  We have measured the backaction of a dc superconducting quantum interference
device (SQUID) position detector on an integrated 1 MHz flexural resonator. The
frequency and quality factor of the micromechanical resonator can be tuned with
bias current and applied magnetic flux. The backaction is caused by the Lorentz
force due to the change in circulating current when the resonator displaces.
The experimental features are reproduced by numerical calculations using the
resistively and capacitively shunted junction (RCSJ) model.
