"We present the earliest observation and first detection of polarized millimeter emission in a γ-ray burst with ALMA Cycle 7 Band 3 (97.5 GHz) observations of GRB 190114C. With observations spanning 2.2 to 5.2 hours after the burst, we detect linear polarization in the GRB afterglow at ~ 5σ, decreasing from Π=(0.87±0.13)% to (0.60±0.19)%, and evolving in polarization position angle from (10±5) degrees to (−44±12) degrees. We show that the optical and X-ray observations between 0.03 days and ∼0.3
days are consistent with a fast cooling forward shock expanding into a wind environment. However, the optical observations at ≲0.03 days, as well as the radio and millimeter observations arise from a separate component, which we interpret as emission from the reverse-shocked jet. Using the measured linear polarization, we constrain the coherence scale of tangled magnetic fields in the GRB jet to an angular size of θ ≈0.001 radian, while the rotation of the polarization angle rules out the presence of large scale, ordered axisymmetric magnetic fields, and in particular the popular toroidal magnetic field model for GRBs outflows. "