1fm/$\surd $Hz Noise Level Low Temperature Atomic Force {\&} Magnetic Force Microscope (LT-AFM/MFM) in 20mK-300K Temperature Range

We describe the design of a new low temperature Fabry-Perot interferometer for LT-AFM/MFM operating in 20mK-300K Temperature range. We used a multilayer dielectric mirror coated optical fiber to achieve 1fm/$\surd $Hz Noise Level, while the shot noise limit was 0.16fm/$\surd $Hz. The fibre can be brought very close to the cantilever using a dedicated 2mm stroke piezo nanopositioner integrated in the piezo tube scanner. The same nanopositioner is used to park the fibre to a safe parking location during cantilever exchange. The LT-AFM/MFM can be used between 6 $\mu$W-3mW laser power. We have demonstrated performance of the LT-AFM/MFM by imaging a hard disk sample between 1.5-300K and Abrikosov vortex lattice in BSCCO single crystal at 4K.