In some aspects, polarization of a spin ensemble can be increased using cavity-based techniques. A resonator applies a drive field to a spin ensemble in a sample in a static magnetic field. The drive field couples the spin ensemble with a cavity, and the coupling increases the polarization of the spin ensemble. In some cases, the sample is thermally insulated from the cavity, for example, to maintain the sample at a higher temperature than the cavity. In some implementations, the spin ensemble achieves a polarization that is higher than its thermal equilibrium polarization.