Temperature-driven ballistic magnon transport (Vol. 49 No.4)

Temperature-driven ballistic magnon transport
Temporal evolution of the spin Seebeck voltage VLSSE for different magnetic film thicknesses. Inset: A platinum-coated magnetic insulator YIG subject to a thermal gradient created by microwave heating of the Pt layer. A thermally actuated magnon spin current Js induces VLSSE.

Application of a temperature gradient to a magnetic medium leads to the generation of a spin current referred to as the longitudinal spin Seebeck effect (LSSE). In a magnetic insulator such a current is created by a flux of thermal magnons. Using spin-dependent electron scattering processes in the adjacent normal metal this current can be converted to an electric voltage. The voltage evolution is determined by the development of the temperature gradient ∇T(x,t) and by the characteristics of the magnon’s motion.
By analysis of the time-dependent LSSE voltages in platinum-coated Yttrium Iron Garnet (YIG) ferrimagnetic films, the authors assumed that thermally-driven magnons with energies above 20 K move through the YIG layer ballistically due to their almost linear quasi-acoustic dispersion law. Consequently, the interaction processes within the ‘acoustic’ magnon mode do not change the magnon propagation velocity, while the number of magnons decays exponentially within an effective propagation length of 425nm. This length was found to be mostly independent on film thickness that proves the ballistic magnon transport scenario.

T. B. Noack and eleven co-authors, Spin Seebeck effect and ballistic transport of quasi-acoustic magnons in room-temperature yttrium iron garnet films, J. Phys. D: Appl. Phys. 51, 234003 (2018)