  Spin current -- a flow of the spin degree of freedom in matter -- has vital
importance in spintronics. Propagation of the spin current ranges over a whole
momentum space; however, generated spin currents are mainly detected in the
long-wavelength limit. To facilitate practical uses of spintronics and
magnonics, microscopic understanding of the spin current is necessary. We here
address yttrium iron garnet, which is a well-employed ferrimagnet for
spintronics, and review {\it in re} the momentum- and energy-resolved
characteristics of its magnetism. Using {\it unpolarized} neutrons, we refined
its detailed crystal and magnetic structure, and examined magnetic excitations
through four decades (10~$\mu$eV-100~meV) using chopper spectrometers in
J-PARC, Japan. We also measured mode-resolved directions of the precessional
motion of the magnetic moment, i.e., magnon polarization, which carries the
spin current in insulators through {\it polarized} neutron scattering, using a
triple-axis spectrometer in ILL, France. The magnon polarization is a hitherto
untested fundamental property of magnets, affecting the thermodynamic
properties of the spin current. Our momentum- and energy-resolved experimental
findings provide an intuitive understanding of the spin current and demonstrate
the importance of neutron scattering techniques for spintronics and magnonics.
