The symmetry of the spin Hamiltonian in herbertsmithite, a spin-1/2 kagom\'{e} lattice

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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 Ising-like exchange anisotropy.