  The new NIKA2 camera at the IRAM 30m radiotelescope was used to observe three
known debris disks in order to constrain the SED of their dust emission in the
millimeter wavelength domain. We have found that the spectral index between the
two NIKA2 bands (1mm and 2mm) is consistent with the Rayleigh-Jeans regime
(lambda^{-2}), unlike the steeper spectra (lambda^{-3}) measured in the
submillimeter-wavelength domain for two of the three disks $-$ around the stars
Vega and HD107146. We provide a succesful proof of concept to model this
spectral inversion in using two populations of dust grains, those smaller and
those larger than a grain radius a0 of 0.5mm. This is obtained in breaking the
slope of the size distribution and the functional form of the absorption
coefficient of the standard model at a0. The third disk - around the star
HR8799 - does not exhibit this spectral inversion but is also the youngest.
