Generation and detection of pure spin currents circulating in magnetic insulator materials have proven its worth by enabling transport of spin information across large distances, much further than in metals, thanks to the abscence of decay mechanism through the delocalized electrons. Spin currents here propagate over localized magnetic moments via spin-waves (SW), or their quanta the magnons, with characteristic frequencies ranging from GHz to THz and associated wavelengths from μm to nm. Most of the work so far has focus on Yttrium Iron garnets (YIG), which is a ferrimagnetic insulator with the lowest known amount of magnetic damping. In this talk, I will present recent result of spin waves transport in high quality ultra-thin films exploiting spin-orbit tools to interconvert the spin signal into an electrical signal. By injecting a high current density in Pt electrodes deposited few microns apart on top of a YIG film, a pure magnon current propagating in the YIG can be induced/detected via the direct/inverse spin Hall effect. In this work, I will mainly focus on investigating the behavior at large energies. We have found that the magnon tranresistance in this system can cross several regimes that involve strong change in the magnon distribution. Throughout various techniques such as Brillouin light scattering spectroscopy, spin Seebeck and spin Hall magnetoresistance measurements, we provide a complete analysis of the different phenomena surrounding the spin transport in thin YIG films and we will show that our experimental findings do not support yet the emergence of new collective behaviors, such as Bose-Einstein condensation at room temperature.
Lucile Soumah, Lilia Qassym, Cécile Carretero, Eric Jacquet, Jamal Ben-Youssef, Richard Lebourgeois, Nathan Beaulieu, Vincent Cros, Paolo Bortolotti, Abdelmadjid Anane
Pulsed laser deposition (PLD) of high quality nanometers thick (YIG) films have recently allowed to open the field of spintronics and magnonics nanostructured magnetic insulators[1,2]. YIG is a versatile material in term of anisotropy or magnetization as doping and growth induced strain can significantly change those properties. We present here the effect of Bi substitution on Bi:YIG PLD grown films with thicknesses ranging from 10 nm to 40 nm. By using lattice matched substituted GGG substrates (sGGG) it is possible to stabilize for specific growth conditions an out-of plane easy magnetization axis. We present comprehensive structural charcterisation using X-ray diffraction and squid magnetometry that shows the transition from an easy in-plane magnetization direction to an out-of plane magnetization. The effect of Bi doping is to significantly increase the Faraday rotation of the films. Using magneto-optical Kerr microscopy, it is therefore possible to observ the shape and the sizes of the magnetic domains for films thicknesses down to 10 nm. Using the Kooy and Enz model it is possible to extract a domain wall energy of 0.49 erg/cm2.
[1] O. D. Kelly et al., Applied Physics Letters 103, 4, 082408 (2013).
[2] M. Collet et al., Nature communications 7, 10377 (2016).
[3] C. Kooy and U. Enz, Philips Res. Rep., vol. 15, pp. 7-29(1960)
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