Study of spin dynamics in magnetic metals using terahertz and optical spectroscopy

Understanding of the ultrafast spin dynamics is essential for application and further development of new spintronic devices. The ultrafast spintronics is a promising path towards faster and less energetically demanding technologies comparing to current electronics. Recent studies have reported on a novel possi- bility of investigation of ultrafast out-of-plane spin transport using the technique of spintronic generation of terahertz (THz) pulses. This work focuses on deter- mining the nature of out-of-plane spin transfer in spintronic THz emitters by direct experimental quantification of the spin current relaxation length and the speed of spin propagation. Our results suggest that the ultrafast spin currents are of the ballistic nature and the spin current relaxation time is limited by the scaterring time and the relaxation length by the mean free path. Another recentely published method, also used in this work, is the the contactless THz measurement of anisotropic magnetoresistance (AMR). In a thin layer of cobalt- iron, we demonstrate that the AMR, a central phenomenon commonly used in the antiferromagnetic spintronics to read the state of magnetic order, can be investigated by the THz spectroscopy even if dominated by the crystalline term. We complement the study by a spectral analysis of the crystalline...Pochopení dynamiky ultrarychlých spinových procesů je zásadním krokem pro aplikace a další vývoj nových spintronických součástek. Ultrarychlá spin- tronika představuje slibnou cestu k rychlejším a energeticky méně náročným technologiím ve srovnání se současnou elektronikou. Nedávné studie naznačují nové možnosti zkoumání ultrarychlého spinového transportu v kolmém směru k rovině vzorku pomocí metody spintronického generování terahertzových (THz) pulzů. Tato práce se zaměřuje na určení povahy tohoto spinového transportu ve spintronických THz emitorech přímým experimentálním vyhodnocením re- laxační délky spinového proudu a rychlosti šíření spinu. Naše výsledky naznačují, že ultrarychlé spinové proudy jsou balistické povahy a doba relaxace spinového proudu je omezena rozptylovým časem a relaxační délka střední volnou dráhou. Další nedávno publikovanou metodou, rovněž použitou v této práci, je bezkon- taktní THz měření anizotropní magnetorezistence (AMR). V tenké vrstvě kobalt- železa demonstrujeme, že AMR, centrální jev běžně používaný v antiferomag- netické spintronice ke čtení stavu magnetického uspořádání, lze zkoumat THz spektroskopií, a to i v případě, kdy dominuje krystalický člen....Department of Chemical Physics and OpticsKatedra chemické fyziky a optikyMatematicko-fyzikální fakultaFaculty of Mathematics and Physic

Accessing ultrafast spin-transport dynamics in copper using broadband terahertz spectroscopy

We study the spatiotemporal dynamics of ultrafast electron spin transport across nanometer-thick copper layers using broadband terahertz spectroscopy. Our analysis of temporal delays, broadening and attenuation of the spin-current pulse revealed ballistic-like propagation of the pulse peak, approaching the Fermi velocity, and diffusive features including a significant velocity dispersion. A comparison to the frequency-dependent Ficks law identified the diffusion-dominated transport regime for distances larger than 2 nm. The findings lie the groundwork for designing future broadband spintronic devices.Comment: Main text consists of 3 figures and 4 pages of tex

Terahertz probing of anisotropic conductivity and morphology of CuMnAs epitaxial thin films

Antiferromagnetic CuMnAs thin films have attracted attention since the discovery of the manipulation of their magnetic structure via electrical, optical, and terahertz pulses of electric fields, enabling convenient approaches to the switching between magnetoresistive states of the film for the information storage. However, the magnetic structure and, thus, the efficiency of the manipulation can be affected by the film morphology and growth defects. In this study, we investigate the properties of CuMnAs thin films by probing the defect-related uniaxial anisotropy of electric conductivity by contact-free terahertz transmission spectroscopy. We show that the terahertz measurements conveniently detect the conductivity anisotropy, that are consistent with conventional DC Hall-bar measurements. Moreover, the terahertz technique allows for considerably finer determination of anisotropy axes and it is less sensitive to the local film degradation. Thanks to the averaging over a large detection area, the THz probing also allows for an analysis of strongly non-uniform thin films. Using scanning near-field terahertz and electron microscopies, we relate the observed anisotropic conductivity of CuMnAs to the elongation and orientation of growth defects, which influence the local microscopic conductivity. We also demonstrate control over the morphology of defects by using vicinal substrates.Comment: 33 pages, 16 figure

