  We propose a new application of single molecule magnet crystals: their use as
"magnetic bubble chambers" for the direct detection of sub-GeV dark matter. The
spins in these macroscopic crystals effectively act as independent nano-scale
magnets. When anti-aligned with an external magnetic field they form
meta-stable states with a relaxation time that can be very long at sufficiently
low temperatures. The Zeeman energy stored in this system can be released
through localized heating, caused for example by the scattering or absorption
of dark matter, resulting in a spin avalanche (or "magnetic deflagration") that
amplifies the effects of the initial heat deposit, enabling detection. Much
like the temperature and pressure in a conventional bubble chamber, the
temperature and external magnetic field set the detection threshold for a
single molecule magnet crystal. We discuss this detector concept for dark
matter detection and propose ways to ameliorate backgrounds. If successfully
developed, this detector concept can search for hidden photon dark matter in
the meV - eV mass range with sensitivities exceeding current bounds by several
orders of magnitude.
