Neutron Scattering Study on Yttrium Iron Garnet for Spintronics

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 in re the momentum- and energy-resolved characteristics of its magnetism. Using unpolarized neutrons, we refined its detailed crystal and magnetic structure, and examined magnetic excitations through four decades (10 µeV–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 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.