Magnetic Resonance Force Microscopy at Millikelvin Temperatures

Magnetic resonance force microscopy (MRFM) combines ultrasensitive force detection with magnetic resonance to manipulate and detect small ensembles of spins. One of the goals of our group is the detection of individual electron spins. In order to achieve single-spin detection, measurements need to be done at very low temperatures, both to reduce the thermo-mechanical noise of the cantilever and to increase the spin relaxation times. In this talk we present results from measurements on ensembles of E' centers in fused silica performed using a MRFM apparatus operating at 100 mK. Considerable technical challenges have been overcome, in particular detecting the cantilever displacement without self-heating, and generating a strong microwave magnetic field within the 400 microwatt power budget of the refrigerator. We have succeeded in demonstrating spin temperatures below 150 mK and spin-lattice relaxation times greater than 1200 seconds. We will also discuss studies of spin dynamics, such as spin-lattice relaxation in the rotating frame and spin-lock lifetimes during cantilever modulation.