  We measure the mode-resolved direction of the precessional motion of the
magnetic order, i.e., magnon polarization, via the chiral term of inelastic
polarized neutron scattering spectra. The magnon polarisation is important in
spintronics, affecting thermodynamic properties such as the magnitude and sign
of the spin Seebeck effect. The observation of both signs of magnon
polarization in Y3Fe5O12 also gives direct proof of its ferrimagnetic nature.
The experiments agree very well with atomistic simulations of the scattering
cross section.
