Oct 06, 2016
Of spin super-conduction spin in electric insulators
Contact : Olivier Klein

Schematic illustration of the Spin Hall effect showing the inter-conversion of a charge current in Pt, Jc, into a pure spin current Js entering in the YIG.

Magnonic is an emerging research field, which aims at exploiting pure spin transport in magnetic materials. The elementary excitations are the propagating spin-waves, also called magnon, which are bosonic quasiparticles. The advantages over conventional electronic devices are a significant reduction in energy consumption thanks to the absence of Joule heating, as well as new features taking advantage of the wave-particle duality. Magnetic materials could be electrical insulators, knowing that their dynamical characteristics are usually much better than their metal counterparts. Among all magnetic insulators, yttrium iron garnet (YIG or Y3Fe5O12) holds a special place for having the lowest known damping factor in nature.
An inherent problem of using a magnetic insulator, is that the inter-conversion of the magnetic signal into an electrical signal (or vise-versa) at the inlet or outlet of the device. Until recently this interconversion was provided by microwave antennas put on the top surface of the material and inductively coupled to the magnetization dynamics. However, this method is not very sensitive and it requires the use of large volumes which prevents extreme miniaturization of devices. This situation has changed radically since the discovery of the spin Hall effect, which capitalizes on the deflection of the electron trajectory according to their spin orientation. When a charge current (Jc) is flowing in a normal metal of strong spin-orbit coupling such as platinum (Pt) it converts into a pure spin current (Js), propagating in the perpendicular direction, where the conversion efficiency is set by the spin Hall angle (see FIG1). If the latter can be transmitted through the interface then it can in principle provides the inter-convertion sought. It turns out that the experimental confirmation of the existence of a spin transfer at the insulator/metal interface has been the subject of a controversy in the community for the past 5 years. The striking signature of the spin transfer process would be the emission of microwave radiation when the system is pumped out of equilibrium by a d.c. current (FIG1). Since the spin transfer torque on the magnetization is collinear to the damping torque, an instability threshold occurs when the natural damping is fully compensated by the external flow of angular momentum, leading to spin-wave amplification through stimulated emission. Using analogy to light, the effect was called spin-wave amplification by stimulated emission of radiation
(SWASER). In a recent paper submitted to Nature Communications [1], we showed for the first time it was possible to induce self-oscillation YIG while flowing a direct electrical current in an adjacent layer of Pt above a critical threshold. To observe this effect, two features were important in order to minimize the amplitude of the threshold current. First it is necessary to use ultra-thin film of YIG of the highest quality, which must then lithographically patterned into micron-size disks in order to lift the degeneracy between normal modes. Direct observation of an RF signal emitted spontaneously will allow better understanding of the spin transfer processes at the interface, but it will possibly open the way to new magnonic functions exploiting the electronic control of magnetic relaxation.

Reference :
[1] M. Collet, X. de Milly, O. d’Allivy Kelly, V.V. Naletov, R. Bernard, P. Bortolotti, J. Ben Youssef, V.E. Demidov, S.O. Demokritov, J.L. Prieto, M. Mun oz, V. Cros, A. Anane, G. de Loubens & O. Klein, Nature Commun. 7, 10377 (2016).







(A) Schematic diagram and (b) microscopic image of the device used to study spin transfer effects in two YIG|Pt microdiscs of respectively 2 and 4μm in diameter. (C) and (d) For each of the disks, measurement with a spectrum analyzer of the fluctuations Vy when z DC current flows between the two contacts Vx. An RF signal appears above a critical current of about -13,5mA and -7,4mA respectively for ecah disk.


Last update : 10/06 2016 (1191)


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