Unraveling relaxation mechanisms in ultra-low damping Fe80Co20 thin films

DANIEL VELÁZQUEZ RODRIGUEZ; JAVIER E. GÓMEZ; GABRIELA ALEJANDRO; MELISSA VAN LANDEGHEM; ETIENNE GOOVAERTS; ALEJANDRO BUTERA

Conferencia; Intermag 21 Virtual Conference; 2021

FeCo alloys have been widely studied for decades because of their unique metallic andmagnetic properties. In recent years, the search of new systems adequate for spintronic devices promoted the study of bilayers consisting of a ferromagnetic film (FM) in contact with a non-magnetic conductive metal (NM).In these bilayers, when the FM is exposed to microwave radiation it can be driven to its ferromagnetic resonance (FMR) condition to produce the injection of a pure spin current across the FM/NM interface. In this framework, the search for new materials and alloys suitable for the fabrication of FM/NM systems, from which an intense pure spin current could be obtained, is one of the main challenges of today?s spintronics. With the purpose of increasing the intensity of the pure spin current density, J , which is inversely proportional to the damping parameter (α) of the ferromagnet (J ∼1/α )[1] during the spin pumping, the search of materials with low damping constants has been very active during the last years. A recent work by M. Schoen et al. [2] reported that Fe Co polycrystalline alloys present an ultra-low magnetic damping near x = 75 ( 20 Oe at 10 GHz), which make them promising candidates for the efficient injection of spin currents. Our results highlight the influence of the sputtering conditions (deposition power and temperature) on the magnetic properties ofFe Co /Ta bilayers deposited on MgO(001) and Si(001) which have been studied using ferromagnetic resonance and magneto-optic Kerr-effect techniques. We found that for the studied fabrication conditions the samples deposited on MgO(001) presented [100] epitaxial growth with a cubic anisotropy field ∼300 Oe, while those sputtered on Si(001) grow polycrystalline with a small uniaxial anisotropy. Moreover, the deposited samples on MgO grow with the axis [100] of the Fe Co rotated by 45 from the axis [100] of the MgO substrate (this was verified using XRD techniques). We have found that the optimal growth conditions to obtain a minimum in the damping constant and films with cubic symmetry are 13 - 16 W of sputtering power and single crystal MgO substrates. Substrate heating during sputtering at temperatures of 150 C also helps to increase the quality of the films. Ferromagnetic resonance measurements at different excitation frequencies (Fig. 1) have determined exceptionally low damping values for ferromagnetic conductors (α∼3×10 ), whichmakes the Fe Co /Ta system an excellent candidate for future applications in spintronic devices. From the dependence of the FMR linewidth on different excitation frequencies, we also found, different relaxation mechanisms act in easy and hard magnetization directions (Fig. 1). From the analysis of the dependence of the FMR linewidth on the orientation of the applied magnetic field and the excitation frequency, we were able to separate the contribution of the different relaxation mechanisms (Gilbert damping, two magnon scattering and mosaicity) to the linewidth and to explain quantitatively the observed behavior (Fig. 2). We made simulations to estimate the mosaicity and obtained relatively small values, consistent with a good quality of the samples. Finally, we also estimated the strength of the two magnon scattering which agrees with values reported in Ferich alloys [3]. The models proposed in this work were corroborated for different excitation frequencies. Our present results are very promising for the development of spintronic devices with improved functionalities.This work was partially supported by Conicet under Grant PIP 201501-00213, ANPCyT Grant PICT 2013- 0401, and U.N. Cuyo Grant 06/C556 all from Argentina. We also received financial support through the international cooperation project between Argentina and Belgium MINCyT-FWO FW/15/01-VS.041.16 N.References: [1] JE Gómez, B. Zerai Tedlla, NR Álvarez, G. Alejandro, E. Goovaerts y A. Butera Phys. Rev. B 90, 184401 (2014)[2] Martin AW Schoen, Danny Thonig, Michael L. Schneider, TJ Silva, Hans T. Nembach, Olle Eriksson, Olof Karis y Justin M. Shaw. [3] K. Lenz, H. Wende, W. Kuch y K.Baberschke, Phys. Rev. B 73, 144424 (2006)