Haydeeite: A spin-1/2 kagome ferromagnet

The mineral haydeeite, alpha-MgCu3(OD)(6)Cl-2, is a S = 1/2 kagome ferromagnet that displays long-range magnetic order below T-C = 4.2 K with a strongly reduced moment. Our inelastic neutron scattering data show clear spin-wave excitations that are well described by a Heisenberg Hamiltonian with a ferromagnetic nearest-neighbor exchange J(1) = -38 K and an antiferromagnetic exchange J(d) = +11 Kacross the hexagons of the kagome lattice. These values place haydeeite in the ferromagnetic part of the phase diagram, close to the cuboc2 type noncoplanar 12-sublattice antiferromagnetic order of cuboctahedral symmetry. Diffuse dynamic short-range ferromagnetic correlations observed above T-C persist well into the ferromagnetically ordered phase with a behavior distinct from critical scattering.