  Magnetic exchange in Kondo lattice systems is of the
Ruderman-Kittel-Kasuya-Yosida type, whose sign depends on the Fermi wave
vector, $k_F$ . In the simplest setting, for small $k_F$ , the interaction is
predominately ferromagnetic, whereas it turns more antiferromagnetic with
growing $k_F$. It is remarkable that even though $k_F$ varies vastly among the
rare-earth systems, an overwhelming majority of lanthanide magnets are in fact
antiferromagnets. To address this puzzle, we investigate the effects of a
p-wave form factor for the Kondo coupling pertinent to nearly all rare-earth
intermetallics. We show that this leads to interference effects which for small
kF are destructive, greatly reducing the size of the RKKY interaction in the
cases where ferromagnetism would otherwise be strongest. By contrast, for large
$k_F$, constructive interference can enhance antiferromagnetic exchange. Based
on this, we propose a new route for designing ferromagnetic rare-earth magnets.
