Magnetic excitations from the two-dimensional interpenetrating Cu framework in Ba2Cu3O4Cl2

We report detailed neutron scattering studies on ${\mathrm{Ba}}₂{\mathrm{Cu}}₃{\mathrm{O}}₄{\mathrm{Cl}}₂$. The compound consists of two interpenetrating sublattices of Cu, labeled as ${\mathrm{Cu}}_{\mathrm{A}}$ and ${\mathrm{Cu}}_{\mathrm{B}}$, each of which forms a square-lattice Heisenberg antiferromagnet. The two sublattices order at different temperatures and effective exchange couplings within the sublattices differ by an order of magnitude. This yields an inelastic neutron spectrum of the ${\mathrm{Cu}}_{\mathrm{A}}$ sublattice extending up to 300 meV and a much weaker dispersion of ${\mathrm{Cu}}_{\mathrm{B}}$ going up to around 20 meV. Using a single-band Hubbard model we derive an effective spin Hamiltonian. From this, we find that linear spin-wave theory gives a good description to the magnetic spectrum. In addition, a magnetic field of 10 T is found to produce effects on the ${\mathrm{Cu}}_{\mathrm{B}}$ dispersion that cannot be explained by conventional spin-wave theory.