Active Microlevers as Miniature Torque Magnetometers

An extremely sensitive miniature torque magnetometer using Si $p$-doped piezoresistive cantilevers is described. The magnetization $\vec{m}$ of very small magnetic or superconducting samples ($\leq$1~$\mu$g) deposited on the cantilever can be measured via the torque $\vec{\tau }=\vec{m}\times \vec{B}$ produced on them by an applied field $B$. The high resolution in the lever deflection of the order of 0.1~\AA{} corresponds to a torque sensitivity of better than $\Delta \tau \leq 10^{-14}$~Nm. In a homogeneous field of 1~Tesla this device allows magnetic moments as small as $\Delta m\simeq 10^{-15}$~Am$^2$ to be measured, a value far higher than that measurable by the best commercial SQUID magnetometers. Measurements performed on microcrystals of the high-$T_{\rm c}$ superconductors Bi$_2$Sr$_2$Ca$_1$Cu$_2$O$_8$ and Hg$_1$Ba$_2$Ca$_3$Cu$_4$O$_{10}$ in the static and dynamic modes demonstrate the excellent performance of this device at low temperature and in magnetic fields ranging between a few millitesla and 5~Tesla.