  We describe a feedback-based dynamic cantilever magnetometry technique
capable of achieving high magnetic moment sensitivity with low applied fields.
Using this technique, we have observed periodic entry of vortices into
mesoscopic Sr2RuO4 rings. The quantized jump in the magnetic moment of the
particle produced by individual vortices was measured with a resolution of
7x10^-16 e.m.u. at an applied field of 1 Oe.
