Session Q2: Invited Session: Manipulating Spin Waves

Magnons, Spin Current and Spin Seebeck Effect

When metals and semiconductors are placed in a temperature gradient, the electric voltage is generated. This mechanism to convert heat into electricity, the so-called Seebeck effect, has attracted much attention recently as the mechanism for utilizing wasted heat energy. [1]. Ferromagnetic insulators are good conductors of spin current, i.e., the flow of electron spins [2]. When they are placed in a temperature gradient, generated are magnons, spin current and the spin voltage [3], i.e., spin accumulation. Once the spin voltage is converted into the electric voltage by inverse spin Hall effect in attached metal films such as Pt, the electric voltage is obtained from heat energy [4-5]. This is called the spin Seebeck effect. Here, we present the linear-response theory of spin Seebeck effect based on the fluctuation-dissipation theorem [6-8] and discuss a variety of the devices. \\[4pt] [1] S. Maekawa et al, \textit{Physics of Transition Metal Oxides} (Springer, 2004). \\[0pt] [2] S. Maekawa: Nature Materials \textbf{\textit{8}}, 777 (2009). \\[0pt] [3] \textit{Concept in Spin Electronics}, eds. S. Maekawa (Oxford University Press, 2006). \\[0pt] [4] K. Uchida et al., Nature \textbf{\textit{455}}, 778 (2008). \\[0pt] [5] K. Uchida et al., Nature Materials \textbf{\textit{9}}, 894 (2010) \\[0pt] [6] H. Adachi et al., APL \textbf{\textit{97}}, 252506 (2010) and Phys. Rev. B \textbf{\textit{83}}, 094410 (2011). \\[0pt] [7] J. Ohe et al., Phys. Rev. B (2011) \\[0pt] [8] K. Uchida et al., Appl. Phys. Lett. \textbf{\textit{97}}, 104419 (2010).   

Spin Wave Transport in Microscopic Magnetic Structures

The coherent transport of spin information is one of the great challenges in condensed matter physics and is of fundamental importance for the development of spintronic devices. Spin waves carry angular momentum and can be utilized to transport spin information over distances much larger than the spin diffusion length in metals. Recent experiments showing that spin waves can be manipulated via spin currents and vice versa due to spin torque, spin pumping, spin Hall and spin Seebeck effects have drawn great attention to the transport properties of spin waves. Fundamental topics are spin-wave propagation characteristics in microstructures with reduced dimensionality\footnote{H. Schultheiss, S. Sch\"afer, P. Candeloro, B. Leven, B. Hillebrands, and A. N. Slavin, Phys. Rev. Lett. \textbf{100}, 047204 (2008).}$^{,}$\footnote{K. Vogt, H. Schultheiss, S. J. Hermsdoerfer, P. Pirro, A. A. Serga, and B. Hillebrands, Appl. Phys. Lett. \textbf{95} 182508 (2009)}, realization of spin-wave transport in two-dimensional waveguides, including directional changes along the spin-wave propagation path\footnote{P. Clausen, K. Vogt, H. Schultheiss, S. Sch\"afer, B. Obry, G. Wolf, P. Pirro, B. Leven, and B. Hillebrands, Appl. Phys. Lett. \textbf{99}, 162505 (2011)}, and the effect of nonlinear damping mechanisms when spin waves are spatially confined in microstructures. We use phase- and time-resolved Brillouin light scattering microscopy to address these topics in micron-sized spin-wave conduits made from permalloy. These experiments allow us to develop a simple model for calculating dispersion relations in spin-wave conduits. This model can be applied to understand how spin waves are transported in conduits with broken translation symmetry and how nonlinear damping via four-magnon-scattering is enhanced due to spatial confinement.   

