Slave fermions and the infinite-U Anderson lattice above quarter-filling: Ferromagnetic spin excitations and antiferromagnetic instabilities at low electron count

We extend our earlier formulation for the Anderson lattice (using auxiliary fermions and bosons to represent the charge and spin, respectively) to calculate the spin wave excitation spectrum for a wide range of model parameters in one and two dimensions. We show that, for the case of nearest neighbor conduction electron hopping, the compensated spin state obtained in mean-field theory is bounded at an upper conduction electron concentration (where the spin wave velocity vanishes) and at a lower concentration (close to quarter-filling) by an antiferromagnetic instability, signaled by a vanishing spin wave energy at the appropriate zone boundary. In addition, the range of stable compensated ferromagnetism vanishes as the local f-level energy passes through the bottom of the band (from below). This suggests that only the Kondo regime where the f level lies well below the Fermi level shows a saturated ferromagnetic state.