Single-ion and exchange anisotropy effects and multiferroic behavior in high-symmetry tetramer single-molecule magnets
Abbreviated Journal Title
Phys. Rev. B
QUANTUM PHASE INTERFERENCE; WEAK FERROMAGNETISM; SPIN ANISOTROPY; ENERGY-BARRIER; MAGNETIZATION; COMPLEXES; CLUSTERS; NI-4; ORIGIN; FAMILY; Physics, Condensed Matter
We study single-ion and exchange anisotropy effects in equal-spin s(1) tetramer single-molecule magnets exhibiting T(d), D(4h), D(2d), C(4h), C(4), or S(4) ionic site point group symmetry. We first write the group-invariant quadratic single-ion and symmetric anisotropic exchange Hamiltonians in the appropriate local coordinates. We then rewrite these local Hamiltonians in the molecular or laboratory representation, along with the group-invariant Dzyaloshinskii-Moriya (DM) and isotropic Heisenberg, biquadratic, and three-center quartic Hamiltonians: Using our exact, compact forms for the single-ion spin matrix elements, we analytically evaluate the eigenstate energies to first order in the microscopic anisotropy interactions, corresponding to the strong exchange limit, and provide tables of simple formulas for the anisotropy energies of the lowest four eigenstate manifolds of ferromagnetic (FM) and antiferromagnetic (AFM) tetramers with arbitrary s(1). For AFM tetramers, we illustrate the first-order level-crossing inductions for s(1) = 1/2, 1, and 3/2, and obtain preliminary estimates of the microscopic parameters in a Ni(4) from fits to magnetization data, indicating the presence of strong symmetric anisotropic exchange interactions. Accurate analytic expressions for the thermodynamics, electron paramagnetic resonance absorption, and inelastic neutron scattering cross section are given, allowing for a determination of three of the microscopic anisotropy interactions from the second excited state manifold of FM tetramers. We also predict that tetramers with symmetries S(4) and D(2d) should exhibit both DM interactions and multiferroic states, and we illustrate our predictions for s(1)= 1/2 and 1.
Physical Review B
"Single-ion and exchange anisotropy effects and multiferroic behavior in high-symmetry tetramer single-molecule magnets" (2008). Faculty Bibliography 2000s. 552.