Using magnons to probe spintronic materials properties

For many spin-based electronic devices, from the read sensors in modern hard disk drives to future spintronic logic concepts, the device physics originates in spin polarized currents in ferromagnetic metals. In this talk, I will describe a novel ``Spin Wave Doppler'' method that uses the interaction of spin waves with spin-polarized currents to determine the spin drift velocity and the spin current polarization [1]. Owing to differences between the band structures of majority-spin and minority-spin electrons, the electrical current also carries an angular momentum current and magnetic moment current. Passing these coupled currents though a magnetic wire changes the linear excitations of the magnetization, i.e spin waves. Interestingly, the excitations can be described as drifting ``downstream'' with the electron flow. We measure this drift velocity by monitoring the spin-wave-mediated transmission between pairs of periodically patterned antennas on magnetic wires as a function of current density in the wire. The transmission frequency resonance shifts by 2$\pi \Delta $f = \textbf{v\textbullet k} where the drift velocity $v$ is proportional to both the current density and the current polarization $P$. I will discuss measurements of the spin polarization of the current in Ni$_{80}$Fe$_{20}$ [2], and novel alloys (CoFe)$_{1-x}$Ga$_{x}$ [3] and (Ni$_{80}$Fe$_{20})_{1-x}$Gd$_{x}$ [4]. \\[4pt] [1] V. Vlaminck and M. Bailleul, Science, \textbf{322}, 410 (2008) \\[0pt] [2] M. Zhu, C. L. Dennis, and R. D. McMichael, Phys. Rev. B, \textbf{81}, 140407 (2010). \\[0pt] [3] M. Zhu, B. D. Soe, R. D. McMichael, M. J. Carey, S. Maat, and J. R. Childress, Appl. Phys. Lett., \textbf{98}, 072510 (2011). \\[0pt] [4] R. L. Thomas, M. Zhu, C. L. Dennis, V. Misra and R. D. McMichael, J. Appl. Phys., \textbf{110}, 033902 (2011).   

Interaction between spin-wave excitations and pure spin currents in magnetic structures

The generation of pure spin current (PSC) in magnetic structures has attracted much attention not only for its fundamental importance in spintronics, but also because it opens up potential applications. One of the most exciting aspects of this area is the interplay between spin-waves (SW) and PSC. Here we report experimental results in which the PSC, generated by both spin pumping (SPE) [1] and spin Seebeck (SSE) [2] effects, can exert a spin-transfer torque sufficient to compensate the SW relaxation in yttrium iron garnet (YIG)/non-magnetic structures. By measuring the propagation of SW packets in single-crystal YIG films we were able to observe the amplification of volume and magnetostatic modes (MSW) by both SSE and SHE [3,4]. The excitation and detection of the SW packets is carried out by using a MSW delay line device. In both cases the amplification is attributed to the spin-transfer torque due to PSC generated by SSE as well as SHE. It will also be presented new results in which PSC are simultaneously excited by SSE and SPE effects in YIG films. While the spin current generated by SPE is obtained by exciting the ferromagnetic resonance (FMR) of the YIG film, the spin current due to SSE is created by applying a temperature gradient along the film plane. The effect of the superposition of both spin currents is characterized by measuring the spin Hall voltage (V$_{H})$ along thin strips of Pt deposited on top of the YIG films. Whereas V$_{H}$ corresponding to the uniform FMR is amplified due the SSE the voltages corresponding to the other magnetostatic spin-wave modes are attenuated [5]. \\[4pt] [1] Y. Tserkovnyak, et al., Rev. Mod. Phys. 77, 1375 (2005).\\[0pt] [2] K. Uchida, et al., Nature 455, 778 (2008).\\[0pt] [3] E. Padr\'{o}n-Hern\'{a}ndez, A. Azevedo, and S. M. Rezende, Phys. Rev. Letts., \textbf{107}, 197203 (2011).\\[0pt] [4] E. Padr\'{o}n-Hern\'{a}ndez, A. Azevedo, and S. M. Rezende, Appl. Phys. Letts., \textbf{99} (2011) in press.\\[0pt] [5] G.L. da Silva, L.H. Vilela-Le\~{a}o, S. M. Rezende and A. Azevedo, (in preparation).   

Manipulation of Spin Waves in Yttrium Iron Garnet Thin Films through Interfacial Spin Scattering

Spin waves in magnetic films have many properties that can be utilized for microwave signal processing and logic operations. These applications, however, are bottlenecked by the damping of spin waves. This presentation reports on a new method for the amplification of spin waves. Specifically, the presentation reports the electric manipulation of spin waves in yttrium iron garnet (YIG) thin films via interfacial spin scattering (ISS). Experiments used a 4.6 $\mu$m-thick YIG film strip with a 20 nm-thick Pt capping layer. A dc pulse was applied to the Pt film that produced a spin current along the Pt thickness direction via the spin-Hall effect. As the spin current scatters off the surface of the YIG film, it exerts a torque on the YIG surface spins. Due to the dipolar and exchange interactions, the effect of this torque is extended to other spins across the YIG thickness and thereby to spin-wave pulses traveling in the YIG film. The net effect of the ISS process depends critically on the relative orientation of (1) the magnetic moments of the electrons in the Pt layer that scatter off the YIG surface and (2) the precession axis of the magnetic moments on the YIG surface. When they are anti-parallel, the spin-wave damping is reduced and the amplitude of a traveling spin-wave pulse is increased. In a parallel configuration, the pulse experiences an enhanced attenuation.