  We report measurements of the microwave surface impedance of the borocarbide
family of superconductors LnNi2B2C (Ln=Y, Er, Tm, Ho). The experiments enable
direct measurements of the superfluid density, and are particularly sensitive
to the influence of magnetic pairbreaking.
  In HoNi2B2C the antiferromagnetic transition is clearly observed at zero
field, and leads to a drastic reduction of the superfluid density, which
recovers at lower temperatures.
  In ErNi2B2C the antiferromagnetic transition is not seen in zero field data.
Magnetic effects are responsible for anomalies in the low temperature surface
impedance below approximately 4K in HoNi2B2C and TmNi2B2C. The temperature
dependence of the microwave impedance disagrees with simple BCS calculations.
