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Tuesday, March 10th 2015

4:30 pm:

The majority of ferrimagnets comprise nonequivalent spin sublattices that produce a net moment, which is an integer fraction of the magnetic moment on one site. We elaborate on an alternative mechanism that yields ferrimagnets with arbitrary net moments produced by a spin canting. This canting originates from the frustration of the spin-1/2 kagome lattice with ferromagnetic nearest-neighbor and antiferromagnetic next-nearest-neighbor couplings, as in the francisite mineral Cu_3 Bi(SeO_3)_2 Cl and its synthetic sibling compounds.

Classical treatment of the francisite spin model with only isotropic (Heisenberg) interactions leads to an infinitely degenerate ground state, where both ferrimagnetic (canted) and antiferromagnetic (spiral) phases have equal energies. Quantum corrections explored by the coupled-cluster method result in a marginal stabilization of the ferrimagnetic state. However, the main driving force behind the unconventional ferrimagnetism turns out to be the magnetic anisotropy of Dzyaloshinsky-Moriya (DM) type. By evaluating both isotropic and anisotropic (DM) terms in the spin Hamiltonian from first-principles DFT calculations, we establish a complete microscopic magnetic model of francisites and compare it to the experiment.

The opportunities for calculating isotropic exchange couplings and magnetic anisotropy parameters from DFT will be discussed.

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