Detection of Individual Vortices in Micron-Size Sr2RuO4 Rings by Phase-Locked Cantilever

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^-19J/T at an applied field of 1 G.