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The mechanisms causing millimeter-wave polarization in protoplanetary disksare under debate. To disentangle the polarization mechanisms, we observe theprotoplanetary disk around HL Tau at 3.1 mm with the Atacama LargeMillimeter/submillimeter Array (ALMA), which had polarization detected withCARMA at 1.3 mm. We successfully detect the ring-like azimuthal polarizedemission at 3.1 mm. This indicates that dust grains are aligned with the majoraxis being in the azimuthal direction, which is consistent with the theory ofradiative alignment of elongated dust grains, where the major axis of dustgrains is perpendicular to the radiation flux. Furthermore, the morphology ofthe polarization vectors at 3.1 mm is completely different from those at 1.3mm. We interpret that the polarization at 3.1 mm to be dominated by the grainalignment with the radiative flux producing azimuthal polarization vectors,while the self-scattering dominates at 1.3 mm and produces the polarizationvectors parallel to the minor axis of the disk. By modeling the totalpolarization fraction with a single grain population model, the maximum grainsize is constrained to be $100{\rm~\mu m}$, which is smaller than the previouspredictions based on the spectral index between ALMA at 3 mm and VLA at 7 mm.

The evidence of radio polarization induced by the radiative grain alignment and self-scattering of dust grains in a protoplanetary disk