Symmetry of the spin Hamiltonian for herbertsmithite: A spin-1/2 kagome lattice

We present magnetization measurements on oriented powder of ZnCu(3)(OH)(6)Cl(2) along and perpendicular to the orienting field. We find a dramatic difference in the magnetization between the two directions. It is biggest at low measurement fields H or high temperatures. We show that the difference at high temperatures must emerge from the Ising-type exchange anisotropy. This allows us to explain muon spin rotation data at T -> 0 in terms of an exotic ferromagnetic ground state.