  A series of compounds M$_{0.1}$Sr$_{0.9}$Cu$_2$(BO$_3$)$_2$ with Sr
substituted by M=Al, La, Na and Y were prepared by solid state reaction. XRD
analysis showed that these doping compounds are isostructural to
SrCu$_2$(BO$_3$)$_2$. The magnetic susceptibility from 1.9K to 300K in an
applied magnetic field of 1.0T and the specific heat from 1.9K to 25K in
applied fields up to 14T were measured. The spin gap is deduced from the low
temperature susceptibility as well as the specific heat. It is found that the
spin gap is strongly suppressed by magnetic fields. No superconductivity is
observed in all four samples.
