  CoSn is a Pauli paramagnet with relatively flat d-bands centered about 100
meV below the Fermi energy Ef. Single crystals of CoSn lightly doped with Fe,
In, or Ni are investigated using x-ray and neutron scattering, magnetic
susceptibility and magnetization, ac susceptibility, specific heat and
resistivity measurements. Within the rigid band approximation, hole doping with
a few percent of Fe or In should move the flat bands closer to Ef, whereas
electron doping with Ni should move the flat bands further away from Ef. We
provide evidence that this indeed occurs. Fe and In doping drive CoSn toward
magnetism, while Ni doping suppresses CoSn's already weak magnetic response.
The resulting ground state is different for Fe versus In doping. For Fe-doped
crystals, Co1-xFexSn, with 0.02 < x < 0.27, the magnetic and specific heat data
are consistent with the formation of a spin glass, with a glass transition
temperature, Tg, ranging from 1 K for x=0.02 to 10 K for x= 0.27. Powder and
single crystal neutron diffraction found no evidence of long-range magnetic
order below Tg with x = 0.17. For In-doped crystals, CoSn1-yIny, both the
magnetic susceptibility and the Sommerfeld coefficient, gamma, increase
substantially relative to pure CoSn, but with no clear indication of a magnetic
transition for 0.05 < y < 0.2. CoSn crystals doped with Ni (Co0.93Ni0.07Sn)
have a significantly smaller magnetic susceptibility and gamma than pure CoSn,
consistent with the flat bands further from Ef.
