Abstract
We studied the transient behavior of the spin current generated by the longitudinal spin Seebeck effect (LSSE) in a set of platinum-coated yttrium iron garnet (YIG) films of different thicknesses. The LSSE was induced by means of pulsed microwave heating of the Pt layer and the spin currents were measured electrically using the inverse spin Hall effect in the same layer. We demonstrate that the time evolution of the LSSE is determined by the evolution of the thermal gradient triggering the flux of thermal magnons in the vicinity of the YIG/Pt interface. These magnons move ballistically within the YIG film with a constant group velocity, while their number decays exponentially within an effective propagation length. The ballistic flight of the magnons with energies above 20 K is a result of their almost linear dispersion law, similar to that of acoustic phonons. By fitting the time-dependent LSSE signal for different film thicknesses varying by almost an order of magnitude, we found that the effective propagation length is practically independent of the YIG film thickness. We consider this fact as strong support of a ballistic transport scenario—the ballistic propagation of quasi-acoustic magnons in room temperature YIG.
Highlights
A permanently growing interest in the field of spin-caloritronics, which combines thermoelectrics with spintronics and nanomagnetism, underlines the importance of spin currents [1] as an alternative to charge currents for the utilization in logic devices [2, 3, 4]
We study the transient evolution of the longitudinal spin Seebeck effect (LSSE) voltage VLSSE in yttrium iron garnet (YIG) films of different thicknesses
Using a modified magnon transport model [20] with the time decay defined by a ballistic flight model, we find that the magnon propagation length is practically independent from the YIG film thickness and represents a material property of YIG
Summary
A permanently growing interest in the field of spin-caloritronics, which combines thermoelectrics with spintronics and nanomagnetism, underlines the importance of spin currents [1] as an alternative to charge currents for the utilization in logic devices [2, 3, 4]. The magnon current can be created by a thermal gradient induced in a ferromagnet exposed to a magnetic field [6, 7] This effect is referred to as the longitudinal spin Seebeck effect (LSSE) [8, 9]. The temporal dynamics of the LSSE is tightly connected with fundamental properties of a magnon gas such as the magnon mean free path This physical quantity is crucial for the understanding of the transport properties of magnetic materials and the general peculiarities of magnon-phonon interaction [10, 11, 12, 13, 14, 15], as well as for the engineering of efficient LSSE-based spin-caloritronic devices [16, 17]
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