= Solution
{c}
Measure several allowed magnetic reflections from an oriented single crystal by <magnetic neutron diffraction>, preferably with polarization analysis. A <neutron> couples to the component of the ordered <magnetic moment> perpendicular to the scattering vector $\mathbf Q$. For a collinear arrangement, the geometric intensity factor is
$$
1-(\widehat{\mathbf Q}\cdot\widehat{\mathbf m})^2.
$$
Compare reflections whose $\mathbf Q$ point in different crystal directions, correcting for the <magnetic form factor> and known experimental geometry. Their relative intensities distinguish candidate spin directions; a moment parallel to $\mathbf Q$ gives no magnetic signal from that component. Polarized <neutrons> help separate nuclear and magnetic contributions. Observing only peak positions establishes periodicity, not the spin direction.
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