Generation of terahertz transients from Co2Fe0.4Mn0.6Si -Heusler-alloy/normal-metal nanobilayers excited by femtosecond optical pulses

Sarah Heidtfeld,Shigemasa Suga,Roman Sobolewski,Martin Mikulics,Carolin Schmitz-Antoniak,Genyu Chen,Koki Takanashi,Roman Adam,Daniel E Bürgler,Derang Cao,Takahide Kubota,Ivan Komissarov,Fangzhou Wang,Claus M Schneider,Hilde Hardtdegen,Christian Greb

doi:10.1103/physrevresearch.3.043025

Abstract

We generated pulses of electromagnetic radiation in the terahertz (THz) frequency range by optical excitation of ${\mathrm{Co}}_{2}{\mathrm{Fe}}_{0.4}{\mathrm{Mn}}_{0.6}\mathrm{Si}$ (CFMS)/normal-metal (NM) bilayer structures. The CFMS is a Heusler alloy showing a band gap in one spin channel and is therefore a half metal. We compared the THz emission efficiency in a systematic manner for four different CFMS/NM bilayers, where NM was either Pt, Ta, Cr, or Al. Our measurements show that the THz intensity is highest for a Pt capping. We also demonstrate the tunability of the THz amplitude by varying the magnetic field for all four bilayers. We attribute the THz generation to the inverse spin Hall effect. In order to investigate the role of the interface in THz generation, we measured the spin mixing conductance for each CFMS/NM bilayer using a ferromagnetic resonance method. We found that the spin-orbit coupling cannot completely describe the THz generation in the bilayers and that the spin transmission efficiency of the CFMS/NM interface and the spin diffusion length, as well as the oxidation of the NM layer, play crucial roles in the THz emission process.

Highlights

The detailed understanding of spin-to-charge conversion has been gaining paramount importance due to intended applications of the effect in low-power, high-speed spin-based electronic devices
If we look at the dependence of the inverse spin Hall effect (ISHE) efficiency on the heavy metal (HM) layer thickness in [39,40], we see that for Ta the ISHE is maximum at approximately 2.7 nm and the efficiency of spin-to-charge conversion sharply drops for thinner films
We have demonstrated the emission of THz transient radiation from bilayers formed by 20 nm of the half-metallic Heusler alloy Co2Fe0.4Mn0.6Si capped with 2-nm-thick metallic layers of either Pt, Ta, Cr, or Al

Summary

Introduction

The detailed understanding of spin-to-charge conversion has been gaining paramount importance due to intended applications of the effect in low-power, high-speed spin-based electronic devices. A variety of phenomena including the spin Hall effect [1,2], the spin Seebeck effect [3,4], and spin pumping [5] have been employed for the generation of spin currents. The spin Hall effect was predicted theoretically in 1971 [6] and the first experimental observation. Of the inverse process was made in 1984 [7]. It has been known that the inverse spin Hall effect (ISHE) can be employed for the conversion of a spin current into a charge current in ferromagnet (FM)/normal-metal (NM) bilayer structures [7,8]. The latter is related to the spin current density Js via

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Results

Conclusion