Excitations and long-range-order in a crystalline ferromagnetic semiconductor*

We present polarized and unpolarized neutron diffraction measurements as well as inelastic neutron scattering measurements examining the dilute ferromagnetic semiconductor Yb$_{14}$MnSb$_{11}$. We find that the system consists of RKKY ferromagnetic exchange coupled Mn$^{2+}$ sites with nearest and next nearest neighbor exchange interactions dominating the magnetic spectrum. We observe a distribution of negative magnetization density throughout the crystal structure with no significant negative magnetization on any single site. The extended spread of a negative magnetization around each of the Mn ions supports a Kondo screening cloud scenario for Yb$_{14}$MnSb$_{11}$.