quantum tunneling of magnetization (QTM) in single-molecule magnets (SMMs), focusing on the interplay between exchange and anisotropy. Careful consideration is given to the relationship between molecular symmetry and the symmetry of the spin Hamiltonian that dictates QTM selection rules. Higher order interactions that can modify the usual selection rules are shown to be very sensitive to the exchange strength. In the strong coupling limit, the spin Hamiltonian possess rigorous $D_{2h}$ symmetry (or $C_{infty}$ in high-symmetry cases). In the case of weaker exchange, additional symmetries may emerge through mixing of excited spin states into the ground state. Group theoretic arguments are introduced to support these ideas, as are extensive results of magnetization hysteresis and electron paramagnetic resonance measurements.
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A Mn30 molecular cluster is established to be the largest single-molecule magnet (SMM) discovered to date. Magnetization versus field measurements show coercive fields of about 0.5 T at low temperatures. Magnetization decay experiments reveal an Arrhenius behavior and temperature-independent relaxation below 0.2 K diagnostic of quantum tunneling of magnetization through the anisotropy barrier.The quantum hole digging method is used to establish resonant quantum tunneling. These results demonstrate that large molecular nanomagnets,having a volume of 15 nm^3, with dimensions approaching the mesoscale can still exhibit the quantum physics of the microscale.
We present low temperature magnetometry measurements on a new Mn3 single-molecule magnet (SMM) in which the quantum tunneling of magnetization (QTM) displays clear evidence for quantum mechanical selection rules. A QTM resonance appearing only at elevated temperatures demonstrates tunneling between excited states with spin projections differing by a multiple of three: this is dictated by the C3 symmetry of the molecule, which forbids pure tunneling from the lowest metastable state. Resonances forbidden by the molecular symmetry are explained by correctly orienting the Jahn-Teller axes of the individual manganese ions, and by including transverse dipolar fields. These factors are likely to be important for QTM in all SMMs.
We present magnetization measurements on the single molecule magnet Mn6, revealing various tunnel transitions inconsistent with a giant-spin description. We propose a dimeric model of the molecule with two coupled spins S=6, which involves crystal-field anisotropy, symmetric Heisenberg exchange interaction, and antisymmetric Dzyaloshinskii-Moriya exchange interaction. We show that this simplified model of the molecule explains the experimentally observed tunnel transitions and that the antisymmetric exchange interaction between the spins gives rise to tunneling processes between spin states belonging to different spin multiplets.
We extend a microscopic theory of polarization and magnetization to include the spin degree of freedom of the electrons. We include a general spin orbit coupling and Zeeman interaction term to account for the modifications to the dynamics upon treating the electrons as spinful particles. We find a contribution to the magnetization due to the intrinsic angular momentum of the electrons. Additionally, the charge current gains a component transverse to both this intrinsic magnetization and the electric field of the crystal lattice. The microscopic polarization and magnetization fields are introduced throughout an extended system using a set of orthogonal orbitals associated with each site. As well free charge and current density fields are introduced associated with charge movement from site to site. The sites act as natural expansion points for the microscopic fields allowing for the evaluation of multipole moments associated with the polarization and magnetization fields. Associated with the dipole moments are the respective macroscopic polarization and magnetization fields, from which we can extract various response tensors. We focus on topologically trivial insulators in the limit of uniform fields to recover the magnetoelectric polarizability (MP) tensor, which contains the accepted expression for the orbital magnetoelectric polarizability (OMP) tensor as well as an added explicitly spin dependent contribution. This general framework can then be extended to finite frequency responses.
Below 360 mK, Fe magnetic molecular clusters are in the pure quantum relaxation regime and we show that the predicted square-root time relaxation is obeyed, allowing us to develop a new method for watching the evolution of the distribution of molecular spin states in the sample. We measure as a function of applied field H the statistical distribution P(xi_H) of magnetic energy bias xi_H$ acting on the molecules. Tunneling initially causes rapid transitions of molecules, thereby digging a hole in P(xi_H) (around the resonant condition xi_H = 0). For small initial magnetization values, the hole width shows an intrinsic broadening which may be due to nuclear spins.
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