[15p-C41-1~7] 応用に向けた新規スピントロニクス現象の物理

▲ [15p-C41-5] THz spin waves in spin caloritronics

〇Joseph Barker¹、Gerrit Bauer^1,2,3 (1.IMR Tohoku Univ.、2.WPI-AIMR Tohoku Univ.、3.Kavli Institute of Nanoscience, Univ. Delft.)

キーワード：spintronics, spincaloritronics, spinwaves

Spin caloritronics is the study of the coupling of spin, charge and heat currents [1]. The experimental demonstration of the spin Seebeck effect in a magnetic insulator [2] triggered much interest due to the potential for applications such as heat scavenging devices [3]. Recent experiments show very long-ranged spin transport by thermal magnons [4], which demands an explanation. Most theory work has been limited by an ultra-simplistic model ferromagnet with a single spin wave mode without giving justice to the actual materials Yttrium Iron Garnet (YIG) or rare-earth doped variants such as Gadolinium Iron Garnet (GdIG). These are complicated ferrimagnets with 20 (32) spin wave modes, respectively. The impact of high-frequency (THz) optical modes in spin caloritronics has hitherto not been considered. We carried out detailed atomistic modelling to calculate the spin wave spectrum and thermal properties of these materials. In agreement with neutron scattering experiments [5], we find the spin wave spectrum itself to be highly temperature dependent. Moreover, the optical and acoustic spin wave modes have opposite chiralities, precessing with or against the magnetization. This has a significant impact on the underlying physics since optical modes increase magnetization but suppress spin pumping and therefore the spin Seebeck effect. In GdIG this plays an important role by inducing a sign change in the spin Seebeck effect due to the changing occupation of these modes [6]. [1] G. E. W. Bauer, E. Saitoh, and B. J. van Wees, Nature Mater. 11, 391 (2012).
[2] K. Uchida, J. Xiao, H. Adachi, et al., Nature Mater. 9, 894 (2010).
[3] K. Uchida, M. Ishida, T. Kikkawa, et al., J. Phys.: Condens. Matter 26, 389601 (2014).
[4] L. J. Cornelissen, J. Liu, R. A. Duine, et al., Nat Phys 11, 1022 (2015).
[5] J. S. Plant, J. Phys. C: Solid State Phys. 10, 4805 (1977).
[6] S. Gepraegs, A. Kehlberger, F. Della Coletta, et al., Nat. Commun. 7, 10452 (2016).