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Register a new userEnhanced THz emission from spintronic Fe/Pt emitters through crystal growth optimization
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Added by
Vassilios Kapaklis Dr.
Publication date
2020
fields
Physics
and research's language is
English
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We investigate the THz emission characteristics of ferromagnetic/non-magnetic metallic heterostructures, focusing on thin Fe/Pt bilayers. In particular, we report on the impact of optimized crystal growth of the epitaxial Fe layers on the THz emission amplitude and spectral bandwidth. We demonstrate an enhancement of the emitted intensity along with an expansion of the emission bandwidth. Both are related to reduced spin scattering and higher interface transmission. Our work provides a pathway for devicing optimal spintronic THz emitters based on epitaxial Fe. It also highlights how THz emission measurements can be utilized to characterize the changes in out-of-equilibrium spin current dynamics in metallic heterostructures, driven by subtle structural refinement.
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An ultra-broadband THz emitter covering a wide range of frequencies from 0.1 to 10 THz is highly desired for spectroscopy applications. So far, spintronic THz emitters have been proven as one class of efficient THz sources with a broadband spectrum while the performance in the lower frequency range (0.1 to 0.5 THz) limits its applications. In this work, we demonstrate a novel concept of a current-enhanced broad spectrum from spintronic THz emitters combined with semiconductor materials. We observe a 2-3 order enhancement of the THz signals in a lower THz frequency range (0.1 to 0.5 THz), in addition to a comparable performance at higher frequencies from this hybrid emitter. With a bias current, there is a photoconduction contribution from semiconductor materials, which can be constructively interfered with the THz signals generated from the magnetic heterostructures driven by the inverse spin Hall effect. Our findings push forward the utilization of metallic heterostructures-based THz emitters on the ultra-broadband THz emission spectroscopy.
THz pulses are generated from femtosecond pulse-excited ferromagnetic/nonmagnetic spintronic heterostructures via inverse spin Hall effect. The contribution from ultrafast demagnetization/remagnetization is extremely weak, in the comparison. The highest possible THz signal strength from spintronic THz emitters is limited by the optical damage threshold of the corresponding heterostructures. The THz generation efficiency does not saturate with the excitation fluence even up till the damage threshold. Bilayer (Fe, CoFeB)/(Pt, Ta) based FM/NM spintronic heterostructures have been studied for an optimized performance for THz generation when pumped by sub-50 fs amplified laser pulses at 800 nm. Among them, CoFeB/Pt is the best combination for an efficient THz source. The optimized FM/NM spintronic heterostructure on a quartz substrate, having alpha-phase Ta as the nonmagnetic layer, show the highest damage threshold as compared to those with Pt, irrespective of their generation efficiency. The damage threshold of the Fe/Ta heterostructure on quartz substrate is ~85 GW/cm2.
Significant progress has been made in answering fundamental questions about how and, more importantly, on what time scales interactions between electrons, spins, and phonons occur in solid state materials. These complex interactions are leading to the first real applications of terahertz (THz) spintronics: THz emitters that can compete with traditional THz sources and provide additional functionalities enabled by the spin degree of freedom. This tutorial article is intended to provide the background necessary to understand, use, and improve THz spintronic emitters. A particular focus is the introduction of the physical effects that underlie the operation of spintronic THz emitters. These effects were, for the most part, first discovered through traditional spin-transport and spintronic studies. We therefore begin with a review of the historical background and current theoretical understanding of ultrafast spin physics that has been developed over the past twenty-five years. We then discuss standard experimental techniques for the characterization of spintronic THz emitters and - more broadly - ultrafast magnetic phenomena. We next present the principles and methods of the synthesis and fabrication of various types of spintronic THz emitters. Finally, we review recent developments in this exciting field including the integration of novel material platforms such as topological insulators as well as antiferromagnets and materials with unconventional spin textures.
In this work, we report our study on the THz emission in Fe/Pt magnetic heterostructures. We have carried out a comprehensive investigation of THz emission from Fe/Pt magnetic heterostructures, employing time-domain THz spectroscopy. We reveal that by properly tuning the thickness of Fe or Pt layer, THz emission can be greatly improved in this type of heterostructure. We further demonstrate that the THz field strength emitted from a newly designed multilayer (Pt/Fe/MgO)$_n$ with n=3 can reach a value of ~1.6 kV/cm, which is comparable to the values from the conventional GaAs antenna with a bias of 4 kV/cm, and the nonlinear crystals, e.g., 100 micrometer GaP and 2 mm ZnTe. For the first time, the intensity and spectrum of THz wave is demonstrated to be tunable by the magnetic field applied on the patterned magnetic Fe/Pt heterostructures. These findings thus promise novel approaches to fabricate powerful and tunable THz emitters based on magnetic heterostructure.
Femtosecond laser excitation of a Co/Pt bilayer results in the efficient emission of picosecond THz pulses. Two known mechanisms for generating THz emission are spin-polarized currents through a Co/Pt interface, resulting in helicity-independent electric currents in the Pt layer due to the inverse spin-Hall effect and helicity-dependent electric currents at the Co/Pt interface due to the inverse spin-orbit torque effect. Here we explore how roughness, crystal structure and intermixing at the Co/Pt interface affect the efficiency of the THz emission. In particular, we varied the roughness of the interface, in the range of 0.1-0.4 nm, by tuning the deposition pressure conditions during the fabrication of the Co/Pt bilayers. To control the intermixing at the Co/Pt interface a 1-2 nm thick CoxPt1-x alloy spacer layer was introduced with various compositions of Co and Pt. Finally, the crystal structure of Co was varied from face centered cubic to hexagonal close packed. Our study shows that the roughness of the interface is of crucial importance for the efficiency of helicity-dependent THz emission induced by femtosecond laser pulses. However, it is puzzling that intermixing while strongly enhancing the helicity-independent THz emission had no effect on the helicity-dependent THz emission which is suppressed and similar to the smooth interfaces.
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