Current-Induced Transverse Spin-Wave Instability in a Thin Nanomagnet

Abstract

We show that an unpolarized electric current incident perpendicular to the plane of a thin ferromagnet can excite a spin-wave instability transverse to the current direction if source and drain contacts are not symmetric. The instability, which is driven by the current-induced "spin-transfer torque", exists for one current direction only.PACS numbers: 75.75.+a, 75.40.Gb, Ferromagnets serve as spin filters for an electrical current passing through the magnet: the spin of the electrons that are transmitted through a ferromagnet becomes partially polarized parallel or antiparallel to the direction of the magnetization whereas spin current perpendicular to the magnetization direction is absorbed. Spin filtering is the root cause for the "spintransfer torque", the phenomenon that a polarized current impinging on a ferromagnet affects its magnetization direction [1,2,3]. The source of the spin polarized current can either be a different ferromagnet, or, for a thick magnet, a region of the same ferromagnet upstream or downstream in the current flow. The "spin-transfer torque" gives rise to magnetization reversal in ferromagnet-normal-metal-ferromagnet trilayers [1, 2], which has been observed experimentally by several groups [4,5,6,7,8,9,10]. Dynamic manifestations of the spin-transfer torque include domain wall motion in bulk ferromagnets [11,12,13,14] and the excitation of spin waves by polarized currents in ferromagnetic multilayers or wires [2,3,4,5,6,7,8,15,16,17]. In all these manifestations, the current-induced spin torque can be distinguished from effects arising from the current induced magnetic field, the main difference being that spintransfer torque effects depend on the current direction, whereas magnetic field induced effects do not.In this letter, we show that an unpolarized current can also exert a spin-transfer torque on a ferromagnet, even if the magnet is so thin that its magnetization direction does not change along the current flow: Although an unpolarized current cannot exert a spin-transfer torque that changes the over-all magnetization direction, it can create a transverse spin wave instability for sufficiently high current densities if the source and drain contacts to the ferromagnet are not symmetric. This spin wave instability can be identified unambiguously as a spin-torque effect because of its dependence on current direction: the spin-wave instability is present for one current direction and absent for the other. The spin-wave instability should lead to a non-hysteretic feature in the currentvoltage characteristic of the ferromagnetic film that exists for one current direction only. In thick ferromagnets, such features have been observed in recent experiments [4,17] instability, asymmetric contacts to source and drain, is generically fulfilled in experiments on nanoscale magnets [5]. The issue of current-induced spin-wave excitation has significant practical relevance for devices based on the spin-torque effect in ferromagnetic multilayers. Whereas, experimentally, the presence of dynamica...

Self-consistent calculation of spin transport and magnetization dynamics

“…Özyilmaz et al [36] experimentally observed current-induced excitation of a single ferromagnetic layer. For an asymmetric Cu/Co/Cu nanopillar structure, current-induced excitations were observed for only one polarity of the current, where, according to the prediction [27], the lateral spin transfer torque should increase the magnetization inhomogeneity. In addition, they did not observe such excitations in a symmetric structure, as expected from the discussion above.…”

“…Spin transfer torques caused by lateral spin diffusion, which we will refer to as "lateral spin transfer torque", were proposed by Polianski and Brouwer [27]. The geometry of the system under consideration is shown in Fig.…”

“…One of us [28] theoretically extended the initial calculation [27] of lateral spin transfer torque to general situations to allow for variation of the magnetization in the direction of the current-flow. Such variation can give instabilities at a single interface, a possible explanation for spin transfer effects seen in point contact experiments [38].…”

“…[27], several experimental [36][37][38] and theoretical [28,[39][40][41] studies have been performed to understand the lateral spin diffusion effect. Özyilmaz et al [36] experimentally observed current-induced excitation of a single ferromagnetic layer.…”

“…In this paper, we call this feedback non-local spin transfer torque because there is a non-local effective relation between the spin torque and the magnetization profile. For the torque acting on the single FM layer, lateral spin diffusion in the two neighboring NM layers [27,28] is an important source for non-locality of the torque.…”