  We report on the preparation of a pure ultracold sample of bosonic DyK
Feshbach molecules, which are composed of the fermionic isotopes 161Dy and 40K.
Employing a magnetic sweep across a resonance located near 7.3 G, we produce up
to 5000 molecules at a temperature of about 50 nK. For purification from the
remaining atoms, we apply a Stern-Gerlach technique based on magnetic
levitation of the molecules in a very weak optical dipole trap. With the
trapped molecules we finally reach a high phase-space density of about 0.1. We
measure the magnetic field dependence of the molecular binding energy and the
magnetic moment, refining our knowledge of the resonance parameters. We also
demonstrate a peculiar anisotropic expansion effect observed when the molecules
are released from the trap and expand freely in the magnetic levitation field.
Moreover, we identify an important lifetime limitation that is imposed by the
1064-nm infrared trap light itself and not by inelastic collisions. The
light-induced decay rate is found to be proportional to the trap light
intensity and the closed-channel fraction of the Feshbach molecule. These
observations suggest a one-photon coupling to electronically excited states to
limit the lifetime and point to the prospect of loss suppression by optimizing
the wavelength of the trapping light. Our results represent important insights
and experimental steps on the way to achieve quantum-degenerate samples of DyK
molecules and novel superfluids based on mass-imbalanced fermion mixtures.
