  In the family of double perovskites, colossal magnetoresistance (CMR) has
been so far observed only in half-metallic ferrimagnets such as the known case
Sr$_2$FeMoO$_6$ where it has been assigned to the tunneling MR at grain
boundaries due to the half-metallic nature. Here we report a new
material-Tl$_2$NiMnO$_6$, a relatively ordered double perovskite stablized by
the high pressure and high temperature synthesis-showing CMR in the vicinity of
its Curie temperature. We explain the origin of such effect with neutron
diffraction experiment and electronic structure calculations that reveal the
material is a ferromagnetic insulator. Hence the ordered Tl$_2$NiMnO$_6$ (~70%
of Ni$^{2+}$/Mn$^{4+}$ cation ordering) represents the first realization of a
ferromagnetic insulating double perovskite, showing CMR. The study of the
relationship between structure and magnetic properties allows us to clarify the
nature of spin glass behaviour in the disordered Tl$_2$NiMnO$_6$ (~31% of
cation ordering), which is related to the clustering of antisite defects and
associated with the short-range spin correlations. Our results highlight the
key role of the cation ordering in establishing the long range magnetic ground
state and lay out new avenues to exploit advanced magnetic materials in double
perovskites.