Study of terahertz radiation emitted using spintronic effects

Effective emission of picosecond terahertz (THz) pulses using optical femtosecond pul- ses is the basis of THz spectroscopy in the time domain. Recent studies have shown that ultrafast optical excitation of thin metal magnetic multilayers leads to effective emission of THz pulses by converting the spin current to electric current. This work focuses on deter- mining the absolute emission and conversion efficiency of spintronic emitters from several manufacturers. Our comparison suggests that efficiency comparable to highly optimized spintronic emitters can be achieved by utilizing multilayer manufacturing capabilites of the Faculty of Mathematics and Physics of Charles University. The work also demon- strates the significant influence of interface quality on the throughput of ultrafast spin currents. Furthermore, the work describes a saturation effect observed in the relation be- tween emission and optical excitation fluency, which defines suitable excitation conditions for scaling the THz emission to higher electric fields. The observed spectral dependence of emission on fluence complements the discussion about the nature of ultrafast spin current formation. 1Efektivní emise pikosekundových terahertzových (THz) pulzů využitím optických fem- tosekundových pulzů je základem THz spektroskopie v časové doméně. Nedávné studie ukázaly, že ultrarychlá optická excitace tenkých kovových magnetických multivrstev vede k efektivní emisi THz pulzů pomocí konverze spinového proudu na elektrický proud. Tato práce se zaměřuje na určení absolutní emise a efektivity konverze spintronických emitorů od několika producentů. Ze srovnání plyne, že lze dosáhnout efektivity srovnatelné s vysoce optimalizovanými spintronickými emitory využitím růstových možností v rámci Matematicko-fyzikální fakulty UK. Práce zároveň demonstruje výrazný vliv kvality roz- hraní na propustnost ultrarychlých spinových proudů. Na závislosti emise na fluenci op- tické excitace je pozorován saturační efekt, který definuje vhodné podmínky excitace pro škálování THz emise do vyšších elektrických polí. Pozorovaná spektrální závislost emise na fluenci doplňuje diskuzi o podstatě vzniku ultrarychlých spinových proudů. 1Department of Chemical Physics and OpticsKatedra chemické fyziky a optikyMatematicko-fyzikální fakultaFaculty of Mathematics and Physic

Study of spin dynamics in magnetic metals using terahertz and optical spectroscopy

Understanding of the ultrafast spin dynamics is essential for application and further development of new spintronic devices. The ultrafast spintronics is a promising path towards faster and less energetically demanding technologies comparing to current electronics. Recent studies have reported on a novel possi- bility of investigation of ultrafast out-of-plane spin transport using the technique of spintronic generation of terahertz (THz) pulses. This work focuses on deter- mining the nature of out-of-plane spin transfer in spintronic THz emitters by direct experimental quantification of the spin current relaxation length and the speed of spin propagation. Our results suggest that the ultrafast spin currents are of the ballistic nature and the spin current relaxation time is limited by the scaterring time and the relaxation length by the mean free path. Another recentely published method, also used in this work, is the the contactless THz measurement of anisotropic magnetoresistance (AMR). In a thin layer of cobalt- iron, we demonstrate that the AMR, a central phenomenon commonly used in the antiferromagnetic spintronics to read the state of magnetic order, can be investigated by the THz spectroscopy even if dominated by the crystalline term. We complement the study by a spectral analysis of the crystalline..

Study of terahertz radiation emitted using spintronic effects

Effective emission of picosecond terahertz (THz) pulses using optical femtosecond pul- ses is the basis of THz spectroscopy in the time domain. Recent studies have shown that ultrafast optical excitation of thin metal magnetic multilayers leads to effective emission of THz pulses by converting the spin current to electric current. This work focuses on deter- mining the absolute emission and conversion efficiency of spintronic emitters from several manufacturers. Our comparison suggests that efficiency comparable to highly optimized spintronic emitters can be achieved by utilizing multilayer manufacturing capabilites of the Faculty of Mathematics and Physics of Charles University. The work also demon- strates the significant influence of interface quality on the throughput of ultrafast spin currents. Furthermore, the work describes a saturation effect observed in the relation be- tween emission and optical excitation fluency, which defines suitable excitation conditions for scaling the THz emission to higher electric fields. The observed spectral dependence of emission on fluence complements the discussion about the nature of ultrafast spin current formation.

Long-term trends in forest bird populations reflect management changes in Central European forests

In areas where forest is under long-term human influence, we can expect that forest bird populations will be affected by changes in forest management. Consequences of such forest management changes can be studied using long-term population trajectories of habitat-defined groups of bird species because various species are associated to different habitat types. Here we focused on 51 species of common forest birds in Czechia, a central European country, using data from a nation-wide breeding bird monitoring scheme collected from 1982 to 2019. Important changes in forest management took place over this time period in Europe and we asked whether the trends in Czech forest bird populations reflected these changes. For this purpose, we defined nine groups according to the species’ habitat associations in respect of stand size, vegetation layers and tree species composition. We calculated annual population indices using log-linear models with Poisson distribution for every species and composed these annual indices into multispecies indicators (MSIs). MSI of all forest birds, as well as MSIs of three groups of habitat generalists showed significant increases over the focal period. The groups of more specialized species showed more variable population trajectories: birds of the shrub layer decreased, whereas birds of the forest canopy and birds of broad-leaved trees increased, and populations of birds of forest edges, forest interior and coniferous trees were stable. These patterns in forest bird populations are consistent with forest maturation and increasing wood volume that took place in Czech forests over several decades. Such changes in forest environment are likely consequences of the change in purposes of forest management from fuel wood to timber harvesting that took place in 20th century resulting in a longer rotation period. Increases in species associated with broad-leaved trees can be explained by replacement of coniferous stands by stands of broad-leaved trees indicating recovery of forest environment towards more natural conditions as majority of the forested area would be naturally covered by broad-leaved vegetation in Czechia. In addition, the occurrence of some broad-leaved trees within coniferous stands, a common practice in Czech forestry, may also provide benefits to populations of bird species associated with broad-leaved trees. Taken together, these results support suitability of current forest management for forest birds that corresponds to recent findings from several other European regions. However, we should take this conclusion with caution because the increases of generalist birds may be also driven by factors outside the forest environment