Resonant Ultrasound Studies of Ferromagnetic Yb$_{14}$MnSb$_{11}$

The compound Yb$_{14}$MnSb$_{11}$ is believed to be a rare example of an under-screened Kondo lattice. In this material the Yb ions are divalent and hence nonmagnetic. The magnetism comes from the Mn 3d electrons and the antiferromagnetic coupling of these electrons to holes in the Sb 5p bands. The antiferromagnetic coupling between a local magnetic moment and extended Bloch states can result in ferromagnetism due to the RKKY interaction, but can also give rise to Kondo physics. Here we report measurements of the elastic moduli (obtained using resonant ultrasound spectroscopy) as a function of temperature (2-300K) and magnetic field (0-2 T) for Yb$_{14}$MnSb$_{11}$. An unusual lattice stiffening is observed below Tc. Attempts to model this behavior using Landau theory will be discussed.