Application of SQUIDs to low temperature and high magnetic field measurements - Ultra low noise torque magnetometry.

Torque magnetometry is a key method to measure the magnetic anisotropy and quantum oscillations in metals. In order to resolve quantum oscillations in sub-millimeter sized samples, piezo-electric micro-cantilevers were introduced. In the case of strongly correlated metals with large Fermi surfaces and high cyclotron masses, magnetic torque resolving powers in excess of $10^4$ are required at temperatures well below $1\,\mathrm{K}$ and magnetic fields beyond $10\,\mathrm{T}$. Here, we present a new broadband read-out scheme for piezo-electric micro-cantilevers via Wheatstone-type resistance measurements in magnetic fields up to $15\,\mathrm{T}$ and temperatures down to $100\,\mathrm{mK}$. By using a two-stage SQUID as null detector of a cold Wheatstone bridge, we were able to achieve a magnetic moment resolution of $\Delta m= 5\times10^{-15}\,\mathrm{J/T}$ at maximal field, outperforming conventional magnetometers by at least two orders of magnitude in this temperature and magnetic field range. Exemplary de Haas-van Alphen measurement of a newly grown delafossite, PdRhO$_2$, were used to show the superior performance of our setup.