Polarized neutron diffraction (PND) is a powerful technique to distinguish a weak magnetic contribution from the total scattering intensity. It can provide a detailed insight into the microscopic spin ordering at the unit cell level, but also into the mesoscopic magnetic ordering, like different types of domain populations. Here we report on the application of this technique to the long-standing problem of determining the absolute direction of the Dzyaloshinskii-Moriya vector in relation to the

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... crystal structure. The proposed PND method, based on the measurement of one representative reflection, is easy to perform and straightforward to interpret. The absolute sign of the Dzyaloshinskii-Moriya interaction (DMI) in MnCO 3 has been independently determined by PND and found to be in agreement with recent results obtained by resonant magnetic synchrotron scattering. This validates the method. In addition, the absolute DMI vector direction in the prototypical room-temperature weak ferromagnet α-Fe 2 O 3 (hematite) has been determined for the first time. To demonstrate the generality of our method, further examples with different symmetries are also presented. Ab initio calculations of the resulting weak noncollinear magnetization using the QUANTUM ESPRESSO package, considering DMI in addition to the symmetric magnetic exchange interaction, were also conducted and found to be in agreement with the experimental results from PND.