  We present polarimetric observations of four Class II protoplanetary disks
(DG Tau, Haro 6-13, RY Tau, and MWC 480) taken with the Atacama Large
Millimeter/submillimeter Array (ALMA) at 3 mm. The polarization morphologies
observed fall into two distinct categories: azimuthal polarization (DG Tau and
Haro 6-13) and polarization parallel to the disk minor axis (RY Tau and MWC
480). The mechanisms responsible for disk polarization at millimeter and
submillimeter wavelengths are currently under debate. In this Letter, we
investigate two mechanisms capable of producing polarized dust emission in
disks: self-scattering and grain alignment to the radiation anisotropy. The
polarization morphologies of DG Tau and Haro 6-13 are broadly consistent with
that expected from radiation alignment (though radiative alignment still does
not account for all of the features seen in these disks), while RY Tau and MWC
480 are more consistent with self-scattering. Such a variation in the polarized
morphology may provide evidence of dust grain size differences between the
sources.
