Microwave-induced spin currents in ferromagnetic-insulator|normal-metal bilayer system

Abstract

A microwave technique is employed to simultaneously examine the spin pumping and the spin Seebeck effect processes in a YIG|Pt bilayer system. The experimental results show that for these two processes, the spin current flows in opposite directions. The temporal dynamics of the longitudinal spin Seebeck effect exhibits that the effect depends on the diffusion of bulk thermal-magnons in the thermal gradient in the ferromagneticinsulator|normal-metal system.Since its discovery in 2008, the spin Seebeck effect 1 -a route to generate a spin current by applying a heat current to ferromagnets-has given a new dimension to the field of spin-caloritronics 2 . In particular, the longitudinal spin Seebeck effect (LSSE) 3 , where the spin current flows along the thermal gradient in the magnetic material, drives the field due to its technologically promising applications in energy harvesting 4 , and in temperature, temperature gradient, and position sensing 5 . Having conceptual understanding and future applications in the centre of attention, a comparative study of the spin current direction and its temporal evolution for different spin-current-generation processes like spin pumping (SP) and spin Seebeck effect (SSE) is very important. In previous experiments 6,7 , these issues have not been explicitly addressed. In this letter, we demonstrate microwaves as a simple-and-controlled tool to investigate both, SP and SSE, processes simultaneously in a single experiment. Such investigations are not possible with other techniques including laser heating 8 or direct-current heating 9 employed to study the SSE. For example, in Ref. 10, the direction of the spin current in the SP and SSE processes has been determined by combining the FMR technique with additional Peltier or dc/ac based heating techniques. Our results reveal that in a ferromagnet|normal metal (paramagnet) system , the spin current flows from the ferromagnet (FM) to the normal metal (NM) in the case of SP process, while the flow reverses for the LSSE provided that the NM is hotter than the FM. The time-resolved measurements show that the spin current dynamics of the LSSE is on submicrosecond timescale compared to nanosecond fast spin pumping process 11 . The experiment was realized using a bilayer of a magnetic insulator, Yttrium Iron Garnet (YIG), and a normal metal, Pt. The sample structure consists a 6.7-µm-thick YIG film of dimensions 14 mm × 3 mm, grown by a) Electronic

Length Scale of the Spin Seebeck Effect

“…5, we evaluate each series with Eq. (1), obtaining mean magnon propagation lengths between 90 and 140 nm for the PLD samples, while for the LPE samples at RT, we can only give an upper limit for a propagation length of 1 μm, in agreement with the time dependent measurements [16,17]. For 50 K, the fit yields a value of ð8 AE 2Þ μm for the YIG/Pt samples and a comparable one of ð6 AE 2Þ μm for the samples with a Cu spacing layer.…”

“…In contrast, for μm thick films, a characteristic rise time was found, and a finite magnon propagation length of the order of several 100 nm was put forward as a possible explanation [16,17]. This clearly calls for study to reveal the origin of this discrepancy as it underlies the fundamental mechanism of the SSE and to determine the intrinsic length scale.…”

Spin Seebeck effect and ballistic transport of quasi-acoustic magnons in room-temperature yttrium iron garnet films

“…In this experiment, the microwave-induced heating of the Pt layer is used to create the thermal gradient in YIG/Pt bilayers [22]. The Pt film is heated by eddy currents, induced in the metal by a microwave field.…”

“…Recently, a number of studies on the time dependency of the LSSE in YIG/Pt bilayers have been presented [20][21][22][23][24][25] and a rise time of the LSSE-induced ISHE voltage (V LSSE ) of a few hundred nanoseconds was reported. Furthermore, a magnon propagation length of about L ∼ 500 nm was determined from the time-dependent measurements [20,22] in a 6.7 μm thick YIG film, using a diffusion model of thermally driven magnons.…”

“…Furthermore, a magnon propagation length of about L ∼ 500 nm was determined from the time-dependent measurements [20,22] in a 6.7 μm thick YIG film, using a diffusion model of thermally driven magnons. However, these values of the propagation length are not unanimous and vary in the literature [20,23,[25][26][27][28] between several hundred nanometers and a few micrometers.…”